[dragonfly.git] / sys / kern / vfs_bio.c

/*
 * Copyright (c) 1994,1997 John S. Dyson
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice immediately at the beginning of the file, without modification,
 *    this list of conditions, and the following disclaimer.
 * 2. Absolutely no warranty of function or purpose is made by the author
 *              John S. Dyson.
 *
 * $FreeBSD: src/sys/kern/vfs_bio.c,v 1.242.2.20 2003/05/28 18:38:10 alc Exp $
 */

/*
 * this file contains a new buffer I/O scheme implementing a coherent
 * VM object and buffer cache scheme.  Pains have been taken to make
 * sure that the performance degradation associated with schemes such
 * as this is not realized.
 *
 * Author:  John S. Dyson
 * Significant help during the development and debugging phases
 * had been provided by David Greenman, also of the FreeBSD core team.
 *
 * see man buf(9) for more info.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/buf.h>
#include <sys/conf.h>
#include <sys/devicestat.h>
#include <sys/eventhandler.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/kernel.h>
#include <sys/kthread.h>
#include <sys/proc.h>
#include <sys/reboot.h>
#include <sys/resourcevar.h>
#include <sys/sysctl.h>
#include <sys/vmmeter.h>
#include <sys/vnode.h>
#include <sys/dsched.h>
#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_kern.h>
#include <vm/vm_pageout.h>
#include <vm/vm_page.h>
#include <vm/vm_object.h>
#include <vm/vm_extern.h>
#include <vm/vm_map.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>

#include <sys/buf2.h>
#include <sys/thread2.h>
#include <sys/spinlock2.h>
#include <sys/mplock2.h>
#include <vm/vm_page2.h>

#include "opt_ddb.h"
#ifdef DDB
#include <ddb/ddb.h>
#endif

/*
 * Buffer queues.
 */
enum bufq_type {
        BQUEUE_NONE,            /* not on any queue */
        BQUEUE_LOCKED,          /* locked buffers */
        BQUEUE_CLEAN,           /* non-B_DELWRI buffers */
        BQUEUE_DIRTY,           /* B_DELWRI buffers */
        BQUEUE_DIRTY_HW,        /* B_DELWRI buffers - heavy weight */
        BQUEUE_EMPTYKVA,        /* empty buffer headers with KVA assignment */
        BQUEUE_EMPTY,           /* empty buffer headers */

        BUFFER_QUEUES           /* number of buffer queues */
};

typedef enum bufq_type bufq_type_t;

#define BD_WAKE_SIZE    16384
#define BD_WAKE_MASK    (BD_WAKE_SIZE - 1)

TAILQ_HEAD(bqueues, buf) bufqueues[BUFFER_QUEUES];
static struct spinlock bufqspin = SPINLOCK_INITIALIZER(&bufqspin);
static struct spinlock bufcspin = SPINLOCK_INITIALIZER(&bufcspin);

static MALLOC_DEFINE(M_BIOBUF, "BIO buffer", "BIO buffer");

struct buf *buf;                /* buffer header pool */

static void vfs_clean_pages(struct buf *bp);
static void vfs_clean_one_page(struct buf *bp, int pageno, vm_page_t m);
static void vfs_dirty_one_page(struct buf *bp, int pageno, vm_page_t m);
static void vfs_vmio_release(struct buf *bp);
static int flushbufqueues(bufq_type_t q);
static vm_page_t bio_page_alloc(vm_object_t obj, vm_pindex_t pg, int deficit);

static void bd_signal(int totalspace);
static void buf_daemon(void);
static void buf_daemon_hw(void);

/*
 * bogus page -- for I/O to/from partially complete buffers
 * this is a temporary solution to the problem, but it is not
 * really that bad.  it would be better to split the buffer
 * for input in the case of buffers partially already in memory,
 * but the code is intricate enough already.
 */
vm_page_t bogus_page;

/*
 * These are all static, but make the ones we export globals so we do
 * not need to use compiler magic.
 */
int bufspace;                   /* locked by buffer_map */
int maxbufspace;
static int bufmallocspace;      /* atomic ops */
int maxbufmallocspace, lobufspace, hibufspace;
static int bufreusecnt, bufdefragcnt, buffreekvacnt;
static int lorunningspace;
static int hirunningspace;
static int runningbufreq;               /* locked by bufcspin */
static int dirtybufspace;               /* locked by bufcspin */
static int dirtybufcount;               /* locked by bufcspin */
static int dirtybufspacehw;             /* locked by bufcspin */
static int dirtybufcounthw;             /* locked by bufcspin */
static int runningbufspace;             /* locked by bufcspin */
static int runningbufcount;             /* locked by bufcspin */
int lodirtybufspace;
int hidirtybufspace;
static int getnewbufcalls;
static int getnewbufrestarts;
static int recoverbufcalls;
static int needsbuffer;         /* locked by bufcspin */
static int bd_request;          /* locked by bufcspin */
static int bd_request_hw;       /* locked by bufcspin */
static u_int bd_wake_ary[BD_WAKE_SIZE];
static u_int bd_wake_index;
static u_int vm_cycle_point = 40; /* 23-36 will migrate more act->inact */
static int debug_commit;

static struct thread *bufdaemon_td;
static struct thread *bufdaemonhw_td;
static u_int lowmempgallocs;
static u_int lowmempgfails;

/*
 * Sysctls for operational control of the buffer cache.
 */
SYSCTL_INT(_vfs, OID_AUTO, lodirtybufspace, CTLFLAG_RW, &lodirtybufspace, 0,
        "Number of dirty buffers to flush before bufdaemon becomes inactive");
SYSCTL_INT(_vfs, OID_AUTO, hidirtybufspace, CTLFLAG_RW, &hidirtybufspace, 0,
        "High watermark used to trigger explicit flushing of dirty buffers");
SYSCTL_INT(_vfs, OID_AUTO, lorunningspace, CTLFLAG_RW, &lorunningspace, 0,
        "Minimum amount of buffer space required for active I/O");
SYSCTL_INT(_vfs, OID_AUTO, hirunningspace, CTLFLAG_RW, &hirunningspace, 0,
        "Maximum amount of buffer space to usable for active I/O");
SYSCTL_UINT(_vfs, OID_AUTO, lowmempgallocs, CTLFLAG_RW, &lowmempgallocs, 0,
        "Page allocations done during periods of very low free memory");
SYSCTL_UINT(_vfs, OID_AUTO, lowmempgfails, CTLFLAG_RW, &lowmempgfails, 0,
        "Page allocations which failed during periods of very low free memory");
SYSCTL_UINT(_vfs, OID_AUTO, vm_cycle_point, CTLFLAG_RW, &vm_cycle_point, 0,
        "Recycle pages to active or inactive queue transition pt 0-64");
/*
 * Sysctls determining current state of the buffer cache.
 */
SYSCTL_INT(_vfs, OID_AUTO, nbuf, CTLFLAG_RD, &nbuf, 0,
        "Total number of buffers in buffer cache");
SYSCTL_INT(_vfs, OID_AUTO, dirtybufspace, CTLFLAG_RD, &dirtybufspace, 0,
        "Pending bytes of dirty buffers (all)");
SYSCTL_INT(_vfs, OID_AUTO, dirtybufspacehw, CTLFLAG_RD, &dirtybufspacehw, 0,
        "Pending bytes of dirty buffers (heavy weight)");
SYSCTL_INT(_vfs, OID_AUTO, dirtybufcount, CTLFLAG_RD, &dirtybufcount, 0,
        "Pending number of dirty buffers");
SYSCTL_INT(_vfs, OID_AUTO, dirtybufcounthw, CTLFLAG_RD, &dirtybufcounthw, 0,
        "Pending number of dirty buffers (heavy weight)");
SYSCTL_INT(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD, &runningbufspace, 0,
        "I/O bytes currently in progress due to asynchronous writes");
SYSCTL_INT(_vfs, OID_AUTO, runningbufcount, CTLFLAG_RD, &runningbufcount, 0,
        "I/O buffers currently in progress due to asynchronous writes");
SYSCTL_INT(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RD, &maxbufspace, 0,
        "Hard limit on maximum amount of memory usable for buffer space");
SYSCTL_INT(_vfs, OID_AUTO, hibufspace, CTLFLAG_RD, &hibufspace, 0,
        "Soft limit on maximum amount of memory usable for buffer space");
SYSCTL_INT(_vfs, OID_AUTO, lobufspace, CTLFLAG_RD, &lobufspace, 0,
        "Minimum amount of memory to reserve for system buffer space");
SYSCTL_INT(_vfs, OID_AUTO, bufspace, CTLFLAG_RD, &bufspace, 0,
        "Amount of memory available for buffers");
SYSCTL_INT(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RD, &maxbufmallocspace,
        0, "Maximum amount of memory reserved for buffers using malloc");
SYSCTL_INT(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD, &bufmallocspace, 0,
        "Amount of memory left for buffers using malloc-scheme");
SYSCTL_INT(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RD, &getnewbufcalls, 0,
        "New buffer header acquisition requests");
SYSCTL_INT(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RD, &getnewbufrestarts,
        0, "New buffer header acquisition restarts");
SYSCTL_INT(_vfs, OID_AUTO, recoverbufcalls, CTLFLAG_RD, &recoverbufcalls, 0,
        "Recover VM space in an emergency");
SYSCTL_INT(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RD, &bufdefragcnt, 0,
        "Buffer acquisition restarts due to fragmented buffer map");
SYSCTL_INT(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RD, &buffreekvacnt, 0,
        "Amount of time KVA space was deallocated in an arbitrary buffer");
SYSCTL_INT(_vfs, OID_AUTO, bufreusecnt, CTLFLAG_RD, &bufreusecnt, 0,
        "Amount of time buffer re-use operations were successful");
SYSCTL_INT(_vfs, OID_AUTO, debug_commit, CTLFLAG_RW, &debug_commit, 0, "");
SYSCTL_INT(_debug_sizeof, OID_AUTO, buf, CTLFLAG_RD, 0, sizeof(struct buf),
        "sizeof(struct buf)");

char *buf_wmesg = BUF_WMESG;

#define VFS_BIO_NEED_ANY        0x01    /* any freeable buffer */
#define VFS_BIO_NEED_UNUSED02   0x02
#define VFS_BIO_NEED_UNUSED04   0x04
#define VFS_BIO_NEED_BUFSPACE   0x08    /* wait for buf space, lo hysteresis */

/*
 * bufspacewakeup:
 *
 *      Called when buffer space is potentially available for recovery.
 *      getnewbuf() will block on this flag when it is unable to free 
 *      sufficient buffer space.  Buffer space becomes recoverable when 
 *      bp's get placed back in the queues.
 */
static __inline void
bufspacewakeup(void)
{
        /*
         * If someone is waiting for BUF space, wake them up.  Even
         * though we haven't freed the kva space yet, the waiting
         * process will be able to now.
         */
        spin_lock(&bufcspin);
        if (needsbuffer & VFS_BIO_NEED_BUFSPACE) {
                needsbuffer &= ~VFS_BIO_NEED_BUFSPACE;
                spin_unlock(&bufcspin);
                wakeup(&needsbuffer);
        } else {
                spin_unlock(&bufcspin);
        }
}

/*
 * runningbufwakeup:
 *
 *      Accounting for I/O in progress.
 *
 */
static __inline void
runningbufwakeup(struct buf *bp)
{
        int totalspace;
        int limit;

        if ((totalspace = bp->b_runningbufspace) != 0) {
                spin_lock(&bufcspin);
                runningbufspace -= totalspace;
                --runningbufcount;
                bp->b_runningbufspace = 0;

                /*
                 * see waitrunningbufspace() for limit test.
                 */
                limit = hirunningspace * 4 / 6;
                if (runningbufreq && runningbufspace <= limit) {
                        runningbufreq = 0;
                        spin_unlock(&bufcspin);
                        wakeup(&runningbufreq);
                } else {
                        spin_unlock(&bufcspin);
                }
                bd_signal(totalspace);
        }
}

/*
 * bufcountwakeup:
 *
 *      Called when a buffer has been added to one of the free queues to
 *      account for the buffer and to wakeup anyone waiting for free buffers.
 *      This typically occurs when large amounts of metadata are being handled
 *      by the buffer cache ( else buffer space runs out first, usually ).
 *
 * MPSAFE
 */
static __inline void
bufcountwakeup(void) 
{
        spin_lock(&bufcspin);
        if (needsbuffer) {
                needsbuffer &= ~VFS_BIO_NEED_ANY;
                spin_unlock(&bufcspin);
                wakeup(&needsbuffer);
        } else {
                spin_unlock(&bufcspin);
        }
}

/*
 * waitrunningbufspace()
 *
 * Wait for the amount of running I/O to drop to hirunningspace * 4 / 6.
 * This is the point where write bursting stops so we don't want to wait
 * for the running amount to drop below it (at least if we still want bioq
 * to burst writes).
 *
 * The caller may be using this function to block in a tight loop, we
 * must block while runningbufspace is greater then or equal to
 * hirunningspace * 4 / 6.
 *
 * And even with that it may not be enough, due to the presence of
 * B_LOCKED dirty buffers, so also wait for at least one running buffer
 * to complete.
 */
void
waitrunningbufspace(void)
{
        int limit = hirunningspace * 4 / 6;
        int dummy;

        spin_lock(&bufcspin);
        if (runningbufspace > limit) {
                while (runningbufspace > limit) {
                        ++runningbufreq;
                        ssleep(&runningbufreq, &bufcspin, 0, "wdrn1", 0);
                }
                spin_unlock(&bufcspin);
        } else if (runningbufspace > limit / 2) {
                ++runningbufreq;
                spin_unlock(&bufcspin);
                tsleep(&dummy, 0, "wdrn2", 1);
        } else {
                spin_unlock(&bufcspin);
        }
}

/*
 * buf_dirty_count_severe:
 *
 *      Return true if we have too many dirty buffers.
 */
int
buf_dirty_count_severe(void)
{
        return (runningbufspace + dirtybufspace >= hidirtybufspace ||
                dirtybufcount >= nbuf / 2);
}

/*
 * Return true if the amount of running I/O is severe and BIOQ should
 * start bursting.
 */
int
buf_runningbufspace_severe(void)
{
        return (runningbufspace >= hirunningspace * 4 / 6);
}

/*
 * vfs_buf_test_cache:
 *
 * Called when a buffer is extended.  This function clears the B_CACHE
 * bit if the newly extended portion of the buffer does not contain
 * valid data.
 *
 * NOTE! Dirty VM pages are not processed into dirty (B_DELWRI) buffer
 * cache buffers.  The VM pages remain dirty, as someone had mmap()'d
 * them while a clean buffer was present.
 */
static __inline__
void
vfs_buf_test_cache(struct buf *bp,
                  vm_ooffset_t foff, vm_offset_t off, vm_offset_t size,
                  vm_page_t m)
{
        if (bp->b_flags & B_CACHE) {
                int base = (foff + off) & PAGE_MASK;
                if (vm_page_is_valid(m, base, size) == 0)
                        bp->b_flags &= ~B_CACHE;
        }
}

/*
 * bd_speedup()
 *
 * Spank the buf_daemon[_hw] if the total dirty buffer space exceeds the
 * low water mark.
 *
 * MPSAFE
 */
static __inline__
void
bd_speedup(void)
{
        if (dirtybufspace < lodirtybufspace && dirtybufcount < nbuf / 2)
                return;

        if (bd_request == 0 &&
            (dirtybufspace - dirtybufspacehw > lodirtybufspace / 2 ||
             dirtybufcount - dirtybufcounthw >= nbuf / 2)) {
                spin_lock(&bufcspin);
                bd_request = 1;
                spin_unlock(&bufcspin);
                wakeup(&bd_request);
        }
        if (bd_request_hw == 0 &&
            (dirtybufspacehw > lodirtybufspace / 2 ||
             dirtybufcounthw >= nbuf / 2)) {
                spin_lock(&bufcspin);
                bd_request_hw = 1;
                spin_unlock(&bufcspin);
                wakeup(&bd_request_hw);
        }
}

/*
 * bd_heatup()
 *
 *      Get the buf_daemon heated up when the number of running and dirty
 *      buffers exceeds the mid-point.
 *
 *      Return the total number of dirty bytes past the second mid point
 *      as a measure of how much excess dirty data there is in the system.
 *
 * MPSAFE
 */
int
bd_heatup(void)
{
        int mid1;
        int mid2;
        int totalspace;

        mid1 = lodirtybufspace + (hidirtybufspace - lodirtybufspace) / 2;

        totalspace = runningbufspace + dirtybufspace;
        if (totalspace >= mid1 || dirtybufcount >= nbuf / 2) {
                bd_speedup();
                mid2 = mid1 + (hidirtybufspace - mid1) / 2;
                if (totalspace >= mid2)
                        return(totalspace - mid2);
        }
        return(0);
}

/*
 * bd_wait()
 *
 *      Wait for the buffer cache to flush (totalspace) bytes worth of
 *      buffers, then return.
 *
 *      Regardless this function blocks while the number of dirty buffers
 *      exceeds hidirtybufspace.
 *
 * MPSAFE
 */
void
bd_wait(int totalspace)
{
        u_int i;
        int count;

        if (curthread == bufdaemonhw_td || curthread == bufdaemon_td)
                return;

        while (totalspace > 0) {
                bd_heatup();
                if (totalspace > runningbufspace + dirtybufspace)
                        totalspace = runningbufspace + dirtybufspace;
                count = totalspace / BKVASIZE;
                if (count >= BD_WAKE_SIZE)
                        count = BD_WAKE_SIZE - 1;

                spin_lock(&bufcspin);
                i = (bd_wake_index + count) & BD_WAKE_MASK;
                ++bd_wake_ary[i];

                /*
                 * This is not a strict interlock, so we play a bit loose
                 * with locking access to dirtybufspace*
                 */
                tsleep_interlock(&bd_wake_ary[i], 0);
                spin_unlock(&bufcspin);
                tsleep(&bd_wake_ary[i], PINTERLOCKED, "flstik", hz);

                totalspace = runningbufspace + dirtybufspace - hidirtybufspace;
        }
}

/*
 * bd_signal()
 * 
 *      This function is called whenever runningbufspace or dirtybufspace
 *      is reduced.  Track threads waiting for run+dirty buffer I/O
 *      complete.
 *
 * MPSAFE
 */
static void
bd_signal(int totalspace)
{
        u_int i;

        if (totalspace > 0) {
                if (totalspace > BKVASIZE * BD_WAKE_SIZE)
                        totalspace = BKVASIZE * BD_WAKE_SIZE;
                spin_lock(&bufcspin);
                while (totalspace > 0) {
                        i = bd_wake_index++;
                        i &= BD_WAKE_MASK;
                        if (bd_wake_ary[i]) {
                                bd_wake_ary[i] = 0;
                                spin_unlock(&bufcspin);
                                wakeup(&bd_wake_ary[i]);
                                spin_lock(&bufcspin);
                        }
                        totalspace -= BKVASIZE;
                }
                spin_unlock(&bufcspin);
        }
}

/*
 * BIO tracking support routines.
 *
 * Release a ref on a bio_track.  Wakeup requests are atomically released
 * along with the last reference so bk_active will never wind up set to
 * only 0x80000000.
 *
 * MPSAFE
 */
static
void
bio_track_rel(struct bio_track *track)
{
        int     active;
        int     desired;

        /*
         * Shortcut
         */
        active = track->bk_active;
        if (active == 1 && atomic_cmpset_int(&track->bk_active, 1, 0))
                return;

        /*
         * Full-on.  Note that the wait flag is only atomically released on
         * the 1->0 count transition.
         *
         * We check for a negative count transition using bit 30 since bit 31
         * has a different meaning.
         */
        for (;;) {
                desired = (active & 0x7FFFFFFF) - 1;
                if (desired)
                        desired |= active & 0x80000000;
                if (atomic_cmpset_int(&track->bk_active, active, desired)) {
                        if (desired & 0x40000000)
                                panic("bio_track_rel: bad count: %p\n", track);
                        if (active & 0x80000000)
                                wakeup(track);
                        break;
                }
                active = track->bk_active;
        }
}

/*
 * Wait for the tracking count to reach 0.
 *
 * Use atomic ops such that the wait flag is only set atomically when
 * bk_active is non-zero.
 *
 * MPSAFE
 */
int
bio_track_wait(struct bio_track *track, int slp_flags, int slp_timo)
{
        int     active;
        int     desired;
        int     error;

        /*
         * Shortcut
         */
        if (track->bk_active == 0)
                return(0);

