[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 $
 * $DragonFly: src/sys/kern/vfs_bio.c,v 1.76 2006/05/25 19:31:13 dillon 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/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/proc.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 <sys/buf2.h>
#include <sys/thread2.h>
#include <vm/vm_page2.h>

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

/*
 * Buffer queues.
 */
#define BUFFER_QUEUES   6
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_EMPTYKVA,        /* empty buffer headers with KVA assignment */
        BQUEUE_EMPTY            /* empty buffer headers */
};
TAILQ_HEAD(bqueues, buf) bufqueues[BUFFER_QUEUES];

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

struct  bio_ops bioops;         /* I/O operation notification */

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

static void vm_hold_free_pages(struct buf *bp, vm_offset_t from,
                vm_offset_t to);
static void vm_hold_load_pages(struct buf *bp, vm_offset_t from,
                vm_offset_t to);
static void vfs_page_set_valid(struct buf *bp, vm_ooffset_t off,
                               int pageno, vm_page_t m);
static void vfs_clean_pages(struct buf *bp);
static void vfs_setdirty(struct buf *bp);
static void vfs_vmio_release(struct buf *bp);
static int flushbufqueues(void);

static int bd_request;

static void buf_daemon (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;
int runningbufspace;

static int bufspace, maxbufspace,
        bufmallocspace, maxbufmallocspace, lobufspace, hibufspace;
static int bufreusecnt, bufdefragcnt, buffreekvacnt;
static int needsbuffer;
static int lorunningspace, hirunningspace, runningbufreq;
static int numdirtybuffers, lodirtybuffers, hidirtybuffers;
static int numfreebuffers, lofreebuffers, hifreebuffers;
static int getnewbufcalls;
static int getnewbufrestarts;

/*
 * Sysctls for operational control of the buffer cache.
 */
SYSCTL_INT(_vfs, OID_AUTO, lodirtybuffers, CTLFLAG_RW, &lodirtybuffers, 0,
        "Number of dirty buffers to flush before bufdaemon becomes inactive");
SYSCTL_INT(_vfs, OID_AUTO, hidirtybuffers, CTLFLAG_RW, &hidirtybuffers, 0,
        "High watermark used to trigger explicit flushing of dirty buffers");
SYSCTL_INT(_vfs, OID_AUTO, lofreebuffers, CTLFLAG_RW, &lofreebuffers, 0,
        "Low watermark for special reserve in low-memory situations");
SYSCTL_INT(_vfs, OID_AUTO, hifreebuffers, CTLFLAG_RW, &hifreebuffers, 0,
        "High watermark for special reserve in low-memory situations");
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");
/*
 * Sysctls determining current state of the buffer cache.
 */
SYSCTL_INT(_vfs, OID_AUTO, numdirtybuffers, CTLFLAG_RD, &numdirtybuffers, 0,
        "Pending number of dirty buffers");
SYSCTL_INT(_vfs, OID_AUTO, numfreebuffers, CTLFLAG_RD, &numfreebuffers, 0,
        "Number of free buffers on the buffer cache free list");
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, 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, 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(_debug_sizeof, OID_AUTO, buf, CTLFLAG_RD, 0, sizeof(struct buf),
        "sizeof(struct buf)");

char *buf_wmesg = BUF_WMESG;

extern int vm_swap_size;

#define VFS_BIO_NEED_ANY        0x01    /* any freeable buffer */
#define VFS_BIO_NEED_DIRTYFLUSH 0x02    /* waiting for dirty buffer flush */
#define VFS_BIO_NEED_FREE       0x04    /* wait for free bufs, hi hysteresis */
#define VFS_BIO_NEED_BUFSPACE   0x08    /* wait for buf space, lo hysteresis */

/*
 * numdirtywakeup:
 *
 *      If someone is blocked due to there being too many dirty buffers,
 *      and numdirtybuffers is now reasonable, wake them up.
 */

static __inline void
numdirtywakeup(int level)
{
        if (numdirtybuffers <= level) {
                if (needsbuffer & VFS_BIO_NEED_DIRTYFLUSH) {
                        needsbuffer &= ~VFS_BIO_NEED_DIRTYFLUSH;
                        wakeup(&needsbuffer);
                }
        }
}

/*
 * 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.
         */
        if (needsbuffer & VFS_BIO_NEED_BUFSPACE) {
                needsbuffer &= ~VFS_BIO_NEED_BUFSPACE;
                wakeup(&needsbuffer);
        }
}

/*
 * runningbufwakeup:
 *
 *      Accounting for I/O in progress.
 *
 */
static __inline void
runningbufwakeup(struct buf *bp)
{
        if (bp->b_runningbufspace) {
                runningbufspace -= bp->b_runningbufspace;
                bp->b_runningbufspace = 0;
                if (runningbufreq && runningbufspace <= lorunningspace) {
                        runningbufreq = 0;
                        wakeup(&runningbufreq);
                }
        }
}

/*
 * 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 ).
 */

static __inline void
bufcountwakeup(void) 
{
        ++numfreebuffers;
        if (needsbuffer) {
                needsbuffer &= ~VFS_BIO_NEED_ANY;
                if (numfreebuffers >= hifreebuffers)
                        needsbuffer &= ~VFS_BIO_NEED_FREE;
                wakeup(&needsbuffer);
        }
}

/*
 * waitrunningbufspace()
 *
 *      runningbufspace is a measure of the amount of I/O currently
 *      running.  This routine is used in async-write situations to
 *      prevent creating huge backups of pending writes to a device.
 *      Only asynchronous writes are governed by this function.  
 *
 *      Reads will adjust runningbufspace, but will not block based on it.
 *      The read load has a side effect of reducing the allowed write load.
 *
 *      This does NOT turn an async write into a sync write.  It waits
 *      for earlier writes to complete and generally returns before the
 *      caller's write has reached the device.
 */
static __inline void
waitrunningbufspace(void)
{
        if (runningbufspace > hirunningspace) {
                crit_enter();
                while (runningbufspace > hirunningspace) {
                        ++runningbufreq;
                        tsleep(&runningbufreq, 0, "wdrain", 0);
                }
                crit_exit();
        }
}

/*
 * 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.
 */
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_wakeup:
 *
 *      Wake up the buffer daemon if the number of outstanding dirty buffers
 *      is above specified threshold 'dirtybuflevel'.
 *
 *      The buffer daemon is explicitly woken up when (a) the pending number
 *      of dirty buffers exceeds the recovery and stall mid-point value,
 *      (b) during bwillwrite() or (c) buf freelist was exhausted.
 */
static __inline__
void
bd_wakeup(int dirtybuflevel)
{
        if (bd_request == 0 && numdirtybuffers >= dirtybuflevel) {
                bd_request = 1;
                wakeup(&bd_request);
        }
}

/*
 * bd_speedup:
 *
 *      Speed up the buffer cache flushing process.
 */

static __inline__
void
bd_speedup(void)
{
        bd_wakeup(1);
}

/*
 * 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);
                LIST_INIT(&bp->b_dep);
                BUF_LOCKINIT(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 = 1024 * 1024;

/*
 * 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.
 */
        hidirtybuffers = nbuf / 4 + 20;
        numdirtybuffers = 0;
/*
 * To support extreme low-memory systems, make sure hidirtybuffers cannot
 * eat up all available buffer space.  This occurs when our minimum cannot
 * be met.  We try to size hidirtybuffers to 3/4 our buffer space assuming
 * BKVASIZE'd (8K) buffers.
 */
        while (hidirtybuffers * BKVASIZE > 3 * hibufspace / 4) {
                hidirtybuffers >>= 1;
        }
        lodirtybuffers = hidirtybuffers / 2;

/*
 * Try to keep the number of free buffers in the specified range,
 * and give special processes (e.g. like buf_daemon) access to an 
 * emergency reserve.
 */
        lofreebuffers = nbuf / 18 + 5;
        hifreebuffers = 2 * lofreebuffers;
        numfreebuffers = nbuf;

/*
 * 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);
        bogus_page = vm_page_alloc(kernel_object,
                        ((bogus_offset - VM_MIN_KERNEL_ADDRESS) >> PAGE_SHIFT),
                        VM_ALLOC_NORMAL);
        vmstats.v_wire_count++;

}

/*
 * Initialize the embedded bio structures
 */
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_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;
}

/*
 * 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;
        }
}

/*
 * 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) {
                        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);
}

void
pop_bio(struct bio *bio)
{
        /* NOP */
}

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().
 */
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;
                bufspacewakeup();
        }
}

/*
 * bremfree:
 *
 *      Remove the buffer from the appropriate free list.
 */
void
bremfree(struct buf *bp)
{
        int old_qindex;

        crit_enter();
        old_qindex = bp->b_qindex;

        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");
        }

        /*
         * Fixup numfreebuffers count.  If the buffer is invalid or not
         * delayed-write, and it was on the EMPTY, LRU, or AGE queues,
         * the buffer was free and we must decrement numfreebuffers.
         */
        if ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0) {
                switch(old_qindex) {
                case BQUEUE_DIRTY:
                case BQUEUE_CLEAN:
                case BQUEUE_EMPTY:
                case BQUEUE_EMPTYKVA:
                        --numfreebuffers;
                        break;
                default:
                        break;
                }
        }
        crit_exit();
}


/*
 * 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)
{
        struct buf *bp;

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

        /* if not found in cache, do some I/O */
        if ((bp->b_flags & B_CACHE) == 0) {
                KASSERT(!(bp->b_flags & B_ASYNC), ("bread: illegal async bp %p", bp));
                bp->b_flags &= ~(B_ERROR | B_INVAL);
                bp->b_cmd = BUF_CMD_READ;
                vfs_busy_pages(vp, bp);
                vn_strategy(vp, &bp->b_bio1);
                return (biowait(bp));
        }
        return (0);
}

/*
 * 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_INVAL);
                bp->b_cmd = BUF_CMD_READ;
                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_ASYNC;
                        rabp->b_flags &= ~(B_ERROR | 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);
        }
        return (rv);
}

/*
 * bwrite:
 *
 *      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 oldflags;

        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return (0);
        }

        oldflags = bp->b_flags;

        if (BUF_REFCNTNB(bp) == 0)
                panic("bwrite: buffer is not busy???");
        crit_enter();

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

        bp->b_flags &= ~B_ERROR;
        bp->b_flags |= B_CACHE;
        bp->b_cmd = BUF_CMD_WRITE;
        vfs_busy_pages(bp->b_vp, bp);