        /*
         * Full-on.  Note that the wait flag may only be atomically set if
         * the active count is non-zero.
         *
         * NOTE: We cannot optimize active == desired since a wakeup could
         *       clear active prior to our tsleep_interlock().
         */
        error = 0;
        while ((active = track->bk_active) != 0) {
                cpu_ccfence();
                desired = active | 0x80000000;
                tsleep_interlock(track, slp_flags);
                if (atomic_cmpset_int(&track->bk_active, active, desired)) {
                        error = tsleep(track, slp_flags | PINTERLOCKED,
                                       "trwait", slp_timo);
                        if (error)
                                break;
                }
        }
        return (error);
}

/*
 * bufinit:
 *
 *      Load time initialisation of the buffer cache, called from machine
 *      dependant initialization code. 
 */
void
bufinit(void)
{
        struct buf *bp;
        vm_offset_t bogus_offset;
        int i;

        /* next, make a null set of free lists */
        for (i = 0; i < BUFFER_QUEUES; i++)
                TAILQ_INIT(&bufqueues[i]);

        /* finally, initialize each buffer header and stick on empty q */
        for (i = 0; i < nbuf; i++) {
                bp = &buf[i];
                bzero(bp, sizeof *bp);
                bp->b_flags = B_INVAL;  /* we're just an empty header */
                bp->b_cmd = BUF_CMD_DONE;
                bp->b_qindex = BQUEUE_EMPTY;
                initbufbio(bp);
                xio_init(&bp->b_xio);
                buf_dep_init(bp);
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_EMPTY], bp, b_freelist);
        }

        /*
         * maxbufspace is the absolute maximum amount of buffer space we are 
         * allowed to reserve in KVM and in real terms.  The absolute maximum
         * is nominally used by buf_daemon.  hibufspace is the nominal maximum
         * used by most other processes.  The differential is required to 
         * ensure that buf_daemon is able to run when other processes might 
         * be blocked waiting for buffer space.
         *
         * maxbufspace is based on BKVASIZE.  Allocating buffers larger then
         * this may result in KVM fragmentation which is not handled optimally
         * by the system.
         */
        maxbufspace = nbuf * BKVASIZE;
        hibufspace = imax(3 * maxbufspace / 4, maxbufspace - MAXBSIZE * 10);
        lobufspace = hibufspace - MAXBSIZE;

        lorunningspace = 512 * 1024;
        /* hirunningspace -- see below */

        /*
         * Limit the amount of malloc memory since it is wired permanently
         * into the kernel space.  Even though this is accounted for in
         * the buffer allocation, we don't want the malloced region to grow
         * uncontrolled.  The malloc scheme improves memory utilization
         * significantly on average (small) directories.
         */
        maxbufmallocspace = hibufspace / 20;

        /*
         * Reduce the chance of a deadlock occuring by limiting the number
         * of delayed-write dirty buffers we allow to stack up.
         *
         * We don't want too much actually queued to the device at once
         * (XXX this needs to be per-mount!), because the buffers will
         * wind up locked for a very long period of time while the I/O
         * drains.
         */
        hidirtybufspace = hibufspace / 2;       /* dirty + running */
        hirunningspace = hibufspace / 16;       /* locked & queued to device */
        if (hirunningspace < 1024 * 1024)
                hirunningspace = 1024 * 1024;

        dirtybufspace = 0;
        dirtybufspacehw = 0;

        lodirtybufspace = hidirtybufspace / 2;

        /*
         * Maximum number of async ops initiated per buf_daemon loop.  This is
         * somewhat of a hack at the moment, we really need to limit ourselves
         * based on the number of bytes of I/O in-transit that were initiated
         * from buf_daemon.
         */

        bogus_offset = kmem_alloc_pageable(&kernel_map, PAGE_SIZE);
        vm_object_hold(&kernel_object);
        bogus_page = vm_page_alloc(&kernel_object,
                                   (bogus_offset >> PAGE_SHIFT),
                                   VM_ALLOC_NORMAL);
        vm_object_drop(&kernel_object);
        vmstats.v_wire_count++;

}

/*
 * Initialize the embedded bio structures, typically used by
 * deprecated code which tries to allocate its own struct bufs.
 */
void
initbufbio(struct buf *bp)
{
        bp->b_bio1.bio_buf = bp;
        bp->b_bio1.bio_prev = NULL;
        bp->b_bio1.bio_offset = NOOFFSET;
        bp->b_bio1.bio_next = &bp->b_bio2;
        bp->b_bio1.bio_done = NULL;
        bp->b_bio1.bio_flags = 0;

        bp->b_bio2.bio_buf = bp;
        bp->b_bio2.bio_prev = &bp->b_bio1;
        bp->b_bio2.bio_offset = NOOFFSET;
        bp->b_bio2.bio_next = NULL;
        bp->b_bio2.bio_done = NULL;
        bp->b_bio2.bio_flags = 0;

        BUF_LOCKINIT(bp);
}

/*
 * Reinitialize the embedded bio structures as well as any additional
 * translation cache layers.
 */
void
reinitbufbio(struct buf *bp)
{
        struct bio *bio;

        for (bio = &bp->b_bio1; bio; bio = bio->bio_next) {
                bio->bio_done = NULL;
                bio->bio_offset = NOOFFSET;
        }
}

/*
 * Undo the effects of an initbufbio().
 */
void
uninitbufbio(struct buf *bp)
{
        dsched_exit_buf(bp);
        BUF_LOCKFREE(bp);
}

/*
 * Push another BIO layer onto an existing BIO and return it.  The new
 * BIO layer may already exist, holding cached translation data.
 */
struct bio *
push_bio(struct bio *bio)
{
        struct bio *nbio;

        if ((nbio = bio->bio_next) == NULL) {
                int index = bio - &bio->bio_buf->b_bio_array[0];
                if (index >= NBUF_BIO - 1) {
                        panic("push_bio: too many layers bp %p\n",
                                bio->bio_buf);
                }
                nbio = &bio->bio_buf->b_bio_array[index + 1];
                bio->bio_next = nbio;
                nbio->bio_prev = bio;
                nbio->bio_buf = bio->bio_buf;
                nbio->bio_offset = NOOFFSET;
                nbio->bio_done = NULL;
                nbio->bio_next = NULL;
        }
        KKASSERT(nbio->bio_done == NULL);
        return(nbio);
}

/*
 * Pop a BIO translation layer, returning the previous layer.  The
 * must have been previously pushed.
 */
struct bio *
pop_bio(struct bio *bio)
{
        return(bio->bio_prev);
}

void
clearbiocache(struct bio *bio)
{
        while (bio) {
                bio->bio_offset = NOOFFSET;
                bio = bio->bio_next;
        }
}

/*
 * bfreekva:
 *
 *      Free the KVA allocation for buffer 'bp'.
 *
 *      Must be called from a critical section as this is the only locking for
 *      buffer_map.
 *
 *      Since this call frees up buffer space, we call bufspacewakeup().
 *
 * MPALMOSTSAFE
 */
static void
bfreekva(struct buf *bp)
{
        int count;

        if (bp->b_kvasize) {
                ++buffreekvacnt;
                count = vm_map_entry_reserve(MAP_RESERVE_COUNT);
                vm_map_lock(&buffer_map);
                bufspace -= bp->b_kvasize;
                vm_map_delete(&buffer_map,
                    (vm_offset_t) bp->b_kvabase,
                    (vm_offset_t) bp->b_kvabase + bp->b_kvasize,
                    &count
                );
                vm_map_unlock(&buffer_map);
                vm_map_entry_release(count);
                bp->b_kvasize = 0;
                bp->b_kvabase = NULL;
                bufspacewakeup();
        }
}

/*
 * bremfree:
 *
 *      Remove the buffer from the appropriate free list.
 */
static __inline void
_bremfree(struct buf *bp)
{
        if (bp->b_qindex != BQUEUE_NONE) {
                KASSERT(BUF_REFCNTNB(bp) == 1, 
                                ("bremfree: bp %p not locked",bp));
                TAILQ_REMOVE(&bufqueues[bp->b_qindex], bp, b_freelist);
                bp->b_qindex = BQUEUE_NONE;
        } else {
                if (BUF_REFCNTNB(bp) <= 1)
                        panic("bremfree: removing a buffer not on a queue");
        }
}

void
bremfree(struct buf *bp)
{
        spin_lock(&bufqspin);
        _bremfree(bp);
        spin_unlock(&bufqspin);
}

static void
bremfree_locked(struct buf *bp)
{
        _bremfree(bp);
}

/*
 * bread:
 *
 *      Get a buffer with the specified data.  Look in the cache first.  We
 *      must clear B_ERROR and B_INVAL prior to initiating I/O.  If B_CACHE
 *      is set, the buffer is valid and we do not have to do anything ( see
 *      getblk() ).
 *
 */
int
bread(struct vnode *vp, off_t loffset, int size, struct buf **bpp)
{
        return (breadn(vp, loffset, size, NULL, NULL, 0, bpp));
}

/*
 * This version of bread issues any required I/O asyncnronously and
 * makes a callback on completion.
 *
 * The callback must check whether BIO_DONE is set in the bio and issue
 * the bpdone(bp, 0) if it isn't.  The callback is responsible for clearing
 * BIO_DONE and disposing of the I/O (bqrelse()ing it).
 */
void
breadcb(struct vnode *vp, off_t loffset, int size,
        void (*func)(struct bio *), void *arg)
{
        struct buf *bp;

        bp = getblk(vp, loffset, size, 0, 0);

        /* if not found in cache, do some I/O */
        if ((bp->b_flags & B_CACHE) == 0) {
                bp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                bp->b_cmd = BUF_CMD_READ;
                bp->b_bio1.bio_done = func;
                bp->b_bio1.bio_caller_info1.ptr = arg;
                vfs_busy_pages(vp, bp);
                BUF_KERNPROC(bp);
                vn_strategy(vp, &bp->b_bio1);
        } else if (func) {
                /*
                 * Since we are issuing the callback synchronously it cannot
                 * race the BIO_DONE, so no need for atomic ops here.
                 */
                /*bp->b_bio1.bio_done = func;*/
                bp->b_bio1.bio_caller_info1.ptr = arg;
                bp->b_bio1.bio_flags |= BIO_DONE;
                func(&bp->b_bio1);
        } else {
                bqrelse(bp);
        }
}

/*
 * breadn:
 *
 *      Operates like bread, but also starts asynchronous I/O on
 *      read-ahead blocks.  We must clear B_ERROR and B_INVAL prior
 *      to initiating I/O . If B_CACHE is set, the buffer is valid 
 *      and we do not have to do anything.
 *
 */
int
breadn(struct vnode *vp, off_t loffset, int size, off_t *raoffset,
        int *rabsize, int cnt, struct buf **bpp)
{
        struct buf *bp, *rabp;
        int i;
        int rv = 0, readwait = 0;

        *bpp = bp = getblk(vp, loffset, size, 0, 0);

        /* if not found in cache, do some I/O */
        if ((bp->b_flags & B_CACHE) == 0) {
                bp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                bp->b_cmd = BUF_CMD_READ;
                bp->b_bio1.bio_done = biodone_sync;
                bp->b_bio1.bio_flags |= BIO_SYNC;
                vfs_busy_pages(vp, bp);
                vn_strategy(vp, &bp->b_bio1);
                ++readwait;
        }

        for (i = 0; i < cnt; i++, raoffset++, rabsize++) {
                if (inmem(vp, *raoffset))
                        continue;
                rabp = getblk(vp, *raoffset, *rabsize, 0, 0);

                if ((rabp->b_flags & B_CACHE) == 0) {
                        rabp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                        rabp->b_cmd = BUF_CMD_READ;
                        vfs_busy_pages(vp, rabp);
                        BUF_KERNPROC(rabp);
                        vn_strategy(vp, &rabp->b_bio1);
                } else {
                        brelse(rabp);
                }
        }
        if (readwait)
                rv = biowait(&bp->b_bio1, "biord");
        return (rv);
}

/*
 * bwrite:
 *
 *      Synchronous write, waits for completion.
 *
 *      Write, release buffer on completion.  (Done by iodone
 *      if async).  Do not bother writing anything if the buffer
 *      is invalid.
 *
 *      Note that we set B_CACHE here, indicating that buffer is
 *      fully valid and thus cacheable.  This is true even of NFS
 *      now so we set it generally.  This could be set either here 
 *      or in biodone() since the I/O is synchronous.  We put it
 *      here.
 */
int
bwrite(struct buf *bp)
{
        int error;

        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return (0);
        }
        if (BUF_REFCNTNB(bp) == 0)
                panic("bwrite: buffer is not busy???");

        /* Mark the buffer clean */
        bundirty(bp);

        bp->b_flags &= ~(B_ERROR | B_EINTR);
        bp->b_flags |= B_CACHE;
        bp->b_cmd = BUF_CMD_WRITE;
        bp->b_bio1.bio_done = biodone_sync;
        bp->b_bio1.bio_flags |= BIO_SYNC;
        vfs_busy_pages(bp->b_vp, bp);

        /*
         * Normal bwrites pipeline writes.  NOTE: b_bufsize is only
         * valid for vnode-backed buffers.
         */
        bsetrunningbufspace(bp, bp->b_bufsize);
        vn_strategy(bp->b_vp, &bp->b_bio1);
        error = biowait(&bp->b_bio1, "biows");
        brelse(bp);

        return (error);
}

/*
 * bawrite:
 *
 *      Asynchronous write.  Start output on a buffer, but do not wait for
 *      it to complete.  The buffer is released when the output completes.
 *
 *      bwrite() ( or the VOP routine anyway ) is responsible for handling
 *      B_INVAL buffers.  Not us.
 */
void
bawrite(struct buf *bp)
{
        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return;
        }
        if (BUF_REFCNTNB(bp) == 0)
                panic("bwrite: buffer is not busy???");

        /* Mark the buffer clean */
        bundirty(bp);

        bp->b_flags &= ~(B_ERROR | B_EINTR);
        bp->b_flags |= B_CACHE;
        bp->b_cmd = BUF_CMD_WRITE;
        KKASSERT(bp->b_bio1.bio_done == NULL);
        vfs_busy_pages(bp->b_vp, bp);

        /*
         * Normal bwrites pipeline writes.  NOTE: b_bufsize is only
         * valid for vnode-backed buffers.
         */
        bsetrunningbufspace(bp, bp->b_bufsize);
        BUF_KERNPROC(bp);
        vn_strategy(bp->b_vp, &bp->b_bio1);
}

/*
 * bowrite:
 *
 *      Ordered write.  Start output on a buffer, and flag it so that the
 *      device will write it in the order it was queued.  The buffer is
 *      released when the output completes.  bwrite() ( or the VOP routine
 *      anyway ) is responsible for handling B_INVAL buffers.
 */
int
bowrite(struct buf *bp)
{
        bp->b_flags |= B_ORDERED;
        bawrite(bp);
        return (0);
}

/*
 * bdwrite:
 *
 *      Delayed write. (Buffer is marked dirty).  Do not bother writing
 *      anything if the buffer is marked invalid.
 *
 *      Note that since the buffer must be completely valid, we can safely
 *      set B_CACHE.  In fact, we have to set B_CACHE here rather then in
 *      biodone() in order to prevent getblk from writing the buffer
 *      out synchronously.
 */
void
bdwrite(struct buf *bp)
{
        if (BUF_REFCNTNB(bp) == 0)
                panic("bdwrite: buffer is not busy");

        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return;
        }
        bdirty(bp);

        if (dsched_is_clear_buf_priv(bp))
                dsched_new_buf(bp);

        /*
         * Set B_CACHE, indicating that the buffer is fully valid.  This is
         * true even of NFS now.
         */
        bp->b_flags |= B_CACHE;

        /*
         * This bmap keeps the system from needing to do the bmap later,
         * perhaps when the system is attempting to do a sync.  Since it
         * is likely that the indirect block -- or whatever other datastructure
         * that the filesystem needs is still in memory now, it is a good
         * thing to do this.  Note also, that if the pageout daemon is
         * requesting a sync -- there might not be enough memory to do
         * the bmap then...  So, this is important to do.
         */
        if (bp->b_bio2.bio_offset == NOOFFSET) {
                VOP_BMAP(bp->b_vp, bp->b_loffset, &bp->b_bio2.bio_offset,
                         NULL, NULL, BUF_CMD_WRITE);
        }

        /*
         * Because the underlying pages may still be mapped and
         * writable trying to set the dirty buffer (b_dirtyoff/end)
         * range here will be inaccurate.
         *
         * However, we must still clean the pages to satisfy the
         * vnode_pager and pageout daemon, so theythink the pages
         * have been "cleaned".  What has really occured is that
         * they've been earmarked for later writing by the buffer
         * cache.
         *
         * So we get the b_dirtyoff/end update but will not actually
         * depend on it (NFS that is) until the pages are busied for
         * writing later on.
         */
        vfs_clean_pages(bp);
        bqrelse(bp);

        /*
         * note: we cannot initiate I/O from a bdwrite even if we wanted to,
         * due to the softdep code.
         */
}

/*
 * Fake write - return pages to VM system as dirty, leave the buffer clean.
 * This is used by tmpfs.
 *
 * It is important for any VFS using this routine to NOT use it for
 * IO_SYNC or IO_ASYNC operations which occur when the system really
 * wants to flush VM pages to backing store.
 */
void
buwrite(struct buf *bp)
{
        vm_page_t m;
        int i;

        /*
         * Only works for VMIO buffers.  If the buffer is already
         * marked for delayed-write we can't avoid the bdwrite().
         */
        if ((bp->b_flags & B_VMIO) == 0 || (bp->b_flags & B_DELWRI)) {
                bdwrite(bp);
                return;
        }

        /*
         * Set valid & dirty.
         */
        for (i = 0; i < bp->b_xio.xio_npages; i++) {
                m = bp->b_xio.xio_pages[i];
                vfs_dirty_one_page(bp, i, m);
        }
        bqrelse(bp);
}

/*
 * bdirty:
 *
 *      Turn buffer into delayed write request by marking it B_DELWRI.
 *      B_RELBUF and B_NOCACHE must be cleared.
 *
 *      We reassign the buffer to itself to properly update it in the
 *      dirty/clean lists. 
 *
 *      Must be called from a critical section.
 *      The buffer must be on BQUEUE_NONE.
 */
void
bdirty(struct buf *bp)
{
        KASSERT(bp->b_qindex == BQUEUE_NONE, ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
        if (bp->b_flags & B_NOCACHE) {
                kprintf("bdirty: clearing B_NOCACHE on buf %p\n", bp);
                bp->b_flags &= ~B_NOCACHE;
        }
        if (bp->b_flags & B_INVAL) {
                kprintf("bdirty: warning, dirtying invalid buffer %p\n", bp);
        }
        bp->b_flags &= ~B_RELBUF;

        if ((bp->b_flags & B_DELWRI) == 0) {
                lwkt_gettoken(&bp->b_vp->v_token);
                bp->b_flags |= B_DELWRI;
                reassignbuf(bp);
                lwkt_reltoken(&bp->b_vp->v_token);

                spin_lock(&bufcspin);
                ++dirtybufcount;
                dirtybufspace += bp->b_bufsize;
                if (bp->b_flags & B_HEAVY) {
                        ++dirtybufcounthw;
                        dirtybufspacehw += bp->b_bufsize;
                }
                spin_unlock(&bufcspin);

                bd_heatup();
        }
}

/*
 * Set B_HEAVY, indicating that this is a heavy-weight buffer that
 * needs to be flushed with a different buf_daemon thread to avoid
 * deadlocks.  B_HEAVY also imposes restrictions in getnewbuf().
 */
void
bheavy(struct buf *bp)
{
        if ((bp->b_flags & B_HEAVY) == 0) {
                bp->b_flags |= B_HEAVY;
                if (bp->b_flags & B_DELWRI) {
                        spin_lock(&bufcspin);
                        ++dirtybufcounthw;
                        dirtybufspacehw += bp->b_bufsize;
                        spin_unlock(&bufcspin);
                }
        }
}

/*
 * bundirty:
 *
 *      Clear B_DELWRI for buffer.
 *
 *      Must be called from a critical section.
 *
 *      The buffer is typically on BQUEUE_NONE but there is one case in 
 *      brelse() that calls this function after placing the buffer on
 *      a different queue.
 *
 * MPSAFE
 */
void
bundirty(struct buf *bp)
{
        if (bp->b_flags & B_DELWRI) {
                lwkt_gettoken(&bp->b_vp->v_token);
                bp->b_flags &= ~B_DELWRI;
                reassignbuf(bp);
                lwkt_reltoken(&bp->b_vp->v_token);

                spin_lock(&bufcspin);
                --dirtybufcount;
                dirtybufspace -= bp->b_bufsize;
                if (bp->b_flags & B_HEAVY) {
                        --dirtybufcounthw;
                        dirtybufspacehw -= bp->b_bufsize;
                }
                spin_unlock(&bufcspin);

                bd_signal(bp->b_bufsize);
        }
        /*
         * Since it is now being written, we can clear its deferred write flag.
         */
        bp->b_flags &= ~B_DEFERRED;
}