        /*
         * Normal bwrites pipeline writes.  NOTE: b_bufsize is only
         * valid for vnode-backed buffers.
         */
        bp->b_runningbufspace = bp->b_bufsize;
        runningbufspace += bp->b_runningbufspace;

        crit_exit();
        if (oldflags & B_ASYNC)
                BUF_KERNPROC(bp);
        vn_strategy(bp->b_vp, &bp->b_bio1);

        if ((oldflags & B_ASYNC) == 0) {
                int rtval = biowait(bp);
                brelse(bp);
                return (rtval);
        } else if ((oldflags & B_NOWDRAIN) == 0) {
                /*
                 * don't allow the async write to saturate the I/O
                 * system.  Deadlocks can occur only if a device strategy
                 * routine (like in VN) turns around and issues another
                 * high-level write, in which case B_NOWDRAIN is expected
                 * to be set.   Otherwise we will not deadlock here because
                 * we are blocking waiting for I/O that is already in-progress
                 * to complete.
                 */
                waitrunningbufspace();
        }

        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);

        /*
         * 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, NULL, &bp->b_bio2.bio_offset,
                         NULL, NULL);
        }

        /*
         * Set the *dirty* buffer range based upon the VM system dirty pages.
         */
        vfs_setdirty(bp);

        /*
         * We need to do this here to satisfy the vnode_pager and the
         * pageout daemon, so that it thinks that the pages have been
         * "cleaned".  Note that since the pages are in a delayed write
         * buffer -- the VFS layer "will" see that the pages get written
         * out on the next sync, or perhaps the cluster will be completed.
         */
        vfs_clean_pages(bp);
        bqrelse(bp);

        /*
         * Wakeup the buffer flushing daemon if we have a lot of dirty
         * buffers (midpoint between our recovery point and our stall
         * point).
         */
        bd_wakeup((lodirtybuffers + hidirtybuffers) / 2);

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

/*
 * 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. 
 *
 *      Since the buffer is not on a queue, we do not update the 
 *      numfreebuffers count.
 *
 *      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) {
                printf("bdirty: clearing B_NOCACHE on buf %p\n", bp);
                bp->b_flags &= ~B_NOCACHE;
        }
        if (bp->b_flags & B_INVAL) {
                printf("bdirty: warning, dirtying invalid buffer %p\n", bp);
        }
        bp->b_flags &= ~B_RELBUF;

        if ((bp->b_flags & B_DELWRI) == 0) {
                bp->b_flags |= B_DELWRI;
                reassignbuf(bp);
                ++numdirtybuffers;
                bd_wakeup((lodirtybuffers + hidirtybuffers) / 2);
        }
}

/*
 * bundirty:
 *
 *      Clear B_DELWRI for buffer.
 *
 *      Since the buffer is not on a queue, we do not update the numfreebuffers
 *      count.
 *      
 *      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.
 */

void
bundirty(struct buf *bp)
{
        if (bp->b_flags & B_DELWRI) {
                bp->b_flags &= ~B_DELWRI;
                reassignbuf(bp);
                --numdirtybuffers;
                numdirtywakeup(lodirtybuffers);
        }
        /*
         * Since it is now being written, we can clear its deferred write flag.
         */
        bp->b_flags &= ~B_DEFERRED;
}

/*
 * 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)
{
        bp->b_flags |= B_ASYNC;
        bwrite(bp);
}

/*
 * 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 | B_ASYNC;
        return (bwrite(bp));
}

/*
 * bwillwrite:
 *
 *      Called prior to the locking of any vnodes when we are expecting to
 *      write.  We do not want to starve the buffer cache with too many
 *      dirty buffers so we block here.  By blocking prior to the locking
 *      of any vnodes we attempt to avoid the situation where a locked vnode
 *      prevents the various system daemons from flushing related buffers.
 */

void
bwillwrite(void)
{
        if (numdirtybuffers >= hidirtybuffers) {
                crit_enter();
                while (numdirtybuffers >= hidirtybuffers) {
                        bd_wakeup(1);
                        needsbuffer |= VFS_BIO_NEED_DIRTYFLUSH;
                        tsleep(&needsbuffer, 0, "flswai", 0);
                }
                crit_exit();
        }
}

/*
 * buf_dirty_count_severe:
 *
 *      Return true if we have too many dirty buffers.
 */
int
buf_dirty_count_severe(void)
{
        return(numdirtybuffers >= hidirtybuffers);
}

/*
 * 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.
 */
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));

        crit_enter();

        /*
         * 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_LOCKED)
                bp->b_flags &= ~B_ERROR;

        /*
         * If a write error occurs and the caller does not want to throw
         * away the buffer, redirty the buffer.  This will also clear
         * B_NOCACHE.
         */
        if (bp->b_cmd == BUF_CMD_WRITE &&
            (bp->b_flags & (B_ERROR | B_INVAL)) == B_ERROR) {
                /*
                 * Failed write, redirty.  Must clear B_ERROR to prevent
                 * pages from being scrapped.  If B_INVAL is set then
                 * this case is not run and the next case is run to 
                 * destroy the buffer.  B_INVAL can occur if the buffer
                 * is outside the range supported by the underlying device.
                 */
                bp->b_flags &= ~B_ERROR;
                bdirty(bp);
        } else if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_ERROR)) ||
                   (bp->b_bufsize <= 0) || bp->b_cmd == BUF_CMD_FREEBLKS) {
                /*
                 * Either a failed I/O or we were asked to free or not
                 * cache the buffer.
                 */
                bp->b_flags |= B_INVAL;
                if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_deallocate)
                        (*bioops.io_deallocate)(bp);
                if (bp->b_flags & B_DELWRI) {
                        --numdirtybuffers;
                        numdirtywakeup(lodirtybuffers);
                }
                bp->b_flags &= ~(B_DELWRI | B_CACHE);
        }