/*
 * Set the b_runningbufspace field, used to track how much I/O is
 * in progress at any given moment.
 */
void
bsetrunningbufspace(struct buf *bp, int bytes)
{
        bp->b_runningbufspace = bytes;
        if (bytes) {
                spin_lock(&bufcspin);
                runningbufspace += bytes;
                ++runningbufcount;
                spin_unlock(&bufcspin);
        }
}

/*
 * brelse:
 *
 *      Release a busy buffer and, if requested, free its resources.  The
 *      buffer will be stashed in the appropriate bufqueue[] allowing it
 *      to be accessed later as a cache entity or reused for other purposes.
 *
 * MPALMOSTSAFE
 */
void
brelse(struct buf *bp)
{
#ifdef INVARIANTS
        int saved_flags = bp->b_flags;
#endif

        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)), ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));

        /*
         * If B_NOCACHE is set we are being asked to destroy the buffer and
         * its backing store.  Clear B_DELWRI.
         *
         * B_NOCACHE is set in two cases: (1) when the caller really wants
         * to destroy the buffer and backing store and (2) when the caller
         * wants to destroy the buffer and backing store after a write 
         * completes.
         */
        if ((bp->b_flags & (B_NOCACHE|B_DELWRI)) == (B_NOCACHE|B_DELWRI)) {
                bundirty(bp);
        }

        if ((bp->b_flags & (B_INVAL | B_DELWRI)) == B_DELWRI) {
                /*
                 * A re-dirtied buffer is only subject to destruction
                 * by B_INVAL.  B_ERROR and B_NOCACHE are ignored.
                 */
                /* leave buffer intact */
        } else if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_ERROR)) ||
                   (bp->b_bufsize <= 0)) {
                /*
                 * Either a failed read or we were asked to free or not
                 * cache the buffer.  This path is reached with B_DELWRI
                 * set only if B_INVAL is already set.  B_NOCACHE governs
                 * backing store destruction.
                 *
                 * NOTE: HAMMER will set B_LOCKED in buf_deallocate if the
                 * buffer cannot be immediately freed.
                 */
                bp->b_flags |= B_INVAL;
                if (LIST_FIRST(&bp->b_dep) != NULL)
                        buf_deallocate(bp);
                if (bp->b_flags & B_DELWRI) {
                        spin_lock(&bufcspin);
                        --dirtybufcount;
                        dirtybufspace -= bp->b_bufsize;
                        if (bp->b_flags & B_HEAVY) {
                                --dirtybufcounthw;
                                dirtybufspacehw -= bp->b_bufsize;
                        }
                        spin_unlock(&bufcspin);

                        bd_signal(bp->b_bufsize);
                }
                bp->b_flags &= ~(B_DELWRI | B_CACHE);
        }

        /*
         * We must clear B_RELBUF if B_DELWRI or B_LOCKED is set,
         * or if b_refs is non-zero.
         *
         * If vfs_vmio_release() is called with either bit set, the
         * underlying pages may wind up getting freed causing a previous
         * write (bdwrite()) to get 'lost' because pages associated with
         * a B_DELWRI bp are marked clean.  Pages associated with a
         * B_LOCKED buffer may be mapped by the filesystem.
         *
         * If we want to release the buffer ourselves (rather then the
         * originator asking us to release it), give the originator a
         * chance to countermand the release by setting B_LOCKED.
         * 
         * We still allow the B_INVAL case to call vfs_vmio_release(), even
         * if B_DELWRI is set.
         *
         * If B_DELWRI is not set we may have to set B_RELBUF if we are low
         * on pages to return pages to the VM page queues.
         */
        if ((bp->b_flags & (B_DELWRI | B_LOCKED)) || bp->b_refs) {
                bp->b_flags &= ~B_RELBUF;
        } else if (vm_page_count_severe()) {
                if (LIST_FIRST(&bp->b_dep) != NULL)
                        buf_deallocate(bp);             /* can set B_LOCKED */
                if (bp->b_flags & (B_DELWRI | B_LOCKED))
                        bp->b_flags &= ~B_RELBUF;
                else
                        bp->b_flags |= B_RELBUF;
        }

        /*
         * Make sure b_cmd is clear.  It may have already been cleared by
         * biodone().
         *
         * At this point destroying the buffer is governed by the B_INVAL 
         * or B_RELBUF flags.
         */
        bp->b_cmd = BUF_CMD_DONE;
        dsched_exit_buf(bp);

        /*
         * VMIO buffer rundown.  Make sure the VM page array is restored
         * after an I/O may have replaces some of the pages with bogus pages
         * in order to not destroy dirty pages in a fill-in read.
         *
         * Note that due to the code above, if a buffer is marked B_DELWRI
         * then the B_RELBUF and B_NOCACHE bits will always be clear.
         * B_INVAL may still be set, however.
         *
         * For clean buffers, B_INVAL or B_RELBUF will destroy the buffer
         * but not the backing store.   B_NOCACHE will destroy the backing
         * store.
         *
         * Note that dirty NFS buffers contain byte-granular write ranges
         * and should not be destroyed w/ B_INVAL even if the backing store
         * is left intact.
         */
        if (bp->b_flags & B_VMIO) {
                /*
                 * Rundown for VMIO buffers which are not dirty NFS buffers.
                 */
                int i, j, resid;
                vm_page_t m;
                off_t foff;
                vm_pindex_t poff;
                vm_object_t obj;
                struct vnode *vp;

                vp = bp->b_vp;

                /*
                 * Get the base offset and length of the buffer.  Note that 
                 * in the VMIO case if the buffer block size is not
                 * page-aligned then b_data pointer may not be page-aligned.
                 * But our b_xio.xio_pages array *IS* page aligned.
                 *
                 * block sizes less then DEV_BSIZE (usually 512) are not 
                 * supported due to the page granularity bits (m->valid,
                 * m->dirty, etc...). 
                 *
                 * See man buf(9) for more information
                 */

                resid = bp->b_bufsize;
                foff = bp->b_loffset;

                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        m = bp->b_xio.xio_pages[i];
                        vm_page_flag_clear(m, PG_ZERO);
                        /*
                         * If we hit a bogus page, fixup *all* of them
                         * now.  Note that we left these pages wired
                         * when we removed them so they had better exist,
                         * and they cannot be ripped out from under us so
                         * no critical section protection is necessary.
                         */
                        if (m == bogus_page) {
                                obj = vp->v_object;
                                poff = OFF_TO_IDX(bp->b_loffset);

                                vm_object_hold(obj);
                                for (j = i; j < bp->b_xio.xio_npages; j++) {
                                        vm_page_t mtmp;

                                        mtmp = bp->b_xio.xio_pages[j];
                                        if (mtmp == bogus_page) {
                                                mtmp = vm_page_lookup(obj, poff + j);
                                                if (!mtmp) {
                                                        panic("brelse: page missing");
                                                }
                                                bp->b_xio.xio_pages[j] = mtmp;
                                        }
                                }
                                bp->b_flags &= ~B_HASBOGUS;
                                vm_object_drop(obj);

                                if ((bp->b_flags & B_INVAL) == 0) {
                                        pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                                bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                                }
                                m = bp->b_xio.xio_pages[i];
                        }

                        /*
                         * Invalidate the backing store if B_NOCACHE is set
                         * (e.g. used with vinvalbuf()).  If this is NFS
                         * we impose a requirement that the block size be
                         * a multiple of PAGE_SIZE and create a temporary
                         * hack to basically invalidate the whole page.  The
                         * problem is that NFS uses really odd buffer sizes
                         * especially when tracking piecemeal writes and
                         * it also vinvalbuf()'s a lot, which would result
                         * in only partial page validation and invalidation
                         * here.  If the file page is mmap()'d, however,
                         * all the valid bits get set so after we invalidate
                         * here we would end up with weird m->valid values
                         * like 0xfc.  nfs_getpages() can't handle this so
                         * we clear all the valid bits for the NFS case
                         * instead of just some of them.
                         *
                         * The real bug is the VM system having to set m->valid
                         * to VM_PAGE_BITS_ALL for faulted-in pages, which
                         * itself is an artifact of the whole 512-byte
                         * granular mess that exists to support odd block 
                         * sizes and UFS meta-data block sizes (e.g. 6144).
                         * A complete rewrite is required.
                         *
                         * XXX
                         */
                        if (bp->b_flags & (B_NOCACHE|B_ERROR)) {
                                int poffset = foff & PAGE_MASK;
                                int presid;

                                presid = PAGE_SIZE - poffset;
                                if (bp->b_vp->v_tag == VT_NFS &&
                                    bp->b_vp->v_type == VREG) {
                                        ; /* entire page */
                                } else if (presid > resid) {
                                        presid = resid;
                                }
                                KASSERT(presid >= 0, ("brelse: extra page"));
                                vm_page_set_invalid(m, poffset, presid);

                                /*
                                 * Also make sure any swap cache is removed
                                 * as it is now stale (HAMMER in particular
                                 * uses B_NOCACHE to deal with buffer
                                 * aliasing).
                                 */
                                swap_pager_unswapped(m);
                        }
                        resid -= PAGE_SIZE - (foff & PAGE_MASK);
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                }
                if (bp->b_flags & (B_INVAL | B_RELBUF))
                        vfs_vmio_release(bp);
        } else {
                /*
                 * Rundown for non-VMIO buffers.
                 */
                if (bp->b_flags & (B_INVAL | B_RELBUF)) {
                        if (bp->b_bufsize)
                                allocbuf(bp, 0);
                        KKASSERT (LIST_FIRST(&bp->b_dep) == NULL);
                        if (bp->b_vp)
                                brelvp(bp);
                }
        }
                        
        if (bp->b_qindex != BQUEUE_NONE)
                panic("brelse: free buffer onto another queue???");
        if (BUF_REFCNTNB(bp) > 1) {
                /* Temporary panic to verify exclusive locking */
                /* This panic goes away when we allow shared refs */
                panic("brelse: multiple refs");
                /* NOT REACHED */
                return;
        }

        /*
         * Figure out the correct queue to place the cleaned up buffer on.
         * Buffers placed in the EMPTY or EMPTYKVA had better already be
         * disassociated from their vnode.
         */
        spin_lock(&bufqspin);
        if (bp->b_flags & B_LOCKED) {
                /*
                 * Buffers that are locked are placed in the locked queue
                 * immediately, regardless of their state.
                 */
                bp->b_qindex = BQUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_LOCKED], bp, b_freelist);
        } else if (bp->b_bufsize == 0) {
                /*
                 * Buffers with no memory.  Due to conditionals near the top
                 * of brelse() such buffers should probably already be
                 * marked B_INVAL and disassociated from their vnode.
                 */
                bp->b_flags |= B_INVAL;
                KASSERT(bp->b_vp == NULL, ("bp1 %p flags %08x/%08x vnode %p unexpectededly still associated!", bp, saved_flags, bp->b_flags, bp->b_vp));
                KKASSERT((bp->b_flags & B_HASHED) == 0);
                if (bp->b_kvasize) {
                        bp->b_qindex = BQUEUE_EMPTYKVA;
                } else {
                        bp->b_qindex = BQUEUE_EMPTY;
                }
                TAILQ_INSERT_HEAD(&bufqueues[bp->b_qindex], bp, b_freelist);
        } else if (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF)) {
                /*
                 * Buffers with junk contents.   Again these buffers had better
                 * already be disassociated from their vnode.
                 */
                KASSERT(bp->b_vp == NULL, ("bp2 %p flags %08x/%08x vnode %p unexpectededly still associated!", bp, saved_flags, bp->b_flags, bp->b_vp));
                KKASSERT((bp->b_flags & B_HASHED) == 0);
                bp->b_flags |= B_INVAL;
                bp->b_qindex = BQUEUE_CLEAN;
                TAILQ_INSERT_HEAD(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
        } else {
                /*
                 * Remaining buffers.  These buffers are still associated with
                 * their vnode.
                 */
                switch(bp->b_flags & (B_DELWRI|B_HEAVY)) {
                case B_DELWRI:
                    bp->b_qindex = BQUEUE_DIRTY;
                    TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_DIRTY], bp, b_freelist);
                    break;
                case B_DELWRI | B_HEAVY:
                    bp->b_qindex = BQUEUE_DIRTY_HW;
                    TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_DIRTY_HW], bp,
                                      b_freelist);
                    break;
                default:
                    /*
                     * NOTE: Buffers are always placed at the end of the
                     * queue.  If B_AGE is not set the buffer will cycle
                     * through the queue twice.
                     */
                    bp->b_qindex = BQUEUE_CLEAN;
                    TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
                    break;
                }
        }
        spin_unlock(&bufqspin);

        /*
         * If B_INVAL, clear B_DELWRI.  We've already placed the buffer
         * on the correct queue.
         */
        if ((bp->b_flags & (B_INVAL|B_DELWRI)) == (B_INVAL|B_DELWRI))
                bundirty(bp);

        /*
         * The bp is on an appropriate queue unless locked.  If it is not
         * locked or dirty we can wakeup threads waiting for buffer space.
         *
         * We've already handled the B_INVAL case ( B_DELWRI will be clear
         * if B_INVAL is set ).
         */
        if ((bp->b_flags & (B_LOCKED|B_DELWRI)) == 0)
                bufcountwakeup();

        /*
         * Something we can maybe free or reuse
         */
        if (bp->b_bufsize || bp->b_kvasize)
                bufspacewakeup();

        /*
         * Clean up temporary flags and unlock the buffer.
         */
        bp->b_flags &= ~(B_ORDERED | B_NOCACHE | B_RELBUF | B_DIRECT);
        BUF_UNLOCK(bp);
}

/*
 * bqrelse:
 *
 *      Release a buffer back to the appropriate queue but do not try to free
 *      it.  The buffer is expected to be used again soon.
 *
 *      bqrelse() is used by bdwrite() to requeue a delayed write, and used by
 *      biodone() to requeue an async I/O on completion.  It is also used when
 *      known good buffers need to be requeued but we think we may need the data
 *      again soon.
 *
 *      XXX we should be able to leave the B_RELBUF hint set on completion.
 *
 * MPSAFE
 */
void
bqrelse(struct buf *bp)
{
        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)), ("bqrelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));

        if (bp->b_qindex != BQUEUE_NONE)
                panic("bqrelse: free buffer onto another queue???");
        if (BUF_REFCNTNB(bp) > 1) {
                /* do not release to free list */
                panic("bqrelse: multiple refs");
                return;
        }

        buf_act_advance(bp);

        spin_lock(&bufqspin);
        if (bp->b_flags & B_LOCKED) {
                /*
                 * Locked buffers are released to the locked queue.  However,
                 * if the buffer is dirty it will first go into the dirty
                 * queue and later on after the I/O completes successfully it
                 * will be released to the locked queue.
                 */
                bp->b_qindex = BQUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_LOCKED], bp, b_freelist);
        } else if (bp->b_flags & B_DELWRI) {
                bp->b_qindex = (bp->b_flags & B_HEAVY) ?
                               BQUEUE_DIRTY_HW : BQUEUE_DIRTY;
                TAILQ_INSERT_TAIL(&bufqueues[bp->b_qindex], bp, b_freelist);
        } else if (vm_page_count_severe()) {
                /*
                 * We are too low on memory, we have to try to free the
                 * buffer (most importantly: the wired pages making up its
                 * backing store) *now*.
                 */
                spin_unlock(&bufqspin);
                brelse(bp);
                return;
        } else {
                bp->b_qindex = BQUEUE_CLEAN;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
        }
        spin_unlock(&bufqspin);

        if ((bp->b_flags & B_LOCKED) == 0 &&
            ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0)) {
                bufcountwakeup();
        }

        /*
         * Something we can maybe free or reuse.
         */
        if (bp->b_bufsize && !(bp->b_flags & B_DELWRI))
                bufspacewakeup();

        /*
         * Final cleanup and unlock.  Clear bits that are only used while a
         * buffer is actively locked.
         */
        bp->b_flags &= ~(B_ORDERED | B_NOCACHE | B_RELBUF);
        dsched_exit_buf(bp);
        BUF_UNLOCK(bp);
}

/*
 * Hold a buffer, preventing it from being reused.  This will prevent
 * normal B_RELBUF operations on the buffer but will not prevent B_INVAL
 * operations.  If a B_INVAL operation occurs the buffer will remain held
 * but the underlying pages may get ripped out.
 *
 * These functions are typically used in VOP_READ/VOP_WRITE functions
 * to hold a buffer during a copyin or copyout, preventing deadlocks
 * or recursive lock panics when read()/write() is used over mmap()'d
 * space.
 *
 * NOTE: bqhold() requires that the buffer be locked at the time of the
 *       hold.  bqdrop() has no requirements other than the buffer having
 *       previously been held.
 */
void
bqhold(struct buf *bp)
{
        atomic_add_int(&bp->b_refs, 1);
}

void
bqdrop(struct buf *bp)
{
        KKASSERT(bp->b_refs > 0);
        atomic_add_int(&bp->b_refs, -1);
}

/*
 * vfs_vmio_release:
 *
 *      Return backing pages held by the buffer 'bp' back to the VM system
 *      if possible.  The pages are freed if they are no longer valid or
 *      attempt to free if it was used for direct I/O otherwise they are
 *      sent to the page cache.
 *
 *      Pages that were marked busy are left alone and skipped.
 *
 *      The KVA mapping (b_data) for the underlying pages is removed by
 *      this function.
 */
static void
vfs_vmio_release(struct buf *bp)
{
        int i;
        vm_page_t m;

        for (i = 0; i < bp->b_xio.xio_npages; i++) {
                m = bp->b_xio.xio_pages[i];
                bp->b_xio.xio_pages[i] = NULL;

                vm_page_busy_wait(m, FALSE, "vmiopg");

                /*
                 * The VFS is telling us this is not a meta-data buffer
                 * even if it is backed by a block device.
                 */
                if (bp->b_flags & B_NOTMETA)
                        vm_page_flag_set(m, PG_NOTMETA);

                /*
                 * This is a very important bit of code.  We try to track
                 * VM page use whether the pages are wired into the buffer
                 * cache or not.  While wired into the buffer cache the
                 * bp tracks the act_count.
                 *
                 * We can choose to place unwired pages on the inactive
                 * queue (0) or active queue (1).  If we place too many
                 * on the active queue the queue will cycle the act_count
                 * on pages we'd like to keep, just from single-use pages
                 * (such as when doing a tar-up or file scan).
                 */
                if (bp->b_act_count < vm_cycle_point)
                        vm_page_unwire(m, 0);
                else
                        vm_page_unwire(m, 1);

                /*
                 * We don't mess with busy pages, it is the responsibility
                 * of the process that busied the pages to deal with them.
                 *
                 * However, the caller may have marked the page invalid and
                 * we must still make sure the page is no longer mapped.
                 */
                if ((m->flags & PG_BUSY) || (m->busy != 0)) {
                        vm_page_protect(m, VM_PROT_NONE);
                        vm_page_wakeup(m);
                        continue;
                }
                        
                if (m->wire_count == 0) {
                        vm_page_flag_clear(m, PG_ZERO);
                        /*
                         * Might as well free the page if we can and it has
                         * no valid data.  We also free the page if the
                         * buffer was used for direct I/O.
                         */
#if 0
                        if ((bp->b_flags & B_ASYNC) == 0 && !m->valid &&
                                        m->hold_count == 0) {
                                vm_page_protect(m, VM_PROT_NONE);
                                vm_page_free(m);
                        } else
#endif
                        /*
                         * Cache the page if we are really low on free
                         * pages.
                         *
                         * Also bypass the active and inactive queues
                         * if B_NOTMETA is set.  This flag is set by HAMMER
                         * on a regular file buffer when double buffering
                         * is enabled or on a block device buffer representing
                         * file data when double buffering is not enabled.
                         * The flag prevents two copies of the same data from
                         * being cached for long periods of time.
                         */
                        if (bp->b_flags & B_DIRECT) {
                                vm_page_wakeup(m);
                                vm_page_try_to_free(m);
                        } else if ((bp->b_flags & B_NOTMETA) ||
                                   vm_page_count_severe()) {
                                m->act_count = bp->b_act_count;
                                vm_page_wakeup(m);
                                vm_page_try_to_cache(m);
                        } else {
                                m->act_count = bp->b_act_count;
                                vm_page_wakeup(m);
                        }
                } else {
                        vm_page_wakeup(m);
                }
        }

        pmap_qremove(trunc_page((vm_offset_t) bp->b_data),
                     bp->b_xio.xio_npages);
        if (bp->b_bufsize) {
                bufspacewakeup();
                bp->b_bufsize = 0;
        }
        bp->b_xio.xio_npages = 0;
        bp->b_flags &= ~B_VMIO;
        KKASSERT (LIST_FIRST(&bp->b_dep) == NULL);
        if (bp->b_vp)
                brelvp(bp);
}

/*
 * vfs_bio_awrite:
 *
 *      Implement clustered async writes for clearing out B_DELWRI buffers.
 *      This is much better then the old way of writing only one buffer at
 *      a time.  Note that we may not be presented with the buffers in the 
 *      correct order, so we search for the cluster in both directions.
 *
 *      The buffer is locked on call.
 */
int
vfs_bio_awrite(struct buf *bp)
{
        int i;
        int j;
        off_t loffset = bp->b_loffset;
        struct vnode *vp = bp->b_vp;
        int nbytes;
        struct buf *bpa;
        int nwritten;
        int size;

        /*
         * right now we support clustered writing only to regular files.  If
         * we find a clusterable block we could be in the middle of a cluster
         * rather then at the beginning.
         *
         * NOTE: b_bio1 contains the logical loffset and is aliased
         * to b_loffset.  b_bio2 contains the translated block number.
         */
        if ((vp->v_type == VREG) && 
            (vp->v_mount != 0) && /* Only on nodes that have the size info */
            (bp->b_flags & (B_CLUSTEROK | B_INVAL)) == B_CLUSTEROK) {

                size = vp->v_mount->mnt_stat.f_iosize;

                for (i = size; i < MAXPHYS; i += size) {
                        if ((bpa = findblk(vp, loffset + i, FINDBLK_TEST)) &&
                            BUF_REFCNT(bpa) == 0 &&
                            ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) ==
                            (B_DELWRI | B_CLUSTEROK)) &&
                            (bpa->b_bufsize == size)) {
                                if ((bpa->b_bio2.bio_offset == NOOFFSET) ||
                                    (bpa->b_bio2.bio_offset !=
                                     bp->b_bio2.bio_offset + i))
                                        break;
                        } else {
                                break;
                        }
                }
                for (j = size; i + j <= MAXPHYS && j <= loffset; j += size) {
                        if ((bpa = findblk(vp, loffset - j, FINDBLK_TEST)) &&
                            BUF_REFCNT(bpa) == 0 &&
                            ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) ==
                            (B_DELWRI | B_CLUSTEROK)) &&
                            (bpa->b_bufsize == size)) {
                                if ((bpa->b_bio2.bio_offset == NOOFFSET) ||
                                    (bpa->b_bio2.bio_offset !=
                                     bp->b_bio2.bio_offset - j))
                                        break;
                        } else {
                                break;
                        }
                }
                j -= size;
                nbytes = (i + j);

                /*
                 * this is a possible cluster write
                 */
                if (nbytes != size) {
                        BUF_UNLOCK(bp);
                        nwritten = cluster_wbuild(vp, size,
                                                  loffset - j, nbytes);
                        return nwritten;
                }
        }

        /*
         * default (old) behavior, writing out only one block
         *
         * XXX returns b_bufsize instead of b_bcount for nwritten?
         */
        nwritten = bp->b_bufsize;
        bremfree(bp);
        bawrite(bp);

        return nwritten;
}

/*
 * getnewbuf:
 *
 *      Find and initialize a new buffer header, freeing up existing buffers 
 *      in the bufqueues as necessary.  The new buffer is returned locked.
 *
 *      Important:  B_INVAL is not set.  If the caller wishes to throw the
 *      buffer away, the caller must set B_INVAL prior to calling brelse().
 *
 *      We block if:
 *              We have insufficient buffer headers
 *              We have insufficient buffer space
 *              buffer_map is too fragmented ( space reservation fails )
 *              If we have to flush dirty buffers ( but we try to avoid this )
 *
 *      To avoid VFS layer recursion we do not flush dirty buffers ourselves.
 *      Instead we ask the buf daemon to do it for us.  We attempt to
 *      avoid piecemeal wakeups of the pageout daemon.
 *
 * MPALMOSTSAFE
 */
struct buf *
getnewbuf(int blkflags, int slptimeo, int size, int maxsize)
{
        struct buf *bp;
        struct buf *nbp;
        int defrag = 0;
        int nqindex;
        int slpflags = (blkflags & GETBLK_PCATCH) ? PCATCH : 0;
        static int flushingbufs;

        /*
         * We can't afford to block since we might be holding a vnode lock,
         * which may prevent system daemons from running.  We deal with
         * low-memory situations by proactively returning memory and running
         * async I/O rather then sync I/O.
         */
        
        ++getnewbufcalls;
        --getnewbufrestarts;
restart:
        ++getnewbufrestarts;

        /*
         * Setup for scan.  If we do not have enough free buffers,
         * we setup a degenerate case that immediately fails.  Note
         * that if we are specially marked process, we are allowed to
         * dip into our reserves.
         *
         * The scanning sequence is nominally:  EMPTY->EMPTYKVA->CLEAN
         *
         * We start with EMPTYKVA.  If the list is empty we backup to EMPTY.
         * However, there are a number of cases (defragging, reusing, ...)
         * where we cannot backup.
         */
        nqindex = BQUEUE_EMPTYKVA;
        spin_lock(&bufqspin);
        nbp = TAILQ_FIRST(&bufqueues[BQUEUE_EMPTYKVA]);

        if (nbp == NULL) {
                /*
                 * If no EMPTYKVA buffers and we are either
                 * defragging or reusing, locate a CLEAN buffer
                 * to free or reuse.  If bufspace useage is low
                 * skip this step so we can allocate a new buffer.
                 */
                if (defrag || bufspace >= lobufspace) {
                        nqindex = BQUEUE_CLEAN;
                        nbp = TAILQ_FIRST(&bufqueues[BQUEUE_CLEAN]);
                }

                /*
                 * If we could not find or were not allowed to reuse a
                 * CLEAN buffer, check to see if it is ok to use an EMPTY
                 * buffer.  We can only use an EMPTY buffer if allocating
                 * its KVA would not otherwise run us out of buffer space.
                 */
                if (nbp == NULL && defrag == 0 &&
                    bufspace + maxsize < hibufspace) {
                        nqindex = BQUEUE_EMPTY;
                        nbp = TAILQ_FIRST(&bufqueues[BQUEUE_EMPTY]);
                }
        }

        /*
         * Run scan, possibly freeing data and/or kva mappings on the fly
         * depending.
         *
         * WARNING!  bufqspin is held!
         */
        while ((bp = nbp) != NULL) {
                int qindex = nqindex;

                nbp = TAILQ_NEXT(bp, b_freelist);

                /*
                 * BQUEUE_CLEAN - B_AGE special case.  If not set the bp
                 * cycles through the queue twice before being selected.
                 */
                if (qindex == BQUEUE_CLEAN && 
                    (bp->b_flags & B_AGE) == 0 && nbp) {
                        bp->b_flags |= B_AGE;
                        TAILQ_REMOVE(&bufqueues[qindex], bp, b_freelist);
                        TAILQ_INSERT_TAIL(&bufqueues[qindex], bp, b_freelist);
                        continue;
                }

                /*
                 * Calculate next bp ( we can only use it if we do not block
                 * or do other fancy things ).
                 */
                if (nbp == NULL) {
                        switch(qindex) {
                        case BQUEUE_EMPTY:
                                nqindex = BQUEUE_EMPTYKVA;
                                if ((nbp = TAILQ_FIRST(&bufqueues[BQUEUE_EMPTYKVA])))
                                        break;
                                /* fall through */
                        case BQUEUE_EMPTYKVA:
                                nqindex = BQUEUE_CLEAN;
                                if ((nbp = TAILQ_FIRST(&bufqueues[BQUEUE_CLEAN])))
                                        break;
                                /* fall through */
                        case BQUEUE_CLEAN:
                                /*
                                 * nbp is NULL. 
                                 */
                                break;
                        }
                }

                /*
                 * Sanity Checks
                 */
                KASSERT(bp->b_qindex == qindex,
                        ("getnewbuf: inconsistent queue %d bp %p", qindex, bp));

                /*
                 * Note: we no longer distinguish between VMIO and non-VMIO
                 * buffers.
                 */
                KASSERT((bp->b_flags & B_DELWRI) == 0,
                        ("delwri buffer %p found in queue %d", bp, qindex));

                /*
                 * Do not try to reuse a buffer with a non-zero b_refs.
                 * This is an unsynchronized test.  A synchronized test
                 * is also performed after we lock the buffer.
                 */
                if (bp->b_refs)
                        continue;

                /*
                 * If we are defragging then we need a buffer with 
                 * b_kvasize != 0.  XXX this situation should no longer
                 * occur, if defrag is non-zero the buffer's b_kvasize
                 * should also be non-zero at this point.  XXX
                 */
                if (defrag && bp->b_kvasize == 0) {
                        kprintf("Warning: defrag empty buffer %p\n", bp);
                        continue;
                }

                /*
                 * Start freeing the bp.  This is somewhat involved.  nbp
                 * remains valid only for BQUEUE_EMPTY[KVA] bp's.  Buffers
                 * on the clean list must be disassociated from their 
                 * current vnode.  Buffers on the empty[kva] lists have
                 * already been disassociated.
                 *
                 * b_refs is checked after locking along with queue changes.
                 * We must check here to deal with zero->nonzero transitions
                 * made by the owner of the buffer lock, which is used by
                 * VFS's to hold the buffer while issuing an unlocked
                 * uiomove()s.  We cannot invalidate the buffer's pages
                 * for this case.  Once we successfully lock a buffer the
                 * only 0->1 transitions of b_refs will occur via findblk().
                 *
                 * We must also check for queue changes after successful
                 * locking as the current lock holder may dispose of the
                 * buffer and change its queue.
                 */
                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
                        spin_unlock(&bufqspin);
                        tsleep(&bd_request, 0, "gnbxxx", (hz + 99) / 100);
                        goto restart;
                }
                if (bp->b_qindex != qindex || bp->b_refs) {
                        spin_unlock(&bufqspin);
                        BUF_UNLOCK(bp);
                        goto restart;
                }
                bremfree_locked(bp);
                spin_unlock(&bufqspin);

                /*
                 * Dependancies must be handled before we disassociate the
                 * vnode.
                 *
                 * NOTE: HAMMER will set B_LOCKED if the buffer cannot
                 * be immediately disassociated.  HAMMER then becomes
                 * responsible for releasing the buffer.
                 *
                 * NOTE: bufqspin is UNLOCKED now.
                 */
                if (LIST_FIRST(&bp->b_dep) != NULL) {
                        buf_deallocate(bp);
                        if (bp->b_flags & B_LOCKED) {
                                bqrelse(bp);
                                goto restart;
                        }
                        KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                }

                if (qindex == BQUEUE_CLEAN) {
                        if (bp->b_flags & B_VMIO)
                                vfs_vmio_release(bp);
                        if (bp->b_vp)
                                brelvp(bp);
                }

                /*
                 * NOTE:  nbp is now entirely invalid.  We can only restart
                 * the scan from this point on.
                 *
                 * Get the rest of the buffer freed up.  b_kva* is still
                 * valid after this operation.
                 */
                KASSERT(bp->b_vp == NULL,
                        ("bp3 %p flags %08x vnode %p qindex %d "
                         "unexpectededly still associated!",
                         bp, bp->b_flags, bp->b_vp, qindex));
                KKASSERT((bp->b_flags & B_HASHED) == 0);

                /*
                 * critical section protection is not required when
                 * scrapping a buffer's contents because it is already 
                 * wired.
                 */
                if (bp->b_bufsize)
                        allocbuf(bp, 0);

                bp->b_flags = B_BNOCLIP;
                bp->b_cmd = BUF_CMD_DONE;
                bp->b_vp = NULL;
                bp->b_error = 0;
                bp->b_resid = 0;
                bp->b_bcount = 0;
                bp->b_xio.xio_npages = 0;
                bp->b_dirtyoff = bp->b_dirtyend = 0;
                bp->b_act_count = ACT_INIT;
                reinitbufbio(bp);
                KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                buf_dep_init(bp);
                if (blkflags & GETBLK_BHEAVY)
                        bp->b_flags |= B_HEAVY;

                /*
                 * If we are defragging then free the buffer.
                 */
                if (defrag) {
                        bp->b_flags |= B_INVAL;
                        bfreekva(bp);
                        brelse(bp);
                        defrag = 0;
                        goto restart;
                }

                /*
                 * If we are overcomitted then recover the buffer and its
                 * KVM space.  This occurs in rare situations when multiple
                 * processes are blocked in getnewbuf() or allocbuf().
                 */
                if (bufspace >= hibufspace)
                        flushingbufs = 1;
                if (flushingbufs && bp->b_kvasize != 0) {
                        bp->b_flags |= B_INVAL;
                        bfreekva(bp);
                        brelse(bp);
                        goto restart;
                }
                if (bufspace < lobufspace)
                        flushingbufs = 0;

                /*
                 * b_refs can transition to a non-zero value while we hold
                 * the buffer locked due to a findblk().  Our brelvp() above
                 * interlocked any future possible transitions due to
                 * findblk()s.
                 *
                 * If we find b_refs to be non-zero we can destroy the
                 * buffer's contents but we cannot yet reuse the buffer.
                 */
                if (bp->b_refs) {
                        bp->b_flags |= B_INVAL;
                        bfreekva(bp);
                        brelse(bp);
                        goto restart;
                }
                break;
                /* NOT REACHED, bufqspin not held */
        }

        /*
         * If we exhausted our list, sleep as appropriate.  We may have to
         * wakeup various daemons and write out some dirty buffers.
         *
         * Generally we are sleeping due to insufficient buffer space.
         *
         * NOTE: bufqspin is held if bp is NULL, else it is not held.
         */
        if (bp == NULL) {
                int flags;
                char *waitmsg;

                spin_unlock(&bufqspin);
                if (defrag) {
                        flags = VFS_BIO_NEED_BUFSPACE;
                        waitmsg = "nbufkv";
                } else if (bufspace >= hibufspace) {
                        waitmsg = "nbufbs";
                        flags = VFS_BIO_NEED_BUFSPACE;
                } else {
                        waitmsg = "newbuf";
                        flags = VFS_BIO_NEED_ANY;
                }

                bd_speedup();   /* heeeelp */
                spin_lock(&bufcspin);
                needsbuffer |= flags;
                while (needsbuffer & flags) {
                        if (ssleep(&needsbuffer, &bufcspin,
                                   slpflags, waitmsg, slptimeo)) {
                                spin_unlock(&bufcspin);
                                return (NULL);
                        }
                }
                spin_unlock(&bufcspin);
        } else {
                /*
                 * We finally have a valid bp.  We aren't quite out of the
                 * woods, we still have to reserve kva space.  In order
                 * to keep fragmentation sane we only allocate kva in
                 * BKVASIZE chunks.
                 *
                 * (bufqspin is not held)
                 */
                maxsize = (maxsize + BKVAMASK) & ~BKVAMASK;

                if (maxsize != bp->b_kvasize) {
                        vm_offset_t addr = 0;
                        int count;

                        bfreekva(bp);

                        count = vm_map_entry_reserve(MAP_RESERVE_COUNT);
                        vm_map_lock(&buffer_map);

                        if (vm_map_findspace(&buffer_map,
                                    vm_map_min(&buffer_map), maxsize,
                                    maxsize, 0, &addr)) {
                                /*
                                 * Uh oh.  Buffer map is too fragmented.  We
                                 * must defragment the map.
                                 */
                                vm_map_unlock(&buffer_map);
                                vm_map_entry_release(count);
                                ++bufdefragcnt;
                                defrag = 1;
                                bp->b_flags |= B_INVAL;
                                brelse(bp);
                                goto restart;
                        }
                        if (addr) {
                                vm_map_insert(&buffer_map, &count,
                                        NULL, 0,
                                        addr, addr + maxsize,
                                        VM_MAPTYPE_NORMAL,
                                        VM_PROT_ALL, VM_PROT_ALL,
                                        MAP_NOFAULT);

                                bp->b_kvabase = (caddr_t) addr;
                                bp->b_kvasize = maxsize;
                                bufspace += bp->b_kvasize;
                                ++bufreusecnt;
                        }
                        vm_map_unlock(&buffer_map);
                        vm_map_entry_release(count);
                }
                bp->b_data = bp->b_kvabase;
        }
        return(bp);
}

/*
 * This routine is called in an emergency to recover VM pages from the
 * buffer cache by cashing in clean buffers.  The idea is to recover
 * enough pages to be able to satisfy a stuck bio_page_alloc().
 *
 * MPSAFE
 */
static int
recoverbufpages(void)
{
        struct buf *bp;
        int bytes = 0;

        ++recoverbufcalls;

        spin_lock(&bufqspin);
        while (bytes < MAXBSIZE) {
                bp = TAILQ_FIRST(&bufqueues[BQUEUE_CLEAN]);
                if (bp == NULL)
                        break;

                /*
                 * BQUEUE_CLEAN - B_AGE special case.  If not set the bp
                 * cycles through the queue twice before being selected.
                 */
                if ((bp->b_flags & B_AGE) == 0 && TAILQ_NEXT(bp, b_freelist)) {
                        bp->b_flags |= B_AGE;
                        TAILQ_REMOVE(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
                        TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_CLEAN],
                                          bp, b_freelist);
                        continue;
                }

                /*
                 * Sanity Checks
                 */
                KKASSERT(bp->b_qindex == BQUEUE_CLEAN);
                KKASSERT((bp->b_flags & B_DELWRI) == 0);

                /*
                 * Start freeing the bp.  This is somewhat involved.
                 *
                 * Buffers on the clean list must be disassociated from
                 * their current vnode
                 */

                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
                        kprintf("recoverbufpages: warning, locked buf %p, "
                                "race corrected\n",
                                bp);
                        ssleep(&bd_request, &bufqspin, 0, "gnbxxx", hz / 100);
                        continue;
                }
                if (bp->b_qindex != BQUEUE_CLEAN) {
                        kprintf("recoverbufpages: warning, BUF_LOCK blocked "
                                "unexpectedly on buf %p index %d, race "
                                "corrected\n",
                                bp, bp->b_qindex);
                        BUF_UNLOCK(bp);
                        continue;
                }
                bremfree_locked(bp);
                spin_unlock(&bufqspin);

                /*
                 * Dependancies must be handled before we disassociate the
                 * vnode.
                 *
                 * NOTE: HAMMER will set B_LOCKED if the buffer cannot
                 * be immediately disassociated.  HAMMER then becomes
                 * responsible for releasing the buffer.
                 */
                if (LIST_FIRST(&bp->b_dep) != NULL) {
                        buf_deallocate(bp);
                        if (bp->b_flags & B_LOCKED) {
                                bqrelse(bp);
                                spin_lock(&bufqspin);
                                continue;
                        }
                        KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                }

                bytes += bp->b_bufsize;

                if (bp->b_flags & B_VMIO) {
                        bp->b_flags |= B_DIRECT;    /* try to free pages */
                        vfs_vmio_release(bp);
                }
                if (bp->b_vp)
                        brelvp(bp);

                KKASSERT(bp->b_vp == NULL);
                KKASSERT((bp->b_flags & B_HASHED) == 0);

                /*
                 * critical section protection is not required when
                 * scrapping a buffer's contents because it is already 
                 * wired.
                 */
                if (bp->b_bufsize)
                        allocbuf(bp, 0);

                bp->b_flags = B_BNOCLIP;
                bp->b_cmd = BUF_CMD_DONE;
                bp->b_vp = NULL;
                bp->b_error = 0;
                bp->b_resid = 0;
                bp->b_bcount = 0;
                bp->b_xio.xio_npages = 0;
                bp->b_dirtyoff = bp->b_dirtyend = 0;
                reinitbufbio(bp);
                KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                buf_dep_init(bp);
                bp->b_flags |= B_INVAL;
                /* bfreekva(bp); */
                brelse(bp);
                spin_lock(&bufqspin);
        }
        spin_unlock(&bufqspin);
        return(bytes);
}

/*
 * buf_daemon:
 *
 *      Buffer flushing daemon.  Buffers are normally flushed by the
 *      update daemon but if it cannot keep up this process starts to
 *      take the load in an attempt to prevent getnewbuf() from blocking.
 *
 *      Once a flush is initiated it does not stop until the number
 *      of buffers falls below lodirtybuffers, but we will wake up anyone
 *      waiting at the mid-point.
 */

static struct kproc_desc buf_kp = {
        "bufdaemon",
        buf_daemon,
        &bufdaemon_td
};
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST,
        kproc_start, &buf_kp)

static struct kproc_desc bufhw_kp = {
        "bufdaemon_hw",
        buf_daemon_hw,
        &bufdaemonhw_td
};
SYSINIT(bufdaemon_hw, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST,
        kproc_start, &bufhw_kp)

/*
 * MPSAFE thread
 */
static void
buf_daemon(void)
{
        int limit;

        /*
         * This process needs to be suspended prior to shutdown sync.
         */
        EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_kproc,
                              bufdaemon_td, SHUTDOWN_PRI_LAST);
        curthread->td_flags |= TDF_SYSTHREAD;