        /*
         * We must clear B_RELBUF if B_DELWRI is set.  If vfs_vmio_release() 
         * is called with B_DELWRI 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.
         * 
         * 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)
                bp->b_flags &= ~B_RELBUF;
        else if (vm_page_count_severe())
                bp->b_flags |= B_RELBUF;

        /*
         * At this point destroying the buffer is governed by the B_INVAL 
         * or B_RELBUF flags.
         */
        bp->b_cmd = BUF_CMD_DONE;

        /*
         * 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);

                                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;
                                        }
                                }

                                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.
                         */
                        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);
                        }
                        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 0
                        if (bp->b_vp)
                                printf("brelse bp %p %08x/%08x: Warning, caught and fixed brelvp bug\n", bp, saved_flags, bp->b_flags);
#endif
                        if (bp->b_bufsize)
                                allocbuf(bp, 0);
                        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");
                /* do not release to free list */
                BUF_UNLOCK(bp);
                crit_exit();
                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.
         */

        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_ERROR | 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 if (bp->b_flags & B_LOCKED) {
                /*
                 * Buffers that are locked.
                 */
                bp->b_qindex = BQUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_LOCKED], bp, b_freelist);
        } else {
                /*
                 * Remaining buffers.  These buffers are still associated with
                 * their vnode.
                 */
                switch(bp->b_flags & (B_DELWRI|B_AGE)) {
                case B_DELWRI | B_AGE:
                    bp->b_qindex = BQUEUE_DIRTY;
                    TAILQ_INSERT_HEAD(&bufqueues[BQUEUE_DIRTY], bp, b_freelist);
                    break;
                case B_DELWRI:
                    bp->b_qindex = BQUEUE_DIRTY;
                    TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_DIRTY], bp, b_freelist);
                    break;
                case B_AGE:
                    bp->b_qindex = BQUEUE_CLEAN;
                    TAILQ_INSERT_HEAD(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
                    break;
                default:
                    bp->b_qindex = BQUEUE_CLEAN;
                    TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
                    break;
                }
        }

        /*
         * 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);

        /*
         * Fixup numfreebuffers count.  The bp is on an appropriate queue
         * unless locked.  We then bump numfreebuffers if it is not B_DELWRI.
         * We've already handled the B_INVAL case ( B_DELWRI will be clear
         * if B_INVAL is set ).
         */
        if ((bp->b_flags & B_LOCKED) == 0 && !(bp->b_flags & B_DELWRI))
                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_ASYNC | B_NOCACHE | B_AGE | B_RELBUF |
                        B_DIRECT | B_NOWDRAIN);
        BUF_UNLOCK(bp);
        crit_exit();
}

/*
 * 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.
 */
void
bqrelse(struct buf *bp)
{
        crit_enter();

        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");
                BUF_UNLOCK(bp);
                crit_exit();
                return;
        }
        if (bp->b_flags & B_LOCKED) {
                bp->b_flags &= ~B_ERROR;
                bp->b_qindex = BQUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_LOCKED], bp, b_freelist);
                /* buffers with stale but valid contents */
        } else if (bp->b_flags & B_DELWRI) {
                bp->b_qindex = BQUEUE_DIRTY;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_DIRTY], 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*.
                 */
                crit_exit();
                brelse(bp);
                return;
        } else {
                bp->b_qindex = BQUEUE_CLEAN;
                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_CLEAN], bp, b_freelist);
        }

        if ((bp->b_flags & B_LOCKED) == 0 &&
            ((bp->b_flags & B_INVAL) || !(bp->b_flags & B_DELWRI))) {
                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_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
        BUF_UNLOCK(bp);
        crit_exit();
}

/*
 * 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;

        crit_enter();
        for (i = 0; i < bp->b_xio.xio_npages; i++) {
                m = bp->b_xio.xio_pages[i];
                bp->b_xio.xio_pages[i] = NULL;
                /*
                 * In order to keep page LRU ordering consistent, put
                 * everything on the inactive queue.
                 */
                vm_page_unwire(m, 0);
                /*
                 * We don't mess with busy pages, it is
                 * the responsibility of the process that
                 * busied the pages to deal with them.
                 */
                if ((m->flags & PG_BUSY) || (m->busy != 0))
                        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 ((bp->b_flags & B_ASYNC) == 0 && !m->valid &&
                                        m->hold_count == 0) {
                                vm_page_busy(m);
                                vm_page_protect(m, VM_PROT_NONE);
                                vm_page_free(m);
                        } else if (bp->b_flags & B_DIRECT) {
                                vm_page_try_to_free(m);
                        } else if (vm_page_count_severe()) {
                                vm_page_try_to_cache(m);
                        }
                }
        }
        crit_exit();
        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;
        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;

        crit_enter();
        /*
         * 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)) &&
                            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)) &&
                            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);
                        crit_exit();
                        return nwritten;
                }
        }

        bremfree(bp);
        bp->b_flags |= B_ASYNC;