        /*
         * This process is allowed to take the buffer cache to the limit
         */
        for (;;) {
                kproc_suspend_loop();

                /*
                 * Do the flush as long as the number of dirty buffers
                 * (including those running) exceeds lodirtybufspace.
                 *
                 * When flushing limit running I/O to hirunningspace
                 * Do the flush.  Limit the amount of in-transit I/O we
                 * allow to build up, otherwise we would completely saturate
                 * the I/O system.  Wakeup any waiting processes before we
                 * normally would so they can run in parallel with our drain.
                 *
                 * Our aggregate normal+HW lo water mark is lodirtybufspace,
                 * but because we split the operation into two threads we
                 * have to cut it in half for each thread.
                 */
                waitrunningbufspace();
                limit = lodirtybufspace / 2;
                while (runningbufspace + dirtybufspace > limit ||
                       dirtybufcount - dirtybufcounthw >= nbuf / 2) {
                        if (flushbufqueues(BQUEUE_DIRTY) == 0)
                                break;
                        if (runningbufspace < hirunningspace)
                                continue;
                        waitrunningbufspace();
                }

                /*
                 * We reached our low water mark, reset the
                 * request and sleep until we are needed again.
                 * The sleep is just so the suspend code works.
                 */
                spin_lock(&bufcspin);
                if (bd_request == 0)
                        ssleep(&bd_request, &bufcspin, 0, "psleep", hz);
                bd_request = 0;
                spin_unlock(&bufcspin);
        }
}

/*
 * MPSAFE thread
 */
static void
buf_daemon_hw(void)
{
        int limit;

        /*
         * This process needs to be suspended prior to shutdown sync.
         */
        EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_kproc,
                              bufdaemonhw_td, SHUTDOWN_PRI_LAST);
        curthread->td_flags |= TDF_SYSTHREAD;

        /*
         * This process is allowed to take the buffer cache to the limit
         */
        for (;;) {
                kproc_suspend_loop();

                /*
                 * Do the flush.  Limit the amount of in-transit I/O we
                 * allow to build up, otherwise we would completely saturate
                 * the I/O system.  Wakeup any waiting processes before we
                 * normally would so they can run in parallel with our drain.
                 *
                 * Once we decide to flush push the queued I/O up to
                 * hirunningspace in order to trigger bursting by the bioq
                 * subsystem.
                 *
                 * Our aggregate normal+HW lo water mark is lodirtybufspace,
                 * but because we split the operation into two threads we
                 * have to cut it in half for each thread.
                 */
                waitrunningbufspace();
                limit = lodirtybufspace / 2;
                while (runningbufspace + dirtybufspacehw > limit ||
                       dirtybufcounthw >= nbuf / 2) {
                        if (flushbufqueues(BQUEUE_DIRTY_HW) == 0)
                                break;
                        if (runningbufspace < hirunningspace)
                                continue;
                        waitrunningbufspace();
                }

                /*
                 * We reached our low water mark, reset the
                 * request and sleep until we are needed again.
                 * The sleep is just so the suspend code works.
                 */
                spin_lock(&bufcspin);
                if (bd_request_hw == 0)
                        ssleep(&bd_request_hw, &bufcspin, 0, "psleep", hz);
                bd_request_hw = 0;
                spin_unlock(&bufcspin);
        }
}

/*
 * flushbufqueues:
 *
 *      Try to flush a buffer in the dirty queue.  We must be careful to
 *      free up B_INVAL buffers instead of write them, which NFS is 
 *      particularly sensitive to.
 *
 *      B_RELBUF may only be set by VFSs.  We do set B_AGE to indicate
 *      that we really want to try to get the buffer out and reuse it
 *      due to the write load on the machine.
 *
 *      We must lock the buffer in order to check its validity before we
 *      can mess with its contents.  bufqspin isn't enough.
 */
static int
flushbufqueues(bufq_type_t q)
{
        struct buf *bp;
        int r = 0;
        int spun;

        spin_lock(&bufqspin);
        spun = 1;

        bp = TAILQ_FIRST(&bufqueues[q]);
        while (bp) {
                if ((bp->b_flags & B_DELWRI) == 0) {
                        kprintf("Unexpected clean buffer %p\n", bp);
                        bp = TAILQ_NEXT(bp, b_freelist);
                        continue;
                }
                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                        bp = TAILQ_NEXT(bp, b_freelist);
                        continue;
                }
                KKASSERT(bp->b_qindex == q);

                /*
                 * Must recheck B_DELWRI after successfully locking
                 * the buffer.
                 */
                if ((bp->b_flags & B_DELWRI) == 0) {
                        BUF_UNLOCK(bp);
                        bp = TAILQ_NEXT(bp, b_freelist);
                        continue;
                }

                if (bp->b_flags & B_INVAL) {
                        _bremfree(bp);
                        spin_unlock(&bufqspin);
                        spun = 0;
                        brelse(bp);
                        ++r;
                        break;
                }

                spin_unlock(&bufqspin);
                spun = 0;

                if (LIST_FIRST(&bp->b_dep) != NULL &&
                    (bp->b_flags & B_DEFERRED) == 0 &&
                    buf_countdeps(bp, 0)) {
                        spin_lock(&bufqspin);
                        spun = 1;
                        TAILQ_REMOVE(&bufqueues[q], bp, b_freelist);
                        TAILQ_INSERT_TAIL(&bufqueues[q], bp, b_freelist);
                        bp->b_flags |= B_DEFERRED;
                        BUF_UNLOCK(bp);
                        bp = TAILQ_FIRST(&bufqueues[q]);
                        continue;
                }

                /*
                 * If the buffer has a dependancy, buf_checkwrite() must
                 * also return 0 for us to be able to initate the write.
                 *
                 * If the buffer is flagged B_ERROR it may be requeued
                 * over and over again, we try to avoid a live lock.
                 *
                 * NOTE: buf_checkwrite is MPSAFE.
                 */
                if (LIST_FIRST(&bp->b_dep) != NULL && buf_checkwrite(bp)) {
                        bremfree(bp);
                        brelse(bp);
                } else if (bp->b_flags & B_ERROR) {
                        tsleep(bp, 0, "bioer", 1);
                        bp->b_flags &= ~B_AGE;
                        vfs_bio_awrite(bp);
                } else {
                        bp->b_flags |= B_AGE;
                        vfs_bio_awrite(bp);
                }
                ++r;
                break;
        }
        if (spun)
                spin_unlock(&bufqspin);
        return (r);
}

/*
 * inmem:
 *
 *      Returns true if no I/O is needed to access the associated VM object.
 *      This is like findblk except it also hunts around in the VM system for
 *      the data.
 *
 *      Note that we ignore vm_page_free() races from interrupts against our
 *      lookup, since if the caller is not protected our return value will not
 *      be any more valid then otherwise once we exit the critical section.
 */
int
inmem(struct vnode *vp, off_t loffset)
{
        vm_object_t obj;
        vm_offset_t toff, tinc, size;
        vm_page_t m;
        int res = 1;

        if (findblk(vp, loffset, FINDBLK_TEST))
                return 1;
        if (vp->v_mount == NULL)
                return 0;
        if ((obj = vp->v_object) == NULL)
                return 0;

        size = PAGE_SIZE;
        if (size > vp->v_mount->mnt_stat.f_iosize)
                size = vp->v_mount->mnt_stat.f_iosize;

        vm_object_hold(obj);
        for (toff = 0; toff < vp->v_mount->mnt_stat.f_iosize; toff += tinc) {
                m = vm_page_lookup(obj, OFF_TO_IDX(loffset + toff));
                if (m == NULL) {
                        res = 0;
                        break;
                }
                tinc = size;
                if (tinc > PAGE_SIZE - ((toff + loffset) & PAGE_MASK))
                        tinc = PAGE_SIZE - ((toff + loffset) & PAGE_MASK);
                if (vm_page_is_valid(m,
                    (vm_offset_t) ((toff + loffset) & PAGE_MASK), tinc) == 0) {
                        res = 0;
                        break;
                }
        }
        vm_object_drop(obj);
        return (res);
}

/*
 * findblk:
 *
 *      Locate and return the specified buffer.  Unless flagged otherwise,
 *      a locked buffer will be returned if it exists or NULL if it does not.
 *
 *      findblk()'d buffers are still on the bufqueues and if you intend
 *      to use your (locked NON-TEST) buffer you need to bremfree(bp)
 *      and possibly do other stuff to it.
 *
 *      FINDBLK_TEST    - Do not lock the buffer.  The caller is responsible
 *                        for locking the buffer and ensuring that it remains
 *                        the desired buffer after locking.
 *
 *      FINDBLK_NBLOCK  - Lock the buffer non-blocking.  If we are unable
 *                        to acquire the lock we return NULL, even if the
 *                        buffer exists.
 *
 *      FINDBLK_REF     - Returns the buffer ref'd, which prevents normal
 *                        reuse by getnewbuf() but does not prevent
 *                        disassociation (B_INVAL).  Used to avoid deadlocks
 *                        against random (vp,loffset)s due to reassignment.
 *
 *      (0)             - Lock the buffer blocking.
 *
 * MPSAFE
 */
struct buf *
findblk(struct vnode *vp, off_t loffset, int flags)
{
        struct buf *bp;
        int lkflags;

        lkflags = LK_EXCLUSIVE;
        if (flags & FINDBLK_NBLOCK)
                lkflags |= LK_NOWAIT;

        for (;;) {
                /*
                 * Lookup.  Ref the buf while holding v_token to prevent
                 * reuse (but does not prevent diassociation).
                 */
                lwkt_gettoken(&vp->v_token);
                bp = buf_rb_hash_RB_LOOKUP(&vp->v_rbhash_tree, loffset);
                if (bp == NULL) {
                        lwkt_reltoken(&vp->v_token);
                        return(NULL);
                }
                bqhold(bp);
                lwkt_reltoken(&vp->v_token);

                /*
                 * If testing only break and return bp, do not lock.
                 */
                if (flags & FINDBLK_TEST)
                        break;

                /*
                 * Lock the buffer, return an error if the lock fails.
                 * (only FINDBLK_NBLOCK can cause the lock to fail).
                 */
                if (BUF_LOCK(bp, lkflags)) {
                        atomic_subtract_int(&bp->b_refs, 1);
                        /* bp = NULL; not needed */
                        return(NULL);
                }

                /*
                 * Revalidate the locked buf before allowing it to be
                 * returned.
                 */
                if (bp->b_vp == vp && bp->b_loffset == loffset)
                        break;
                atomic_subtract_int(&bp->b_refs, 1);
                BUF_UNLOCK(bp);
        }

        /*
         * Success
         */
        if ((flags & FINDBLK_REF) == 0)
                atomic_subtract_int(&bp->b_refs, 1);
        return(bp);
}

/*
 * getcacheblk:
 *
 *      Similar to getblk() except only returns the buffer if it is
 *      B_CACHE and requires no other manipulation.  Otherwise NULL
 *      is returned.
 *
 *      If B_RAM is set the buffer might be just fine, but we return
 *      NULL anyway because we want the code to fall through to the
 *      cluster read.  Otherwise read-ahead breaks.
 *
 *      If blksize is 0 the buffer cache buffer must already be fully
 *      cached.
 *
 *      If blksize is non-zero getblk() will be used, allowing a buffer
 *      to be reinstantiated from its VM backing store.  The buffer must
 *      still be fully cached after reinstantiation to be returned.
 */
struct buf *
getcacheblk(struct vnode *vp, off_t loffset, int blksize)
{
        struct buf *bp;

        if (blksize) {
                bp = getblk(vp, loffset, blksize, 0, 0);
                if (bp) {
                        if ((bp->b_flags & (B_INVAL | B_CACHE | B_RAM)) ==
                            B_CACHE) {
                                bp->b_flags &= ~B_AGE;
                        } else {
                                brelse(bp);
                                bp = NULL;
                        }
                }
        } else {
                bp = findblk(vp, loffset, 0);
                if (bp) {
                        if ((bp->b_flags & (B_INVAL | B_CACHE | B_RAM)) ==
                            B_CACHE) {
                                bp->b_flags &= ~B_AGE;
                                bremfree(bp);
                        } else {
                                BUF_UNLOCK(bp);
                                bp = NULL;
                        }
                }
        }
        return (bp);
}

/*
 * getblk:
 *
 *      Get a block given a specified block and offset into a file/device.
 *      B_INVAL may or may not be set on return.  The caller should clear
 *      B_INVAL prior to initiating a READ.
 *
 *      IT IS IMPORTANT TO UNDERSTAND THAT IF YOU CALL GETBLK() AND B_CACHE
 *      IS NOT SET, YOU MUST INITIALIZE THE RETURNED BUFFER, ISSUE A READ,
 *      OR SET B_INVAL BEFORE RETIRING IT.  If you retire a getblk'd buffer
 *      without doing any of those things the system will likely believe
 *      the buffer to be valid (especially if it is not B_VMIO), and the
 *      next getblk() will return the buffer with B_CACHE set.
 *
 *      For a non-VMIO buffer, B_CACHE is set to the opposite of B_INVAL for
 *      an existing buffer.
 *
 *      For a VMIO buffer, B_CACHE is modified according to the backing VM.
 *      If getblk()ing a previously 0-sized invalid buffer, B_CACHE is set
 *      and then cleared based on the backing VM.  If the previous buffer is
 *      non-0-sized but invalid, B_CACHE will be cleared.
 *
 *      If getblk() must create a new buffer, the new buffer is returned with
 *      both B_INVAL and B_CACHE clear unless it is a VMIO buffer, in which
 *      case it is returned with B_INVAL clear and B_CACHE set based on the
 *      backing VM.
 *
 *      getblk() also forces a bwrite() for any B_DELWRI buffer whos
 *      B_CACHE bit is clear.
 *      
 *      What this means, basically, is that the caller should use B_CACHE to
 *      determine whether the buffer is fully valid or not and should clear
 *      B_INVAL prior to issuing a read.  If the caller intends to validate
 *      the buffer by loading its data area with something, the caller needs
 *      to clear B_INVAL.  If the caller does this without issuing an I/O, 
 *      the caller should set B_CACHE ( as an optimization ), else the caller
 *      should issue the I/O and biodone() will set B_CACHE if the I/O was
 *      a write attempt or if it was a successfull read.  If the caller 
 *      intends to issue a READ, the caller must clear B_INVAL and B_ERROR
 *      prior to issuing the READ.  biodone() will *not* clear B_INVAL.
 *
 *      getblk flags:
 *
 *      GETBLK_PCATCH - catch signal if blocked, can cause NULL return
 *      GETBLK_BHEAVY - heavy-weight buffer cache buffer
 *
 * MPALMOSTSAFE
 */
struct buf *
getblk(struct vnode *vp, off_t loffset, int size, int blkflags, int slptimeo)
{
        struct buf *bp;
        int slpflags = (blkflags & GETBLK_PCATCH) ? PCATCH : 0;
        int error;
        int lkflags;

        if (size > MAXBSIZE)
                panic("getblk: size(%d) > MAXBSIZE(%d)", size, MAXBSIZE);
        if (vp->v_object == NULL)
                panic("getblk: vnode %p has no object!", vp);

loop:
        if ((bp = findblk(vp, loffset, FINDBLK_REF | FINDBLK_TEST)) != NULL) {
                /*
                 * The buffer was found in the cache, but we need to lock it.
                 * We must acquire a ref on the bp to prevent reuse, but
                 * this will not prevent disassociation (brelvp()) so we
                 * must recheck (vp,loffset) after acquiring the lock.
                 *
                 * Without the ref the buffer could potentially be reused
                 * before we acquire the lock and create a deadlock
                 * situation between the thread trying to reuse the buffer
                 * and us due to the fact that we would wind up blocking
                 * on a random (vp,loffset).
                 */
                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                        if (blkflags & GETBLK_NOWAIT) {
                                bqdrop(bp);
                                return(NULL);
                        }
                        lkflags = LK_EXCLUSIVE | LK_SLEEPFAIL;
                        if (blkflags & GETBLK_PCATCH)
                                lkflags |= LK_PCATCH;
                        error = BUF_TIMELOCK(bp, lkflags, "getblk", slptimeo);
                        if (error) {
                                bqdrop(bp);
                                if (error == ENOLCK)
                                        goto loop;
                                return (NULL);
                        }
                        /* buffer may have changed on us */
                }
                bqdrop(bp);

                /*
                 * Once the buffer has been locked, make sure we didn't race
                 * a buffer recyclement.  Buffers that are no longer hashed
                 * will have b_vp == NULL, so this takes care of that check
                 * as well.
                 */
                if (bp->b_vp != vp || bp->b_loffset != loffset) {
                        kprintf("Warning buffer %p (vp %p loffset %lld) "
                                "was recycled\n",
                                bp, vp, (long long)loffset);
                        BUF_UNLOCK(bp);
                        goto loop;
                }

                /*
                 * If SZMATCH any pre-existing buffer must be of the requested
                 * size or NULL is returned.  The caller absolutely does not
                 * want getblk() to bwrite() the buffer on a size mismatch.
                 */
                if ((blkflags & GETBLK_SZMATCH) && size != bp->b_bcount) {
                        BUF_UNLOCK(bp);
                        return(NULL);
                }

                /*
                 * All vnode-based buffers must be backed by a VM object.
                 */
                KKASSERT(bp->b_flags & B_VMIO);
                KKASSERT(bp->b_cmd == BUF_CMD_DONE);
                bp->b_flags &= ~B_AGE;

                /*
                 * Make sure that B_INVAL buffers do not have a cached
                 * block number translation.
                 */
                if ((bp->b_flags & B_INVAL) && (bp->b_bio2.bio_offset != NOOFFSET)) {
                        kprintf("Warning invalid buffer %p (vp %p loffset %lld)"
                                " did not have cleared bio_offset cache\n",
                                bp, vp, (long long)loffset);
                        clearbiocache(&bp->b_bio2);
                }

                /*
                 * The buffer is locked.  B_CACHE is cleared if the buffer is 
                 * invalid.
                 */
                if (bp->b_flags & B_INVAL)
                        bp->b_flags &= ~B_CACHE;
                bremfree(bp);

                /*
                 * Any size inconsistancy with a dirty buffer or a buffer
                 * with a softupdates dependancy must be resolved.  Resizing
                 * the buffer in such circumstances can lead to problems.
                 *
                 * Dirty or dependant buffers are written synchronously.
                 * Other types of buffers are simply released and
                 * reconstituted as they may be backed by valid, dirty VM
                 * pages (but not marked B_DELWRI).
                 *
                 * NFS NOTE: NFS buffers which straddle EOF are oddly-sized
                 * and may be left over from a prior truncation (and thus
                 * no longer represent the actual EOF point), so we
                 * definitely do not want to B_NOCACHE the backing store.
                 */
                if (size != bp->b_bcount) {
                        if (bp->b_flags & B_DELWRI) {
                                bp->b_flags |= B_RELBUF;
                                bwrite(bp);
                        } else if (LIST_FIRST(&bp->b_dep)) {
                                bp->b_flags |= B_RELBUF;
                                bwrite(bp);
                        } else {
                                bp->b_flags |= B_RELBUF;
                                brelse(bp);
                        }
                        goto loop;
                }
                KKASSERT(size <= bp->b_kvasize);
                KASSERT(bp->b_loffset != NOOFFSET, 
                        ("getblk: no buffer offset"));