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

static struct buf *
getnewbuf(int slpflag, int slptimeo, int size, int maxsize)
{
        struct buf *bp;
        struct buf *nbp;
        int defrag = 0;
        int nqindex;
        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;
        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.
         */

        while ((bp = nbp) != NULL) {
                int qindex = nqindex;

                /*
                 * Calculate next bp ( we can only use it if we do not block
                 * or do other fancy things ).
                 */
                if ((nbp = TAILQ_NEXT(bp, b_freelist)) == 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: inconsistant 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));

                /*
                 * 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) {
                        printf("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.
                 */

                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
                        printf("getnewbuf: warning, locked buf %p, race corrected\n", bp);
                        tsleep(&bd_request, 0, "gnbxxx", hz / 100);
                        goto restart;
                }
                if (bp->b_qindex != qindex) {
                        printf("getnewbuf: warning, BUF_LOCK blocked unexpectedly on buf %p index %d->%d, race corrected\n", bp, qindex, bp->b_qindex);
                        BUF_UNLOCK(bp);
                        goto restart;
                }
                bremfree(bp);

                if (qindex == BQUEUE_CLEAN) {
                        if (bp->b_flags & B_VMIO) {
                                bp->b_flags &= ~B_ASYNC;
                                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);
                if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_deallocate)
                        (*bioops.io_deallocate)(bp);

                /*
                 * 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);

                LIST_INIT(&bp->b_dep);

                /*
                 * 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;
                break;
        }

        /*
         * 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.
         */

        if (bp == NULL) {
                int flags;
                char *waitmsg;

                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 */

                needsbuffer |= flags;
                while (needsbuffer & flags) {
                        if (tsleep(&needsbuffer, slpflag, waitmsg, slptimeo))
                                return (NULL);
                }
        } 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.
                 */
                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, &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_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);
}

/*
 * 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.
 */

static struct thread *bufdaemonthread;

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

static void
buf_daemon()
{
        /*
         * This process needs to be suspended prior to shutdown sync.
         */
        EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_kproc,
            bufdaemonthread, SHUTDOWN_PRI_LAST);

        /*
         * This process is allowed to take the buffer cache to the limit
         */
        crit_enter();

        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.
                 */
                while (numdirtybuffers > lodirtybuffers) {
                        if (flushbufqueues() == 0)
                                break;
                        waitrunningbufspace();
                        numdirtywakeup((lodirtybuffers + hidirtybuffers) / 2);
                }

                /*
                 * Only clear bd_request if we have reached our low water
                 * mark.  The buf_daemon normally waits 5 seconds and
                 * then incrementally flushes any dirty buffers that have
                 * built up, within reason.
                 *
                 * If we were unable to hit our low water mark and couldn't
                 * find any flushable buffers, we sleep half a second. 
                 * Otherwise we loop immediately.
                 */
                if (numdirtybuffers <= lodirtybuffers) {
                        /*
                         * 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.
                         */
                        bd_request = 0;
                        tsleep(&bd_request, 0, "psleep", hz);
                } else {
                        /*
                         * We couldn't find any flushable dirty buffers but
                         * still have too many dirty buffers, we
                         * have to sleep and try again.  (rare)
                         */
                        tsleep(&bd_request, 0, "qsleep", hz / 2);
                }
        }
}

/*
 * 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.
 */

static int
flushbufqueues(void)
{
        struct buf *bp;
        int r = 0;

        bp = TAILQ_FIRST(&bufqueues[BQUEUE_DIRTY]);

        while (bp) {
                KASSERT((bp->b_flags & B_DELWRI), ("unexpected clean buffer %p", bp));
                if (bp->b_flags & B_DELWRI) {
                        if (bp->b_flags & B_INVAL) {
                                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0)
                                        panic("flushbufqueues: locked buf");
                                bremfree(bp);
                                brelse(bp);
                                ++r;
                                break;
                        }
                        if (LIST_FIRST(&bp->b_dep) != NULL &&
                            bioops.io_countdeps &&
                            (bp->b_flags & B_DEFERRED) == 0 &&
                            (*bioops.io_countdeps)(bp, 0)) {
                                TAILQ_REMOVE(&bufqueues[BQUEUE_DIRTY],
                                             bp, b_freelist);
                                TAILQ_INSERT_TAIL(&bufqueues[BQUEUE_DIRTY],
                                                  bp, b_freelist);
                                bp->b_flags |= B_DEFERRED;
                                bp = TAILQ_FIRST(&bufqueues[BQUEUE_DIRTY]);
                                continue;
                        }

                        /*
                         * Only write it out if we can successfully lock
                         * it.
                         */
                        if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
                                vfs_bio_awrite(bp);
                                ++r;
                                break;
                        }
                }
                bp = TAILQ_NEXT(bp, b_freelist);
        }
        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;

        if (findblk(vp, loffset))
                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;

        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)
                        return 0;
                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)
                        return 0;
        }
        return 1;
}

/*
 * vfs_setdirty:
 *
 *      Sets the dirty range for a buffer based on the status of the dirty
 *      bits in the pages comprising the buffer.
 *
 *      The range is limited to the size of the buffer.
 *
 *      This routine is primarily used by NFS, but is generalized for the
 *      B_VMIO case.
 */
static void
vfs_setdirty(struct buf *bp) 
{
        int i;
        vm_object_t object;

        /*
         * Degenerate case - empty buffer
         */

        if (bp->b_bufsize == 0)
                return;

        /*
         * We qualify the scan for modified pages on whether the
         * object has been flushed yet.  The OBJ_WRITEABLE flag
         * is not cleared simply by protecting pages off.
         */

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

        object = bp->b_xio.xio_pages[0]->object;

        if ((object->flags & OBJ_WRITEABLE) && !(object->flags & OBJ_MIGHTBEDIRTY))
                printf("Warning: object %p writeable but not mightbedirty\n", object);
        if (!(object->flags & OBJ_WRITEABLE) && (object->flags & OBJ_MIGHTBEDIRTY))
                printf("Warning: object %p mightbedirty but not writeable\n", object);

        if (object->flags & (OBJ_MIGHTBEDIRTY|OBJ_CLEANING)) {
                vm_offset_t boffset;
                vm_offset_t eoffset;

                /*
                 * test the pages to see if they have been modified directly
                 * by users through the VM system.
                 */
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_flag_clear(bp->b_xio.xio_pages[i], PG_ZERO);
                        vm_page_test_dirty(bp->b_xio.xio_pages[i]);
                }

                /*
                 * Calculate the encompassing dirty range, boffset and eoffset,
                 * (eoffset - boffset) bytes.
                 */

                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        if (bp->b_xio.xio_pages[i]->dirty)
                                break;
                }
                boffset = (i << PAGE_SHIFT) - (bp->b_loffset & PAGE_MASK);

                for (i = bp->b_xio.xio_npages - 1; i >= 0; --i) {
                        if (bp->b_xio.xio_pages[i]->dirty) {
                                break;
                        }
                }
                eoffset = ((i + 1) << PAGE_SHIFT) - (bp->b_loffset & PAGE_MASK);

                /*
                 * Fit it to the buffer.
                 */

                if (eoffset > bp->b_bcount)
                        eoffset = bp->b_bcount;

                /*
                 * If we have a good dirty range, merge with the existing
                 * dirty range.
                 */

                if (boffset < eoffset) {
                        if (bp->b_dirtyoff > boffset)
                                bp->b_dirtyoff = boffset;
                        if (bp->b_dirtyend < eoffset)
                                bp->b_dirtyend = eoffset;
                }
        }
}

/*
 * findblk:
 *
 *      Locate and return the specified buffer, or NULL if the buffer does
 *      not exist.  Do not attempt to lock the buffer or manipulate it in
 *      any way.  The caller must validate that the correct buffer has been
 *      obtain after locking it.
 */
struct buf *
findblk(struct vnode *vp, off_t loffset)
{
        struct buf *bp;

        crit_enter();
        bp = buf_rb_hash_RB_LOOKUP(&vp->v_rbhash_tree, loffset);
        crit_exit();
        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.
 */
struct buf *
getblk(struct vnode *vp, off_t loffset, int size, int slpflag, int slptimeo)
{
        struct buf *bp;

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

        crit_enter();
loop:
        /*
         * Block if we are low on buffers.   Certain processes are allowed
         * to completely exhaust the buffer cache.
         *
         * If this check ever becomes a bottleneck it may be better to
         * move it into the else, when findblk() fails.  At the moment
         * it isn't a problem.
         *
         * XXX remove, we cannot afford to block anywhere if holding a vnode
         * lock in low-memory situation, so take it to the max.
         */
        if (numfreebuffers == 0) {
                if (!curproc)
                        return NULL;
                needsbuffer |= VFS_BIO_NEED_ANY;
                tsleep(&needsbuffer, slpflag, "newbuf", slptimeo);
        }

        if ((bp = findblk(vp, loffset))) {
                /*
                 * The buffer was found in the cache, but we need to lock it.
                 * Even with LK_NOWAIT the lockmgr may break our critical
                 * section, so double-check the validity of the buffer
                 * once the lock has been obtained.
                 */
                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                        int lkflags = LK_EXCLUSIVE | LK_SLEEPFAIL;
                        if (slpflag & PCATCH)
                                lkflags |= LK_PCATCH;
                        if (BUF_TIMELOCK(bp, lkflags, "getblk", slptimeo) ==
                            ENOLCK) {
                                goto loop;
                        }
                        crit_exit();
                        return (NULL);
                }

                /*
                 * 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) {
                        printf("Warning buffer %p (vp %p loffset %lld) was recycled\n", bp, vp, loffset);
                        BUF_UNLOCK(bp);
                        goto loop;
                }

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

                /*
                 * 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)) {
                        printf("Warning invalid buffer %p (vp %p loffset %lld) did not have cleared bio_offset cache\n", bp, vp, 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.
                 */
                if (size != bp->b_bcount) {
                        if (bp->b_flags & B_DELWRI) {
                                bp->b_flags |= B_NOCACHE;
                                bwrite(bp);
                        } else if (LIST_FIRST(&bp->b_dep)) {
                                bp->b_flags |= B_NOCACHE;
                                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.
                 *
                 * 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) {
                        bp->b_flags |= B_NOCACHE;
                        bwrite(bp);
                        goto loop;
                }
                crit_exit();
        } 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 is such that the underlying directory (with a
                 * non-NULL v_mountedhere) 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);