                /*
                 * A buffer with B_DELWRI set and B_CACHE clear must
                 * be committed before we can return the buffer in
                 * order to prevent the caller from issuing a read
                 * ( due to B_CACHE not being set ) and overwriting
                 * it.
                 *
                 * Most callers, including NFS and FFS, need this to
                 * operate properly either because they assume they
                 * can issue a read if B_CACHE is not set, or because
                 * ( for example ) an uncached B_DELWRI might loop due 
                 * to softupdates re-dirtying the buffer.  In the latter
                 * case, B_CACHE is set after the first write completes,
                 * preventing further loops.
                 *
                 * NOTE!  b*write() sets B_CACHE.  If we cleared B_CACHE
                 * above while extending the buffer, we cannot allow the
                 * buffer to remain with B_CACHE set after the write
                 * completes or it will represent a corrupt state.  To
                 * deal with this we set B_NOCACHE to scrap the buffer
                 * after the write.
                 *
                 * XXX Should this be B_RELBUF instead of B_NOCACHE?
                 *     I'm not even sure this state is still possible
                 *     now that getblk() writes out any dirty buffers
                 *     on size changes.
                 *
                 * We might be able to do something fancy, like setting
                 * B_CACHE in bwrite() except if B_DELWRI is already set,
                 * so the below call doesn't set B_CACHE, but that gets real
                 * confusing.  This is much easier.
                 */

                if ((bp->b_flags & (B_CACHE|B_DELWRI)) == B_DELWRI) {
                        kprintf("getblk: Warning, bp %p loff=%jx DELWRI set "
                                "and CACHE clear, b_flags %08x\n",
                                bp, (intmax_t)bp->b_loffset, bp->b_flags);
                        bp->b_flags |= B_NOCACHE;
                        bwrite(bp);
                        goto loop;
                }
        } else {
                /*
                 * Buffer is not in-core, create new buffer.  The buffer
                 * returned by getnewbuf() is locked.  Note that the returned
                 * buffer is also considered valid (not marked B_INVAL).
                 *
                 * Calculating the offset for the I/O requires figuring out
                 * the block size.  We use DEV_BSIZE for VBLK or VCHR and
                 * the mount's f_iosize otherwise.  If the vnode does not
                 * have an associated mount we assume that the passed size is 
                 * the block size.  
                 *
                 * Note that vn_isdisk() cannot be used here since it may
                 * return a failure for numerous reasons.   Note that the
                 * buffer size may be larger then the block size (the caller
                 * will use block numbers with the proper multiple).  Beware
                 * of using any v_* fields which are part of unions.  In
                 * particular, in DragonFly the mount point overloading 
                 * mechanism uses the namecache only and the underlying
                 * directory vnode is not a special case.
                 */
                int bsize, maxsize;

                if (vp->v_type == VBLK || vp->v_type == VCHR)
                        bsize = DEV_BSIZE;
                else if (vp->v_mount)
                        bsize = vp->v_mount->mnt_stat.f_iosize;
                else
                        bsize = size;

                maxsize = size + (loffset & PAGE_MASK);
                maxsize = imax(maxsize, bsize);

                bp = getnewbuf(blkflags, slptimeo, size, maxsize);
                if (bp == NULL) {
                        if (slpflags || slptimeo)
                                return NULL;
                        goto loop;
                }

                /*
                 * Atomically insert the buffer into the hash, so that it can
                 * be found by findblk().
                 *
                 * If bgetvp() returns non-zero a collision occured, and the
                 * bp will not be associated with the vnode.
                 *
                 * Make sure the translation layer has been cleared.
                 */
                bp->b_loffset = loffset;
                bp->b_bio2.bio_offset = NOOFFSET;
                /* bp->b_bio2.bio_next = NULL; */

                if (bgetvp(vp, bp, size)) {
                        bp->b_flags |= B_INVAL;
                        brelse(bp);
                        goto loop;
                }

                /*
                 * All vnode-based buffers must be backed by a VM object.
                 */
                KKASSERT(vp->v_object != NULL);
                bp->b_flags |= B_VMIO;
                KKASSERT(bp->b_cmd == BUF_CMD_DONE);

                allocbuf(bp, size);
        }
        KKASSERT(dsched_is_clear_buf_priv(bp));
        return (bp);
}

/*
 * regetblk(bp)
 *
 * Reacquire a buffer that was previously released to the locked queue,
 * or reacquire a buffer which is interlocked by having bioops->io_deallocate
 * set B_LOCKED (which handles the acquisition race).
 *
 * To this end, either B_LOCKED must be set or the dependancy list must be
 * non-empty.
 *
 * MPSAFE
 */
void
regetblk(struct buf *bp)
{
        KKASSERT((bp->b_flags & B_LOCKED) || LIST_FIRST(&bp->b_dep) != NULL);
        BUF_LOCK(bp, LK_EXCLUSIVE | LK_RETRY);
        bremfree(bp);
}

/*
 * geteblk:
 *
 *      Get an empty, disassociated buffer of given size.  The buffer is
 *      initially set to B_INVAL.
 *
 *      critical section protection is not required for the allocbuf()
 *      call because races are impossible here.
 *
 * MPALMOSTSAFE
 */
struct buf *
geteblk(int size)
{
        struct buf *bp;
        int maxsize;

        maxsize = (size + BKVAMASK) & ~BKVAMASK;

        while ((bp = getnewbuf(0, 0, size, maxsize)) == 0)
                ;
        allocbuf(bp, size);
        bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
        KKASSERT(dsched_is_clear_buf_priv(bp));
        return (bp);
}


/*
 * allocbuf:
 *
 *      This code constitutes the buffer memory from either anonymous system
 *      memory (in the case of non-VMIO operations) or from an associated
 *      VM object (in the case of VMIO operations).  This code is able to
 *      resize a buffer up or down.
 *
 *      Note that this code is tricky, and has many complications to resolve
 *      deadlock or inconsistant data situations.  Tread lightly!!! 
 *      There are B_CACHE and B_DELWRI interactions that must be dealt with by 
 *      the caller.  Calling this code willy nilly can result in the loss of
 *      data.
 *
 *      allocbuf() only adjusts B_CACHE for VMIO buffers.  getblk() deals with
 *      B_CACHE for the non-VMIO case.
 *
 *      This routine does not need to be called from a critical section but you
 *      must own the buffer.
 *
 * MPSAFE
 */
int
allocbuf(struct buf *bp, int size)
{
        int newbsize, mbsize;
        int i;

        if (BUF_REFCNT(bp) == 0)
                panic("allocbuf: buffer not busy");

        if (bp->b_kvasize < size)
                panic("allocbuf: buffer too small");

        if ((bp->b_flags & B_VMIO) == 0) {
                caddr_t origbuf;
                int origbufsize;
                /*
                 * Just get anonymous memory from the kernel.  Don't
                 * mess with B_CACHE.
                 */
                mbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                if (bp->b_flags & B_MALLOC)
                        newbsize = mbsize;
                else
                        newbsize = round_page(size);

                if (newbsize < bp->b_bufsize) {
                        /*
                         * Malloced buffers are not shrunk
                         */
                        if (bp->b_flags & B_MALLOC) {
                                if (newbsize) {
                                        bp->b_bcount = size;
                                } else {
                                        kfree(bp->b_data, M_BIOBUF);
                                        if (bp->b_bufsize) {
                                                atomic_subtract_int(&bufmallocspace, bp->b_bufsize);
                                                bufspacewakeup();
                                                bp->b_bufsize = 0;
                                        }
                                        bp->b_data = bp->b_kvabase;
                                        bp->b_bcount = 0;
                                        bp->b_flags &= ~B_MALLOC;
                                }
                                return 1;
                        }               
                        vm_hold_free_pages(
                            bp,
                            (vm_offset_t) bp->b_data + newbsize,
                            (vm_offset_t) bp->b_data + bp->b_bufsize);
                } else if (newbsize > bp->b_bufsize) {
                        /*
                         * We only use malloced memory on the first allocation.
                         * and revert to page-allocated memory when the buffer
                         * grows.
                         */
                        if ((bufmallocspace < maxbufmallocspace) &&
                                (bp->b_bufsize == 0) &&
                                (mbsize <= PAGE_SIZE/2)) {

                                bp->b_data = kmalloc(mbsize, M_BIOBUF, M_WAITOK);
                                bp->b_bufsize = mbsize;
                                bp->b_bcount = size;
                                bp->b_flags |= B_MALLOC;
                                atomic_add_int(&bufmallocspace, mbsize);
                                return 1;
                        }
                        origbuf = NULL;
                        origbufsize = 0;
                        /*
                         * If the buffer is growing on its other-than-first
                         * allocation, then we revert to the page-allocation
                         * scheme.
                         */
                        if (bp->b_flags & B_MALLOC) {
                                origbuf = bp->b_data;
                                origbufsize = bp->b_bufsize;
                                bp->b_data = bp->b_kvabase;
                                if (bp->b_bufsize) {
                                        atomic_subtract_int(&bufmallocspace,
                                                            bp->b_bufsize);
                                        bufspacewakeup();
                                        bp->b_bufsize = 0;
                                }
                                bp->b_flags &= ~B_MALLOC;
                                newbsize = round_page(newbsize);
                        }
                        vm_hold_load_pages(
                            bp,
                            (vm_offset_t) bp->b_data + bp->b_bufsize,
                            (vm_offset_t) bp->b_data + newbsize);
                        if (origbuf) {
                                bcopy(origbuf, bp->b_data, origbufsize);
                                kfree(origbuf, M_BIOBUF);
                        }
                }
        } else {
                vm_page_t m;
                int desiredpages;

                newbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                desiredpages = ((int)(bp->b_loffset & PAGE_MASK) +
                                newbsize + PAGE_MASK) >> PAGE_SHIFT;
                KKASSERT(desiredpages <= XIO_INTERNAL_PAGES);

                if (bp->b_flags & B_MALLOC)
                        panic("allocbuf: VMIO buffer can't be malloced");
                /*
                 * Set B_CACHE initially if buffer is 0 length or will become
                 * 0-length.
                 */
                if (size == 0 || bp->b_bufsize == 0)
                        bp->b_flags |= B_CACHE;

                if (newbsize < bp->b_bufsize) {
                        /*
                         * DEV_BSIZE aligned new buffer size is less then the
                         * DEV_BSIZE aligned existing buffer size.  Figure out
                         * if we have to remove any pages.
                         */
                        if (desiredpages < bp->b_xio.xio_npages) {
                                for (i = desiredpages; i < bp->b_xio.xio_npages; i++) {
                                        /*
                                         * the page is not freed here -- it
                                         * is the responsibility of 
                                         * vnode_pager_setsize
                                         */
                                        m = bp->b_xio.xio_pages[i];
                                        KASSERT(m != bogus_page,
                                            ("allocbuf: bogus page found"));
                                        vm_page_busy_wait(m, TRUE, "biodep");
                                        bp->b_xio.xio_pages[i] = NULL;
                                        vm_page_unwire(m, 0);
                                        vm_page_wakeup(m);
                                }
                                pmap_qremove((vm_offset_t) trunc_page((vm_offset_t)bp->b_data) +
                                    (desiredpages << PAGE_SHIFT), (bp->b_xio.xio_npages - desiredpages));
                                bp->b_xio.xio_npages = desiredpages;
                        }
                } else if (size > bp->b_bcount) {
                        /*
                         * We are growing the buffer, possibly in a 
                         * byte-granular fashion.
                         */
                        struct vnode *vp;
                        vm_object_t obj;
                        vm_offset_t toff;
                        vm_offset_t tinc;

                        /*
                         * Step 1, bring in the VM pages from the object, 
                         * allocating them if necessary.  We must clear
                         * B_CACHE if these pages are not valid for the 
                         * range covered by the buffer.
                         *
                         * critical section protection is required to protect
                         * against interrupts unbusying and freeing pages
                         * between our vm_page_lookup() and our
                         * busycheck/wiring call.
                         */
                        vp = bp->b_vp;
                        obj = vp->v_object;

                        vm_object_hold(obj);
                        while (bp->b_xio.xio_npages < desiredpages) {
                                vm_page_t m;
                                vm_pindex_t pi;
                                int error;

                                pi = OFF_TO_IDX(bp->b_loffset) +
                                     bp->b_xio.xio_npages;

                                /*
                                 * Blocking on m->busy might lead to a
                                 * deadlock:
                                 *
                                 *  vm_fault->getpages->cluster_read->allocbuf
                                 */
                                m = vm_page_lookup_busy_try(obj, pi, FALSE,
                                                            &error);
                                if (error) {
                                        vm_page_sleep_busy(m, FALSE, "pgtblk");
                                        continue;
                                }
                                if (m == NULL) {
                                        /*
                                         * note: must allocate system pages
                                         * since blocking here could intefere
                                         * with paging I/O, no matter which
                                         * process we are.
                                         */
                                        m = bio_page_alloc(obj, pi, desiredpages - bp->b_xio.xio_npages);
                                        if (m) {
                                                vm_page_wire(m);
                                                vm_page_flag_clear(m, PG_ZERO);
                                                vm_page_wakeup(m);
                                                bp->b_flags &= ~B_CACHE;
                                                bp->b_xio.xio_pages[bp->b_xio.xio_npages] = m;
                                                ++bp->b_xio.xio_npages;
                                        }
                                        continue;
                                }

                                /*
                                 * We found a page and were able to busy it.
                                 */
                                vm_page_flag_clear(m, PG_ZERO);
                                vm_page_wire(m);
                                vm_page_wakeup(m);
                                bp->b_xio.xio_pages[bp->b_xio.xio_npages] = m;
                                ++bp->b_xio.xio_npages;
                                if (bp->b_act_count < m->act_count)
                                        bp->b_act_count = m->act_count;
                        }
                        vm_object_drop(obj);

                        /*
                         * Step 2.  We've loaded the pages into the buffer,
                         * we have to figure out if we can still have B_CACHE
                         * set.  Note that B_CACHE is set according to the
                         * byte-granular range ( bcount and size ), not the
                         * aligned range ( newbsize ).
                         *
                         * The VM test is against m->valid, which is DEV_BSIZE
                         * aligned.  Needless to say, the validity of the data
                         * needs to also be DEV_BSIZE aligned.  Note that this
                         * fails with NFS if the server or some other client
                         * extends the file's EOF.  If our buffer is resized, 
                         * B_CACHE may remain set! XXX
                         */

                        toff = bp->b_bcount;
                        tinc = PAGE_SIZE - ((bp->b_loffset + toff) & PAGE_MASK);

                        while ((bp->b_flags & B_CACHE) && toff < size) {
                                vm_pindex_t pi;

                                if (tinc > (size - toff))
                                        tinc = size - toff;

                                pi = ((bp->b_loffset & PAGE_MASK) + toff) >> 
                                    PAGE_SHIFT;

                                vfs_buf_test_cache(
                                    bp, 
                                    bp->b_loffset,
                                    toff, 
                                    tinc, 
                                    bp->b_xio.xio_pages[pi]
                                );
                                toff += tinc;
                                tinc = PAGE_SIZE;
                        }

                        /*
                         * Step 3, fixup the KVM pmap.  Remember that
                         * bp->b_data is relative to bp->b_loffset, but 
                         * bp->b_loffset may be offset into the first page.
                         */

                        bp->b_data = (caddr_t)
                            trunc_page((vm_offset_t)bp->b_data);
                        pmap_qenter(
                            (vm_offset_t)bp->b_data,
                            bp->b_xio.xio_pages, 
                            bp->b_xio.xio_npages
                        );
                        bp->b_data = (caddr_t)((vm_offset_t)bp->b_data | 
                            (vm_offset_t)(bp->b_loffset & PAGE_MASK));
                }
        }

        /* adjust space use on already-dirty buffer */
        if (bp->b_flags & B_DELWRI) {
                spin_lock(&bufcspin);
                dirtybufspace += newbsize - bp->b_bufsize;
                if (bp->b_flags & B_HEAVY)
                        dirtybufspacehw += newbsize - bp->b_bufsize;
                spin_unlock(&bufcspin);
        }
        if (newbsize < bp->b_bufsize)
                bufspacewakeup();
        bp->b_bufsize = newbsize;       /* actual buffer allocation     */
        bp->b_bcount = size;            /* requested buffer size        */
        return 1;
}

/*
 * biowait:
 *
 *      Wait for buffer I/O completion, returning error status. B_EINTR
 *      is converted into an EINTR error but not cleared (since a chain
 *      of biowait() calls may occur).
 *
 *      On return bpdone() will have been called but the buffer will remain
 *      locked and will not have been brelse()'d.
 *
 *      NOTE!  If a timeout is specified and ETIMEDOUT occurs the I/O is
 *      likely still in progress on return.
 *
 *      NOTE!  This operation is on a BIO, not a BUF.
 *
 *      NOTE!  BIO_DONE is cleared by vn_strategy()
 *
 * MPSAFE
 */
static __inline int
_biowait(struct bio *bio, const char *wmesg, int to)
{
        struct buf *bp = bio->bio_buf;
        u_int32_t flags;
        u_int32_t nflags;
        int error;

        KKASSERT(bio == &bp->b_bio1);
        for (;;) {
                flags = bio->bio_flags;
                if (flags & BIO_DONE)
                        break;
                nflags = flags | BIO_WANT;
                tsleep_interlock(bio, 0);
                if (atomic_cmpset_int(&bio->bio_flags, flags, nflags)) {
                        if (wmesg)
                                error = tsleep(bio, PINTERLOCKED, wmesg, to);
                        else if (bp->b_cmd == BUF_CMD_READ)
                                error = tsleep(bio, PINTERLOCKED, "biord", to);
                        else
                                error = tsleep(bio, PINTERLOCKED, "biowr", to);
                        if (error) {
                                kprintf("tsleep error biowait %d\n", error);
                                return (error);
                        }
                }
        }

        /*
         * Finish up.
         */
        KKASSERT(bp->b_cmd == BUF_CMD_DONE);
        bio->bio_flags &= ~(BIO_DONE | BIO_SYNC);
        if (bp->b_flags & B_EINTR)
                return (EINTR);
        if (bp->b_flags & B_ERROR)
                return (bp->b_error ? bp->b_error : EIO);
        return (0);
}

int
biowait(struct bio *bio, const char *wmesg)
{
        return(_biowait(bio, wmesg, 0));
}

int
biowait_timeout(struct bio *bio, const char *wmesg, int to)
{
        return(_biowait(bio, wmesg, to));
}

/*
 * This associates a tracking count with an I/O.  vn_strategy() and
 * dev_dstrategy() do this automatically but there are a few cases
 * where a vnode or device layer is bypassed when a block translation
 * is cached.  In such cases bio_start_transaction() may be called on
 * the bypassed layers so the system gets an I/O in progress indication 
 * for those higher layers.
 */
void
bio_start_transaction(struct bio *bio, struct bio_track *track)
{
        bio->bio_track = track;
        if (dsched_is_clear_buf_priv(bio->bio_buf))
                dsched_new_buf(bio->bio_buf);
        bio_track_ref(track);
}

/*
 * Initiate I/O on a vnode.
 *
 * SWAPCACHE OPERATION:
 *
 *      Real buffer cache buffers have a non-NULL bp->b_vp.  Unfortunately
 *      devfs also uses b_vp for fake buffers so we also have to check
 *      that B_PAGING is 0.  In this case the passed 'vp' is probably the
 *      underlying block device.  The swap assignments are related to the
 *      buffer cache buffer's b_vp, not the passed vp.
 *
 *      The passed vp == bp->b_vp only in the case where the strategy call
 *      is made on the vp itself for its own buffers (a regular file or
 *      block device vp).  The filesystem usually then re-calls vn_strategy()
 *      after translating the request to an underlying device.
 *
 *      Cluster buffers set B_CLUSTER and the passed vp is the vp of the
 *      underlying buffer cache buffers.
 *
 *      We can only deal with page-aligned buffers at the moment, because
 *      we can't tell what the real dirty state for pages straddling a buffer
 *      are.
 *
 *      In order to call swap_pager_strategy() we must provide the VM object
 *      and base offset for the underlying buffer cache pages so it can find
 *      the swap blocks.
 */
void
vn_strategy(struct vnode *vp, struct bio *bio)
{
        struct bio_track *track;
        struct buf *bp = bio->bio_buf;

        KKASSERT(bp->b_cmd != BUF_CMD_DONE);

        /*
         * Set when an I/O is issued on the bp.  Cleared by consumers
         * (aka HAMMER), allowing the consumer to determine if I/O had
         * actually occurred.
         */
        bp->b_flags |= B_IODEBUG;

        /*
         * Handle the swap cache intercept.
         */
        if (vn_cache_strategy(vp, bio))
                return;

        /*
         * Otherwise do the operation through the filesystem
         */
        if (bp->b_cmd == BUF_CMD_READ)
                track = &vp->v_track_read;
        else
                track = &vp->v_track_write;
        KKASSERT((bio->bio_flags & BIO_DONE) == 0);
        bio->bio_track = track;
        if (dsched_is_clear_buf_priv(bio->bio_buf))
                dsched_new_buf(bio->bio_buf);
        bio_track_ref(track);
        vop_strategy(*vp->v_ops, vp, bio);
}

static void vn_cache_strategy_callback(struct bio *bio);

int
vn_cache_strategy(struct vnode *vp, struct bio *bio)
{
        struct buf *bp = bio->bio_buf;
        struct bio *nbio;
        vm_object_t object;
        vm_page_t m;
        int i;

        /*
         * Is this buffer cache buffer suitable for reading from
         * the swap cache?
         */
        if (vm_swapcache_read_enable == 0 ||
            bp->b_cmd != BUF_CMD_READ ||
            ((bp->b_flags & B_CLUSTER) == 0 &&
             (bp->b_vp == NULL || (bp->b_flags & B_PAGING))) ||
            ((int)bp->b_loffset & PAGE_MASK) != 0 ||
            (bp->b_bcount & PAGE_MASK) != 0) {
                return(0);
        }

        /*
         * Figure out the original VM object (it will match the underlying
         * VM pages).  Note that swap cached data uses page indices relative
         * to that object, not relative to bio->bio_offset.
         */
        if (bp->b_flags & B_CLUSTER)
                object = vp->v_object;
        else
                object = bp->b_vp->v_object;