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

                /*
                 * This code is used to make sure that a buffer is not
                 * created while the getnewbuf routine is blocked.
                 * This can be a problem whether the vnode is locked or not.
                 * If the buffer is created out from under us, we have to
                 * throw away the one we just created.  There is now window
                 * race because we are safely running in a critical section
                 * from the point of the duplicate buffer creation through
                 * to here, and we've locked the buffer.
                 */
                if (findblk(vp, loffset)) {
                        bp->b_flags |= B_INVAL;
                        brelse(bp);
                        goto loop;
                }

                /*
                 * Insert the buffer into the hash, so that it can
                 * be found by findblk(). 
                 *
                 * Make sure the translation layer has been cleared.
                 */
                bp->b_loffset = loffset;
                bp->b_bio2.bio_offset = NOOFFSET;
                /* bp->b_bio2.bio_next = NULL; */

                bgetvp(vp, bp);

                /*
                 * 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);

                crit_exit();
        }
        return (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.
 */
struct buf *
geteblk(int size)
{
        struct buf *bp;
        int maxsize;

        maxsize = (size + BKVAMASK) & ~BKVAMASK;

        crit_enter();
        while ((bp = getnewbuf(0, 0, size, maxsize)) == 0)
                ;
        crit_exit();
        allocbuf(bp, size);
        bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
        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.
 */
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 {
                                        free(bp->b_data, M_BIOBUF);
                                        if (bp->b_bufsize) {
                                                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 = malloc(mbsize, M_BIOBUF, M_WAITOK);
                                bp->b_bufsize = mbsize;
                                bp->b_bcount = size;
                                bp->b_flags |= B_MALLOC;
                                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) {
                                        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);
                                free(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"));
                                        while (vm_page_sleep_busy(m, TRUE, "biodep"))
                                                ;

                                        bp->b_xio.xio_pages[i] = NULL;
                                        vm_page_unwire(m, 0);
                                }
                                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;

                        crit_enter();
                        while (bp->b_xio.xio_npages < desiredpages) {
                                vm_page_t m;
                                vm_pindex_t pi;

                                pi = OFF_TO_IDX(bp->b_loffset) + bp->b_xio.xio_npages;
                                if ((m = vm_page_lookup(obj, pi)) == NULL) {
                                        /*
                                         * note: must allocate system pages
                                         * since blocking here could intefere
                                         * with paging I/O, no matter which
                                         * process we are.
                                         */
                                        m = vm_page_alloc(obj, pi, VM_ALLOC_NORMAL | VM_ALLOC_SYSTEM);
                                        if (m == NULL) {
                                                vm_wait();
                                                vm_pageout_deficit += desiredpages -
                                                        bp->b_xio.xio_npages;
                                        } else {
                                                vm_page_wire(m);
                                                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.  If we have to sleep on it,
                                 * retry because it might have gotten freed out
                                 * from under us.
                                 *
                                 * We can only test PG_BUSY here.  Blocking on
                                 * m->busy might lead to a deadlock:
                                 *
                                 *  vm_fault->getpages->cluster_read->allocbuf
                                 *
                                 */

                                if (vm_page_sleep_busy(m, FALSE, "pgtblk"))
                                        continue;

                                /*
                                 * We have a good page.  Should we wakeup the
                                 * page daemon?
                                 */
                                if ((curthread != pagethread) &&
                                    ((m->queue - m->pc) == PQ_CACHE) &&
                                    ((vmstats.v_free_count + vmstats.v_cache_count) <
                                        (vmstats.v_free_min + vmstats.v_cache_min))) {
                                        pagedaemon_wakeup();
                                }
                                vm_page_flag_clear(m, PG_ZERO);
                                vm_page_wire(m);
                                bp->b_xio.xio_pages[bp->b_xio.xio_npages] = m;
                                ++bp->b_xio.xio_npages;
                        }
                        crit_exit();

                        /*
                         * 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));
                }
        }
        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.  The buffer
 *      is left locked on return.  B_EINTR is converted into an EINTR error
 *      and cleared.
 *
 *      NOTE!  The original b_cmd is lost on return, since b_cmd will be
 *      set to BUF_CMD_DONE.
 */
int
biowait(struct buf *bp)
{
        crit_enter();
        while (bp->b_cmd != BUF_CMD_DONE) {
                if (bp->b_cmd == BUF_CMD_READ)
                        tsleep(bp, 0, "biord", 0);
                else
                        tsleep(bp, 0, "biowr", 0);
        }
        crit_exit();
        if (bp->b_flags & B_EINTR) {
                bp->b_flags &= ~B_EINTR;
                return (EINTR);
        }
        if (bp->b_flags & B_ERROR) {
                return (bp->b_error ? bp->b_error : EIO);
        } else {
                return (0);
        }
}

/*
 * 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;
        atomic_add_int(&track->bk_active, 1);
}

/*
 * Initiate I/O on a vnode.
 */
void
vn_strategy(struct vnode *vp, struct bio *bio)
{
        struct bio_track *track;