        /*
         * In order to be able to use the swap cache all underlying VM
         * pages must be marked as such, and we can't have any bogus pages.
         */
        for (i = 0; i < bp->b_xio.xio_npages; ++i) {
                m = bp->b_xio.xio_pages[i];
                if ((m->flags & PG_SWAPPED) == 0)
                        break;
                if (m == bogus_page)
                        break;
        }

        /*
         * If we are good then issue the I/O using swap_pager_strategy().
         *
         * We can only do this if the buffer actually supports object-backed
         * I/O.  If it doesn't npages will be 0.
         */
        if (i && i == bp->b_xio.xio_npages) {
                m = bp->b_xio.xio_pages[0];
                nbio = push_bio(bio);
                nbio->bio_done = vn_cache_strategy_callback;
                nbio->bio_offset = ptoa(m->pindex);
                KKASSERT(m->object == object);
                swap_pager_strategy(object, nbio);
                return(1);
        }
        return(0);
}

/*
 * This is a bit of a hack but since the vn_cache_strategy() function can
 * override a VFS's strategy function we must make sure that the bio, which
 * is probably bio2, doesn't leak an unexpected offset value back to the
 * filesystem.  The filesystem (e.g. UFS) might otherwise assume that the
 * bio went through its own file strategy function and the the bio2 offset
 * is a cached disk offset when, in fact, it isn't.
 */
static void
vn_cache_strategy_callback(struct bio *bio)
{
        bio->bio_offset = NOOFFSET;
        biodone(pop_bio(bio));
}

/*
 * bpdone:
 *
 *      Finish I/O on a buffer after all BIOs have been processed.
 *      Called when the bio chain is exhausted or by biowait.  If called
 *      by biowait, elseit is typically 0.
 *
 *      bpdone is also responsible for setting B_CACHE in a B_VMIO bp.
 *      In a non-VMIO bp, B_CACHE will be set on the next getblk() 
 *      assuming B_INVAL is clear.
 *
 *      For the VMIO case, we set B_CACHE if the op was a read and no
 *      read error occured, or if the op was a write.  B_CACHE is never
 *      set if the buffer is invalid or otherwise uncacheable.
 *
 *      bpdone does not mess with B_INVAL, allowing the I/O routine or the
 *      initiator to leave B_INVAL set to brelse the buffer out of existance
 *      in the biodone routine.
 */
void
bpdone(struct buf *bp, int elseit)
{
        buf_cmd_t cmd;

        KASSERT(BUF_REFCNTNB(bp) > 0, 
                ("biodone: bp %p not busy %d", bp, BUF_REFCNTNB(bp)));
        KASSERT(bp->b_cmd != BUF_CMD_DONE, 
                ("biodone: bp %p already done!", bp));

        /*
         * No more BIOs are left.  All completion functions have been dealt
         * with, now we clean up the buffer.
         */
        cmd = bp->b_cmd;
        bp->b_cmd = BUF_CMD_DONE;

        /*
         * Only reads and writes are processed past this point.
         */
        if (cmd != BUF_CMD_READ && cmd != BUF_CMD_WRITE) {
                if (cmd == BUF_CMD_FREEBLKS)
                        bp->b_flags |= B_NOCACHE;
                if (elseit)
                        brelse(bp);
                return;
        }

        /*
         * Warning: softupdates may re-dirty the buffer, and HAMMER can do
         * a lot worse.  XXX - move this above the clearing of b_cmd
         */
        if (LIST_FIRST(&bp->b_dep) != NULL)
                buf_complete(bp);       /* MPSAFE */

        /*
         * A failed write must re-dirty the buffer unless B_INVAL
         * was set.  Only applicable to normal buffers (with VPs).
         * vinum buffers may not have a vp.
         */
        if (cmd == BUF_CMD_WRITE &&
            (bp->b_flags & (B_ERROR | B_INVAL)) == B_ERROR) {
                bp->b_flags &= ~B_NOCACHE;
                if (bp->b_vp)
                        bdirty(bp);
        }

        if (bp->b_flags & B_VMIO) {
                int i;
                vm_ooffset_t foff;
                vm_page_t m;
                vm_object_t obj;
                int iosize;
                struct vnode *vp = bp->b_vp;

                obj = vp->v_object;

#if defined(VFS_BIO_DEBUG)
                if (vp->v_auxrefs == 0)
                        panic("biodone: zero vnode hold count");
                if ((vp->v_flag & VOBJBUF) == 0)
                        panic("biodone: vnode is not setup for merged cache");
#endif

                foff = bp->b_loffset;
                KASSERT(foff != NOOFFSET, ("biodone: no buffer offset"));
                KASSERT(obj != NULL, ("biodone: missing VM object"));

#if defined(VFS_BIO_DEBUG)
                if (obj->paging_in_progress < bp->b_xio.xio_npages) {
                        kprintf("biodone: paging in progress(%d) < "
                                "bp->b_xio.xio_npages(%d)\n",
                                obj->paging_in_progress,
                                bp->b_xio.xio_npages);
                }
#endif

                /*
                 * Set B_CACHE if the op was a normal read and no error
                 * occured.  B_CACHE is set for writes in the b*write()
                 * routines.
                 */
                iosize = bp->b_bcount - bp->b_resid;
                if (cmd == BUF_CMD_READ &&
                    (bp->b_flags & (B_INVAL|B_NOCACHE|B_ERROR)) == 0) {
                        bp->b_flags |= B_CACHE;
                }

                vm_object_hold(obj);
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        int bogusflag = 0;
                        int resid;

                        resid = ((foff + PAGE_SIZE) & ~(off_t)PAGE_MASK) - foff;
                        if (resid > iosize)
                                resid = iosize;

                        /*
                         * cleanup bogus pages, restoring the originals.  Since
                         * the originals should still be wired, we don't have
                         * to worry about interrupt/freeing races destroying
                         * the VM object association.
                         */
                        m = bp->b_xio.xio_pages[i];
                        if (m == bogus_page) {
                                bogusflag = 1;
                                m = vm_page_lookup(obj, OFF_TO_IDX(foff));
                                if (m == NULL)
                                        panic("biodone: page disappeared");
                                bp->b_xio.xio_pages[i] = m;
                                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                        bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                        }
#if defined(VFS_BIO_DEBUG)
                        if (OFF_TO_IDX(foff) != m->pindex) {
                                kprintf("biodone: foff(%lu)/m->pindex(%ld) "
                                        "mismatch\n",
                                        (unsigned long)foff, (long)m->pindex);
                        }
#endif

                        /*
                         * In the write case, the valid and clean bits are
                         * already changed correctly (see bdwrite()), so we
                         * only need to do this here in the read case.
                         */
                        vm_page_busy_wait(m, FALSE, "bpdpgw");
                        if (cmd == BUF_CMD_READ && !bogusflag && resid > 0) {
                                vfs_clean_one_page(bp, i, m);
                        }
                        vm_page_flag_clear(m, PG_ZERO);

                        /*
                         * when debugging new filesystems or buffer I/O
                         * methods, this is the most common error that pops
                         * up.  if you see this, you have not set the page
                         * busy flag correctly!!!
                         */
                        if (m->busy == 0) {
                                kprintf("biodone: page busy < 0, "
                                    "pindex: %d, foff: 0x(%x,%x), "
                                    "resid: %d, index: %d\n",
                                    (int) m->pindex, (int)(foff >> 32),
                                                (int) foff & 0xffffffff, resid, i);
                                if (!vn_isdisk(vp, NULL))
                                        kprintf(" iosize: %ld, loffset: %lld, "
                                                "flags: 0x%08x, npages: %d\n",
                                            bp->b_vp->v_mount->mnt_stat.f_iosize,
                                            (long long)bp->b_loffset,
                                            bp->b_flags, bp->b_xio.xio_npages);
                                else
                                        kprintf(" VDEV, loffset: %lld, flags: 0x%08x, npages: %d\n",
                                            (long long)bp->b_loffset,
                                            bp->b_flags, bp->b_xio.xio_npages);
                                kprintf(" valid: 0x%x, dirty: 0x%x, wired: %d\n",
                                    m->valid, m->dirty, m->wire_count);
                                panic("biodone: page busy < 0");
                        }
                        vm_page_io_finish(m);
                        vm_page_wakeup(m);
                        vm_object_pip_wakeup(obj);
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                        iosize -= resid;
                }
                bp->b_flags &= ~B_HASBOGUS;
                vm_object_drop(obj);
        }

        /*
         * Finish up by releasing the buffer.  There are no more synchronous
         * or asynchronous completions, those were handled by bio_done
         * callbacks.
         */
        if (elseit) {
                if (bp->b_flags & (B_NOCACHE|B_INVAL|B_ERROR|B_RELBUF))
                        brelse(bp);
                else
                        bqrelse(bp);
        }
}

/*
 * Normal biodone.
 */
void
biodone(struct bio *bio)
{
        struct buf *bp = bio->bio_buf;

        runningbufwakeup(bp);

        /*
         * Run up the chain of BIO's.   Leave b_cmd intact for the duration.
         */
        while (bio) {
                biodone_t *done_func;
                struct bio_track *track;

                /*
                 * BIO tracking.  Most but not all BIOs are tracked.
                 */
                if ((track = bio->bio_track) != NULL) {
                        bio_track_rel(track);
                        bio->bio_track = NULL;
                }

                /*
                 * A bio_done function terminates the loop.  The function
                 * will be responsible for any further chaining and/or
                 * buffer management.
                 *
                 * WARNING!  The done function can deallocate the buffer!
                 */
                if ((done_func = bio->bio_done) != NULL) {
                        bio->bio_done = NULL;
                        done_func(bio);
                        return;
                }
                bio = bio->bio_prev;
        }

        /*
         * If we've run out of bio's do normal [a]synchronous completion.
         */
        bpdone(bp, 1);
}

/*
 * Synchronous biodone - this terminates a synchronous BIO.
 *
 * bpdone() is called with elseit=FALSE, leaving the buffer completed
 * but still locked.  The caller must brelse() the buffer after waiting
 * for completion.
 */
void
biodone_sync(struct bio *bio)
{
        struct buf *bp = bio->bio_buf;
        int flags;
        int nflags;

        KKASSERT(bio == &bp->b_bio1);
        bpdone(bp, 0);

        for (;;) {
                flags = bio->bio_flags;
                nflags = (flags | BIO_DONE) & ~BIO_WANT;

                if (atomic_cmpset_int(&bio->bio_flags, flags, nflags)) {
                        if (flags & BIO_WANT)
                                wakeup(bio);
                        break;
                }
        }
}

/*
 * vfs_unbusy_pages:
 *
 *      This routine is called in lieu of iodone in the case of
 *      incomplete I/O.  This keeps the busy status for pages
 *      consistant.
 */
void
vfs_unbusy_pages(struct buf *bp)
{
        int i;

        runningbufwakeup(bp);

        if (bp->b_flags & B_VMIO) {
                struct vnode *vp = bp->b_vp;
                vm_object_t obj;

                obj = vp->v_object;
                vm_object_hold(obj);

                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];

                        /*
                         * When restoring bogus changes the original pages
                         * should still be wired, so we are in no danger of
                         * losing the object association and do not need
                         * critical section protection particularly.
                         */
                        if (m == bogus_page) {
                                m = vm_page_lookup(obj, OFF_TO_IDX(bp->b_loffset) + i);
                                if (!m) {
                                        panic("vfs_unbusy_pages: page missing");
                                }
                                bp->b_xio.xio_pages[i] = m;
                                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                        bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                        }
                        vm_page_busy_wait(m, FALSE, "bpdpgw");
                        vm_page_flag_clear(m, PG_ZERO);
                        vm_page_io_finish(m);
                        vm_page_wakeup(m);
                        vm_object_pip_wakeup(obj);
                }
                bp->b_flags &= ~B_HASBOGUS;
                vm_object_drop(obj);
        }
}

/*
 * vfs_busy_pages:
 *
 *      This routine is called before a device strategy routine.
 *      It is used to tell the VM system that paging I/O is in
 *      progress, and treat the pages associated with the buffer
 *      almost as being PG_BUSY.  Also the object 'paging_in_progress'
 *      flag is handled to make sure that the object doesn't become
 *      inconsistant.
 *
 *      Since I/O has not been initiated yet, certain buffer flags
 *      such as B_ERROR or B_INVAL may be in an inconsistant state
 *      and should be ignored.
 *
 * MPSAFE
 */
void
vfs_busy_pages(struct vnode *vp, struct buf *bp)
{
        int i, bogus;
        struct lwp *lp = curthread->td_lwp;

        /*
         * The buffer's I/O command must already be set.  If reading,
         * B_CACHE must be 0 (double check against callers only doing
         * I/O when B_CACHE is 0).
         */
        KKASSERT(bp->b_cmd != BUF_CMD_DONE);
        KKASSERT(bp->b_cmd == BUF_CMD_WRITE || (bp->b_flags & B_CACHE) == 0);

        if (bp->b_flags & B_VMIO) {
                vm_object_t obj;

                obj = vp->v_object;
                KASSERT(bp->b_loffset != NOOFFSET,
                        ("vfs_busy_pages: no buffer offset"));

                /*
                 * Busy all the pages.  We have to busy them all at once
                 * to avoid deadlocks.
                 */
retry:
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];

                        if (vm_page_busy_try(m, FALSE)) {
                                vm_page_sleep_busy(m, FALSE, "vbpage");
                                while (--i >= 0)
                                        vm_page_wakeup(bp->b_xio.xio_pages[i]);
                                goto retry;
                        }
                }

                /*
                 * Setup for I/O, soft-busy the page right now because
                 * the next loop may block.
                 */
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];

                        vm_page_flag_clear(m, PG_ZERO);
                        if ((bp->b_flags & B_CLUSTER) == 0) {
                                vm_object_pip_add(obj, 1);
                                vm_page_io_start(m);
                        }
                }

                /*
                 * Adjust protections for I/O and do bogus-page mapping.
                 * Assume that vm_page_protect() can block (it can block
                 * if VM_PROT_NONE, don't take any chances regardless).
                 *
                 * In particular note that for writes we must incorporate
                 * page dirtyness from the VM system into the buffer's
                 * dirty range.
                 *
                 * For reads we theoretically must incorporate page dirtyness
                 * from the VM system to determine if the page needs bogus
                 * replacement, but we shortcut the test by simply checking
                 * that all m->valid bits are set, indicating that the page
                 * is fully valid and does not need to be re-read.  For any
                 * VM system dirtyness the page will also be fully valid
                 * since it was mapped at one point.
                 */
                bogus = 0;
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];

                        vm_page_flag_clear(m, PG_ZERO); /* XXX */
                        if (bp->b_cmd == BUF_CMD_WRITE) {
                                /*
                                 * When readying a vnode-backed buffer for
                                 * a write we must zero-fill any invalid
                                 * portions of the backing VM pages, mark
                                 * it valid and clear related dirty bits.
                                 *
                                 * vfs_clean_one_page() incorporates any
                                 * VM dirtyness and updates the b_dirtyoff
                                 * range (after we've made the page RO).
                                 *
                                 * It is also expected that the pmap modified
                                 * bit has already been cleared by the
                                 * vm_page_protect().  We may not be able
                                 * to clear all dirty bits for a page if it
                                 * was also memory mapped (NFS).
                                 *
                                 * Finally be sure to unassign any swap-cache
                                 * backing store as it is now stale.
                                 */
                                vm_page_protect(m, VM_PROT_READ);
                                vfs_clean_one_page(bp, i, m);
                                swap_pager_unswapped(m);
                        } else if (m->valid == VM_PAGE_BITS_ALL) {
                                /*
                                 * When readying a vnode-backed buffer for
                                 * read we must replace any dirty pages with
                                 * a bogus page so dirty data is not destroyed
                                 * when filling gaps.
                                 *
                                 * To avoid testing whether the page is
                                 * dirty we instead test that the page was
                                 * at some point mapped (m->valid fully
                                 * valid) with the understanding that
                                 * this also covers the dirty case.
                                 */
                                bp->b_xio.xio_pages[i] = bogus_page;
                                bp->b_flags |= B_HASBOGUS;
                                bogus++;
                        } else if (m->valid & m->dirty) {
                                /*
                                 * This case should not occur as partial
                                 * dirtyment can only happen if the buffer
                                 * is B_CACHE, and this code is not entered
                                 * if the buffer is B_CACHE.
                                 */
                                kprintf("Warning: vfs_busy_pages - page not "
                                        "fully valid! loff=%jx bpf=%08x "
                                        "idx=%d val=%02x dir=%02x\n",
                                        (intmax_t)bp->b_loffset, bp->b_flags,
                                        i, m->valid, m->dirty);
                                vm_page_protect(m, VM_PROT_NONE);
                        } else {
                                /*
                                 * The page is not valid and can be made
                                 * part of the read.
                                 */
                                vm_page_protect(m, VM_PROT_NONE);
                        }
                        vm_page_wakeup(m);
                }
                if (bogus) {
                        pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                }
        }

        /*
         * This is the easiest place to put the process accounting for the I/O
         * for now.
         */
        if (lp != NULL) {
                if (bp->b_cmd == BUF_CMD_READ)
                        lp->lwp_ru.ru_inblock++;
                else
                        lp->lwp_ru.ru_oublock++;
        }
}

/*
 * Tell the VM system that the pages associated with this buffer
 * are clean.  This is used for delayed writes where the data is
 * going to go to disk eventually without additional VM intevention.
 *
 * NOTE: While we only really need to clean through to b_bcount, we
 *       just go ahead and clean through to b_bufsize.
 */
static void
vfs_clean_pages(struct buf *bp)
{
        vm_page_t m;
        int i;

        if ((bp->b_flags & B_VMIO) == 0)
                return;

        KASSERT(bp->b_loffset != NOOFFSET,
                ("vfs_clean_pages: no buffer offset"));

        for (i = 0; i < bp->b_xio.xio_npages; i++) {
                m = bp->b_xio.xio_pages[i];
                vfs_clean_one_page(bp, i, m);
        }
}

/*
 * vfs_clean_one_page:
 *
 *      Set the valid bits and clear the dirty bits in a page within a
 *      buffer.  The range is restricted to the buffer's size and the
 *      buffer's logical offset might index into the first page.
 *
 *      The caller has busied or soft-busied the page and it is not mapped,
 *      test and incorporate the dirty bits into b_dirtyoff/end before
 *      clearing them.  Note that we need to clear the pmap modified bits
 *      after determining the the page was dirty, vm_page_set_validclean()
 *      does not do it for us.
 *
 *      This routine is typically called after a read completes (dirty should
 *      be zero in that case as we are not called on bogus-replace pages),
 *      or before a write is initiated.
 */
static void
vfs_clean_one_page(struct buf *bp, int pageno, vm_page_t m)
{
        int bcount;
        int xoff;
        int soff;
        int eoff;

        /*
         * Calculate offset range within the page but relative to buffer's
         * loffset.  loffset might be offset into the first page.
         */
        xoff = (int)bp->b_loffset & PAGE_MASK;  /* loffset offset into pg 0 */
        bcount = bp->b_bcount + xoff;           /* offset adjusted */

        if (pageno == 0) {
                soff = xoff;
                eoff = PAGE_SIZE;
        } else {
                soff = (pageno << PAGE_SHIFT);
                eoff = soff + PAGE_SIZE;
        }
        if (eoff > bcount)
                eoff = bcount;
        if (soff >= eoff)
                return;

        /*
         * Test dirty bits and adjust b_dirtyoff/end.
         *
         * If dirty pages are incorporated into the bp any prior
         * B_NEEDCOMMIT state (NFS) must be cleared because the
         * caller has not taken into account the new dirty data.
         *
         * If the page was memory mapped the dirty bits might go beyond the
         * end of the buffer, but we can't really make the assumption that
         * a file EOF straddles the buffer (even though this is the case for
         * NFS if B_NEEDCOMMIT is also set).  So for the purposes of clearing
         * B_NEEDCOMMIT we only test the dirty bits covered by the buffer.
         * This also saves some console spam.
         *
         * When clearing B_NEEDCOMMIT we must also clear B_CLUSTEROK,
         * NFS can handle huge commits but not huge writes.
         */
        vm_page_test_dirty(m);
        if (m->dirty) {
                if ((bp->b_flags & B_NEEDCOMMIT) &&
                    (m->dirty & vm_page_bits(soff & PAGE_MASK, eoff - soff))) {
                        if (debug_commit)
                        kprintf("Warning: vfs_clean_one_page: bp %p "
                                "loff=%jx,%d flgs=%08x clr B_NEEDCOMMIT"
                                " cmd %d vd %02x/%02x x/s/e %d %d %d "
                                "doff/end %d %d\n",
                                bp, (intmax_t)bp->b_loffset, bp->b_bcount,
                                bp->b_flags, bp->b_cmd,
                                m->valid, m->dirty, xoff, soff, eoff,
                                bp->b_dirtyoff, bp->b_dirtyend);
                        bp->b_flags &= ~(B_NEEDCOMMIT | B_CLUSTEROK);
                        if (debug_commit)
                                print_backtrace(-1);
                }
                /*
                 * Only clear the pmap modified bits if ALL the dirty bits
                 * are set, otherwise the system might mis-clear portions
                 * of a page.
                 */
                if (m->dirty == VM_PAGE_BITS_ALL &&
                    (bp->b_flags & B_NEEDCOMMIT) == 0) {
                        pmap_clear_modify(m);
                }
                if (bp->b_dirtyoff > soff - xoff)
                        bp->b_dirtyoff = soff - xoff;
                if (bp->b_dirtyend < eoff - xoff)
                        bp->b_dirtyend = eoff - xoff;
        }

        /*
         * Set related valid bits, clear related dirty bits.
         * Does not mess with the pmap modified bit.
         *
         * WARNING!  We cannot just clear all of m->dirty here as the
         *           buffer cache buffers may use a DEV_BSIZE'd aligned
         *           block size, or have an odd size (e.g. NFS at file EOF).
         *           The putpages code can clear m->dirty to 0.
         *
         *           If a VOP_WRITE generates a buffer cache buffer which
         *           covers the same space as mapped writable pages the
         *           buffer flush might not be able to clear all the dirty
         *           bits and still require a putpages from the VM system
         *           to finish it off.
         *
         * WARNING!  vm_page_set_validclean() currently assumes vm_token
         *           is held.  The page might not be busied (bdwrite() case).
         *           XXX remove this comment once we've validated that this
         *           is no longer an issue.
         */
        vm_page_set_validclean(m, soff & PAGE_MASK, eoff - soff);
}

/*
 * Similar to vfs_clean_one_page() but sets the bits to valid and dirty.
 * The page data is assumed to be valid (there is no zeroing here).
 */
static void
vfs_dirty_one_page(struct buf *bp, int pageno, vm_page_t m)
{
        int bcount;
        int xoff;
        int soff;
        int eoff;

        /*
         * Calculate offset range within the page but relative to buffer's
         * loffset.  loffset might be offset into the first page.
         */
        xoff = (int)bp->b_loffset & PAGE_MASK;  /* loffset offset into pg 0 */
        bcount = bp->b_bcount + xoff;           /* offset adjusted */

        if (pageno == 0) {
                soff = xoff;
                eoff = PAGE_SIZE;
        } else {
                soff = (pageno << PAGE_SHIFT);
                eoff = soff + PAGE_SIZE;
        }
        if (eoff > bcount)
                eoff = bcount;
        if (soff >= eoff)
                return;
        vm_page_set_validdirty(m, soff & PAGE_MASK, eoff - soff);
}

/*
 * vfs_bio_clrbuf:
 *
 *      Clear a buffer.  This routine essentially fakes an I/O, so we need
 *      to clear B_ERROR and B_INVAL.
 *
 *      Note that while we only theoretically need to clear through b_bcount,
 *      we go ahead and clear through b_bufsize.
 */

void
vfs_bio_clrbuf(struct buf *bp)
{
        int i, mask = 0;
        caddr_t sa, ea;
        if ((bp->b_flags & (B_VMIO | B_MALLOC)) == B_VMIO) {
                bp->b_flags &= ~(B_INVAL | B_EINTR | B_ERROR);
                if ((bp->b_xio.xio_npages == 1) && (bp->b_bufsize < PAGE_SIZE) &&
                    (bp->b_loffset & PAGE_MASK) == 0) {
                        mask = (1 << (bp->b_bufsize / DEV_BSIZE)) - 1;
                        if ((bp->b_xio.xio_pages[0]->valid & mask) == mask) {
                                bp->b_resid = 0;
                                return;
                        }
                        if (((bp->b_xio.xio_pages[0]->flags & PG_ZERO) == 0) &&
                            ((bp->b_xio.xio_pages[0]->valid & mask) == 0)) {
                                bzero(bp->b_data, bp->b_bufsize);
                                bp->b_xio.xio_pages[0]->valid |= mask;
                                bp->b_resid = 0;
                                return;
                        }
                }
                sa = bp->b_data;
                for(i=0;i<bp->b_xio.xio_npages;i++,sa=ea) {
                        int j = ((vm_offset_t)sa & PAGE_MASK) / DEV_BSIZE;
                        ea = (caddr_t)trunc_page((vm_offset_t)sa + PAGE_SIZE);
                        ea = (caddr_t)(vm_offset_t)ulmin(
                            (u_long)(vm_offset_t)ea,
                            (u_long)(vm_offset_t)bp->b_data + bp->b_bufsize);
                        mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                        if ((bp->b_xio.xio_pages[i]->valid & mask) == mask)
                                continue;
                        if ((bp->b_xio.xio_pages[i]->valid & mask) == 0) {
                                if ((bp->b_xio.xio_pages[i]->flags & PG_ZERO) == 0) {
                                        bzero(sa, ea - sa);
                                }
                        } else {
                                for (; sa < ea; sa += DEV_BSIZE, j++) {
                                        if (((bp->b_xio.xio_pages[i]->flags & PG_ZERO) == 0) &&
                                                (bp->b_xio.xio_pages[i]->valid & (1<<j)) == 0)
                                                bzero(sa, DEV_BSIZE);
                                }
                        }
                        bp->b_xio.xio_pages[i]->valid |= mask;
                        vm_page_flag_clear(bp->b_xio.xio_pages[i], PG_ZERO);
                }
                bp->b_resid = 0;
        } else {
                clrbuf(bp);
        }
}

/*
 * vm_hold_load_pages:
 *
 *      Load pages into the buffer's address space.  The pages are
 *      allocated from the kernel object in order to reduce interference
 *      with the any VM paging I/O activity.  The range of loaded
 *      pages will be wired.
 *
 *      If a page cannot be allocated, the 'pagedaemon' is woken up to
 *      retrieve the full range (to - from) of pages.
 *
 * MPSAFE
 */
void
vm_hold_load_pages(struct buf *bp, vm_offset_t from, vm_offset_t to)
{
        vm_offset_t pg;
        vm_page_t p;
        int index;

        to = round_page(to);
        from = round_page(from);
        index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;

        pg = from;
        while (pg < to) {
                /*
                 * Note: must allocate system pages since blocking here
                 * could intefere with paging I/O, no matter which
                 * process we are.
                 */
                vm_object_hold(&kernel_object);
                p = bio_page_alloc(&kernel_object, pg >> PAGE_SHIFT,
                                   (vm_pindex_t)((to - pg) >> PAGE_SHIFT));
                vm_object_drop(&kernel_object);
                if (p) {
                        vm_page_wire(p);
                        p->valid = VM_PAGE_BITS_ALL;
                        vm_page_flag_clear(p, PG_ZERO);
                        pmap_kenter(pg, VM_PAGE_TO_PHYS(p));
                        bp->b_xio.xio_pages[index] = p;
                        vm_page_wakeup(p);

                        pg += PAGE_SIZE;
                        ++index;
                }
        }
        bp->b_xio.xio_npages = index;
}

/*
 * Allocate pages for a buffer cache buffer.
 *
 * Under extremely severe memory conditions even allocating out of the
 * system reserve can fail.  If this occurs we must allocate out of the
 * interrupt reserve to avoid a deadlock with the pageout daemon.
 *
 * The pageout daemon can run (putpages -> VOP_WRITE -> getblk -> allocbuf).
 * If the buffer cache's vm_page_alloc() fails a vm_wait() can deadlock
 * against the pageout daemon if pages are not freed from other sources.
 *
 * MPSAFE
 */
static
vm_page_t
bio_page_alloc(vm_object_t obj, vm_pindex_t pg, int deficit)
{
        vm_page_t p;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(obj));

        /*
         * Try a normal allocation, allow use of system reserve.
         */
        p = vm_page_alloc(obj, pg, VM_ALLOC_NORMAL | VM_ALLOC_SYSTEM);
        if (p)
                return(p);

        /*
         * The normal allocation failed and we clearly have a page
         * deficit.  Try to reclaim some clean VM pages directly
         * from the buffer cache.
         */
        vm_pageout_deficit += deficit;
        recoverbufpages();

        /*
         * We may have blocked, the caller will know what to do if the
         * page now exists.
         */
        if (vm_page_lookup(obj, pg)) {
                return(NULL);
        }

        /*
         * Only system threads can use the interrupt reserve
         */
        if ((curthread->td_flags & TDF_SYSTHREAD) == 0) {
                vm_wait(hz);
                return(NULL);
        }


        /*
         * Allocate and allow use of the interrupt reserve.
         *
         * If after all that we still can't allocate a VM page we are
         * in real trouble, but we slog on anyway hoping that the system
         * won't deadlock.
         */
        p = vm_page_alloc(obj, pg, VM_ALLOC_NORMAL | VM_ALLOC_SYSTEM |
                                   VM_ALLOC_INTERRUPT);
        if (p) {
                if (vm_page_count_severe()) {
                        ++lowmempgallocs;
                        vm_wait(hz / 20 + 1);
                }
        } else {
                kprintf("bio_page_alloc: Memory exhausted during bufcache "
                        "page allocation\n");
                ++lowmempgfails;
                vm_wait(hz);
        }
        return(p);
}

/*
 * vm_hold_free_pages:
 *
 *      Return pages associated with the buffer back to the VM system.
 *
 *      The range of pages underlying the buffer's address space will
 *      be unmapped and un-wired.
 *
 * MPSAFE
 */
void
vm_hold_free_pages(struct buf *bp, vm_offset_t from, vm_offset_t to)
{
        vm_offset_t pg;
        vm_page_t p;
        int index, newnpages;

        from = round_page(from);
        to = round_page(to);
        index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
        newnpages = index;

        for (pg = from; pg < to; pg += PAGE_SIZE, index++) {
                p = bp->b_xio.xio_pages[index];
                if (p && (index < bp->b_xio.xio_npages)) {
                        if (p->busy) {
                                kprintf("vm_hold_free_pages: doffset: %lld, "
                                        "loffset: %lld\n",
                                        (long long)bp->b_bio2.bio_offset,
                                        (long long)bp->b_loffset);
                        }
                        bp->b_xio.xio_pages[index] = NULL;
                        pmap_kremove(pg);
                        vm_page_busy_wait(p, FALSE, "vmhldpg");
                        vm_page_unwire(p, 0);
                        vm_page_free(p);
                }
        }
        bp->b_xio.xio_npages = newnpages;
}

/*
 * vmapbuf:
 *
 *      Map a user buffer into KVM via a pbuf.  On return the buffer's
 *      b_data, b_bufsize, and b_bcount will be set, and its XIO page array
 *      initialized.
 */
int
vmapbuf(struct buf *bp, caddr_t udata, int bytes)
{
        caddr_t addr;
        vm_offset_t va;
        vm_page_t m;
        int vmprot;
        int error;
        int pidx;
        int i;

        /* 
         * bp had better have a command and it better be a pbuf.
         */
        KKASSERT(bp->b_cmd != BUF_CMD_DONE);
        KKASSERT(bp->b_flags & B_PAGING);
        KKASSERT(bp->b_kvabase);

        if (bytes < 0)
                return (-1);

        /*
         * Map the user data into KVM.  Mappings have to be page-aligned.
         */
        addr = (caddr_t)trunc_page((vm_offset_t)udata);
        pidx = 0;

        vmprot = VM_PROT_READ;
        if (bp->b_cmd == BUF_CMD_READ)
                vmprot |= VM_PROT_WRITE;

        while (addr < udata + bytes) {
                /*
                 * Do the vm_fault if needed; do the copy-on-write thing
                 * when reading stuff off device into memory.
                 *
                 * vm_fault_page*() returns a held VM page.
                 */
                va = (addr >= udata) ? (vm_offset_t)addr : (vm_offset_t)udata;
                va = trunc_page(va);

                m = vm_fault_page_quick(va, vmprot, &error);
                if (m == NULL) {
                        for (i = 0; i < pidx; ++i) {
                            vm_page_unhold(bp->b_xio.xio_pages[i]);
                            bp->b_xio.xio_pages[i] = NULL;
                        }
                        return(-1);
                }
                bp->b_xio.xio_pages[pidx] = m;
                addr += PAGE_SIZE;
                ++pidx;
        }

        /*
         * Map the page array and set the buffer fields to point to
         * the mapped data buffer.
         */
        if (pidx > btoc(MAXPHYS))
                panic("vmapbuf: mapped more than MAXPHYS");
        pmap_qenter((vm_offset_t)bp->b_kvabase, bp->b_xio.xio_pages, pidx);

        bp->b_xio.xio_npages = pidx;
        bp->b_data = bp->b_kvabase + ((int)(intptr_t)udata & PAGE_MASK);
        bp->b_bcount = bytes;
        bp->b_bufsize = bytes;
        return(0);
}

/*
 * vunmapbuf:
 *
 *      Free the io map PTEs associated with this IO operation.
 *      We also invalidate the TLB entries and restore the original b_addr.
 */
void
vunmapbuf(struct buf *bp)
{
        int pidx;
        int npages;

        KKASSERT(bp->b_flags & B_PAGING);

        npages = bp->b_xio.xio_npages;
        pmap_qremove(trunc_page((vm_offset_t)bp->b_data), npages);
        for (pidx = 0; pidx < npages; ++pidx) {
                vm_page_unhold(bp->b_xio.xio_pages[pidx]);
                bp->b_xio.xio_pages[pidx] = NULL;
        }
        bp->b_xio.xio_npages = 0;
        bp->b_data = bp->b_kvabase;
}

/*
 * Scan all buffers in the system and issue the callback.
 */
int
scan_all_buffers(int (*callback)(struct buf *, void *), void *info)
{
        int count = 0;
        int error;
        int n;

        for (n = 0; n < nbuf; ++n) {
                if ((error = callback(&buf[n], info)) < 0) {
                        count = error;
                        break;
                }
                count += error;
        }
        return (count);
}

/*
 * nestiobuf_iodone: biodone callback for nested buffers and propagate
 * completion to the master buffer.
 */
static void
nestiobuf_iodone(struct bio *bio)
{
        struct bio *mbio;
        struct buf *mbp, *bp;
        struct devstat *stats;
        int error;
        int donebytes;

        bp = bio->bio_buf;
        mbio = bio->bio_caller_info1.ptr;
        stats = bio->bio_caller_info2.ptr;
        mbp = mbio->bio_buf;

        KKASSERT(bp->b_bcount <= bp->b_bufsize);
        KKASSERT(mbp != bp);

        error = bp->b_error;
        if (bp->b_error == 0 &&
            (bp->b_bcount < bp->b_bufsize || bp->b_resid > 0)) {
                /*
                 * Not all got transfered, raise an error. We have no way to
                 * propagate these conditions to mbp.
                 */
                error = EIO;
        }

        donebytes = bp->b_bufsize;

        relpbuf(bp, NULL);

        nestiobuf_done(mbio, donebytes, error, stats);
}

void
nestiobuf_done(struct bio *mbio, int donebytes, int error, struct devstat *stats)
{
        struct buf *mbp;

        mbp = mbio->bio_buf;    

        KKASSERT((int)(intptr_t)mbio->bio_driver_info > 0);

        /*
         * If an error occured, propagate it to the master buffer.
         *
         * Several biodone()s may wind up running concurrently so
         * use an atomic op to adjust b_flags.
         */
        if (error) {
                mbp->b_error = error;
                atomic_set_int(&mbp->b_flags, B_ERROR);
        }

        /*
         * Decrement the operations in progress counter and terminate the
         * I/O if this was the last bit.
         */
        if (atomic_fetchadd_int((int *)&mbio->bio_driver_info, -1) == 1) {
                mbp->b_resid = 0;
                if (stats)
                        devstat_end_transaction_buf(stats, mbp);
                biodone(mbio);
        }
}

/*
 * Initialize a nestiobuf for use.  Set an initial count of 1 to prevent
 * the mbio from being biodone()'d while we are still adding sub-bios to
 * it.
 */
void
nestiobuf_init(struct bio *bio)
{
        bio->bio_driver_info = (void *)1;
}

/*
 * The BIOs added to the nestedio have already been started, remove the
 * count that placeheld our mbio and biodone() it if the count would
 * transition to 0.
 */
void
nestiobuf_start(struct bio *mbio)
{
        struct buf *mbp = mbio->bio_buf;

        /*
         * Decrement the operations in progress counter and terminate the
         * I/O if this was the last bit.
         */
        if (atomic_fetchadd_int((int *)&mbio->bio_driver_info, -1) == 1) {
                if (mbp->b_flags & B_ERROR)
                        mbp->b_resid = mbp->b_bcount;
                else
                        mbp->b_resid = 0;
                biodone(mbio);
        }
}

/*
 * Set an intermediate error prior to calling nestiobuf_start()
 */
void
nestiobuf_error(struct bio *mbio, int error)
{
        struct buf *mbp = mbio->bio_buf;

        if (error) {
                mbp->b_error = error;
                atomic_set_int(&mbp->b_flags, B_ERROR);
        }
}

/*
 * nestiobuf_add: setup a "nested" buffer.
 *
 * => 'mbp' is a "master" buffer which is being divided into sub pieces.
 * => 'bp' should be a buffer allocated by getiobuf.
 * => 'offset' is a byte offset in the master buffer.
 * => 'size' is a size in bytes of this nested buffer.
 */
void
nestiobuf_add(struct bio *mbio, struct buf *bp, int offset, size_t size, struct devstat *stats)
{
        struct buf *mbp = mbio->bio_buf;
        struct vnode *vp = mbp->b_vp;

        KKASSERT(mbp->b_bcount >= offset + size);

        atomic_add_int((int *)&mbio->bio_driver_info, 1);

        /* kernel needs to own the lock for it to be released in biodone */
        BUF_KERNPROC(bp);
        bp->b_vp = vp;
        bp->b_cmd = mbp->b_cmd;
        bp->b_bio1.bio_done = nestiobuf_iodone;
        bp->b_data = (char *)mbp->b_data + offset;
        bp->b_resid = bp->b_bcount = size;
        bp->b_bufsize = bp->b_bcount;

        bp->b_bio1.bio_track = NULL;
        bp->b_bio1.bio_caller_info1.ptr = mbio;
        bp->b_bio1.bio_caller_info2.ptr = stats;
}

/*
 * print out statistics from the current status of the buffer pool
 * this can be toggeled by the system control option debug.syncprt
 */
#ifdef DEBUG
void
vfs_bufstats(void)
{
        int i, j, count;
        struct buf *bp;
        struct bqueues *dp;
        int counts[(MAXBSIZE / PAGE_SIZE) + 1];
        static char *bname[3] = { "LOCKED", "LRU", "AGE" };

        for (dp = bufqueues, i = 0; dp < &bufqueues[3]; dp++, i++) {
                count = 0;
                for (j = 0; j <= MAXBSIZE/PAGE_SIZE; j++)
                        counts[j] = 0;

                spin_lock(&bufqspin);
                TAILQ_FOREACH(bp, dp, b_freelist) {
                        counts[bp->b_bufsize/PAGE_SIZE]++;
                        count++;
                }
                spin_unlock(&bufqspin);

                kprintf("%s: total-%d", bname[i], count);
                for (j = 0; j <= MAXBSIZE/PAGE_SIZE; j++)
                        if (counts[j] != 0)
                                kprintf(", %d-%d", j * PAGE_SIZE, counts[j]);
                kprintf("\n");
        }
}
#endif

#ifdef DDB

DB_SHOW_COMMAND(buffer, db_show_buffer)
{
        /* get args */
        struct buf *bp = (struct buf *)addr;

        if (!have_addr) {
                db_printf("usage: show buffer <addr>\n");
                return;
        }

        db_printf("b_flags = 0x%b\n", (u_int)bp->b_flags, PRINT_BUF_FLAGS);
        db_printf("b_cmd = %d\n", bp->b_cmd);
        db_printf("b_error = %d, b_bufsize = %d, b_bcount = %d, "
                  "b_resid = %d\n, b_data = %p, "
                  "bio_offset(disk) = %lld, bio_offset(phys) = %lld\n",
                  bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
                  bp->b_data,
                  (long long)bp->b_bio2.bio_offset,
                  (long long)(bp->b_bio2.bio_next ?
                                bp->b_bio2.bio_next->bio_offset : (off_t)-1));
        if (bp->b_xio.xio_npages) {
                int i;
                db_printf("b_xio.xio_npages = %d, pages(OBJ, IDX, PA): ",
                        bp->b_xio.xio_npages);
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m;
                        m = bp->b_xio.xio_pages[i];
                        db_printf("(%p, 0x%lx, 0x%lx)", (void *)m->object,
                            (u_long)m->pindex, (u_long)VM_PAGE_TO_PHYS(m));
                        if ((i + 1) < bp->b_xio.xio_npages)
                                db_printf(",");
                }
                db_printf("\n");
        }
}
#endif /* DDB */