        KKASSERT(bio->bio_buf->b_cmd != BUF_CMD_DONE);
        if (bio->bio_buf->b_cmd == BUF_CMD_READ)
                track = &vp->v_track_read;
        else
                track = &vp->v_track_write;
        bio->bio_track = track;
        atomic_add_int(&track->bk_active, 1);
        vop_strategy(*vp->v_ops, vp, bio);
}


/*
 * biodone:
 *
 *      Finish I/O on a buffer, optionally calling a completion function.
 *      This is usually called from an interrupt so process blocking is
 *      not allowed.
 *
 *      biodone 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.
 *
 *      biodone 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
biodone(struct bio *bio)
{
        struct buf *bp = bio->bio_buf;
        buf_cmd_t cmd;

        crit_enter();

        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));

        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) {
                        atomic_subtract_int(&track->bk_active, 1);
                        if (track->bk_active < 0) {
                                panic("biodone: bad active count bio %p\n",
                                      bio);
                        }
                        if (track->bk_waitflag) {
                                track->bk_waitflag = 0;
                                wakeup(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);
                        crit_exit();
                        return;
                }
                bio = bio->bio_prev;
        }

        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) {
                brelse(bp);
                crit_exit();
                return;
        }

        /*
         * Warning: softupdates may re-dirty the buffer.
         */
        if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_complete)
                (*bioops.io_complete)(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_holdcnt == 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) {
                        printf("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;
                }

                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) {
                                printf(
"biodone: foff(%lu)/m->pindex(%d) mismatch\n",
                                    (unsigned long)foff, 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.
                         */
                        if (cmd == BUF_CMD_READ && !bogusflag && resid > 0) {
                                vfs_page_set_valid(bp, foff, 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) {
                                printf("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))
                                        printf(" iosize: %ld, loffset: %lld, flags: 0x%08x, npages: %d\n",
                                            bp->b_vp->v_mount->mnt_stat.f_iosize,
                                            bp->b_loffset,
                                            bp->b_flags, bp->b_xio.xio_npages);
                                else
                                        printf(" VDEV, loffset: %lld, flags: 0x%08x, npages: %d\n",
                                            bp->b_loffset,
                                            bp->b_flags, bp->b_xio.xio_npages);
                                printf(" 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_object_pip_subtract(obj, 1);
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                        iosize -= resid;
                }
                if (obj)
                        vm_object_pip_wakeupn(obj, 0);
        }

        /*
         * For asynchronous completions, release the buffer now. The brelse
         * will do a wakeup there if necessary - so no need to do a wakeup
         * here in the async case. The sync case always needs to do a wakeup.
         */

        if (bp->b_flags & B_ASYNC) {
                if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_ERROR | B_RELBUF)) != 0)
                        brelse(bp);
                else
                        bqrelse(bp);
        } else {
                wakeup(bp);
        }
        crit_exit();
}

/*
 * 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;

                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_object_pip_subtract(obj, 1);
                        vm_page_flag_clear(m, PG_ZERO);
                        vm_page_io_finish(m);
                }
                vm_object_pip_wakeupn(obj, 0);
        }
}

/*
 * vfs_page_set_valid:
 *
 *      Set the valid bits in a page based on the supplied offset.   The
 *      range is restricted to the buffer's size.
 *
 *      This routine is typically called after a read completes.
 */
static void
vfs_page_set_valid(struct buf *bp, vm_ooffset_t off, int pageno, vm_page_t m)
{
        vm_ooffset_t soff, eoff;

        /*
         * Start and end offsets in buffer.  eoff - soff may not cross a
         * page boundry or cross the end of the buffer.  The end of the
         * buffer, in this case, is our file EOF, not the allocation size
         * of the buffer.
         */
        soff = off;
        eoff = (off + PAGE_SIZE) & ~(off_t)PAGE_MASK;
        if (eoff > bp->b_loffset + bp->b_bcount)
                eoff = bp->b_loffset + bp->b_bcount;

        /*
         * Set valid range.  This is typically the entire buffer and thus the
         * entire page.
         */
        if (eoff > soff) {
                vm_page_set_validclean(
                    m,
                   (vm_offset_t) (soff & PAGE_MASK),
                   (vm_offset_t) (eoff - soff)
                );
        }
}

/*
 * 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.
 */
void
vfs_busy_pages(struct vnode *vp, struct buf *bp)
{
        int i, bogus;
        struct proc *p = curthread->td_proc;

        /*
         * 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;
                vm_ooffset_t foff;

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

retry:
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];
                        if (vm_page_sleep_busy(m, FALSE, "vbpage"))
                                goto retry;
                }

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

                        /*
                         * When readying a vnode-backed buffer for a write
                         * we must zero-fill any invalid portions of the
                         * backing VM pages.
                         *
                         * When readying a vnode-backed buffer for a read
                         * we must replace any dirty pages with a bogus
                         * page so we do not destroy dirty data when
                         * filling in gaps.  Dirty pages might not
                         * necessarily be marked dirty yet, so use m->valid
                         * as a reasonable test.
                         *
                         * Bogus page replacement is, uh, bogus.  We need
                         * to find a better way.
                         */
                        vm_page_protect(m, VM_PROT_NONE);
                        if (bp->b_cmd == BUF_CMD_WRITE) {
                                vfs_page_set_valid(bp, foff, i, m);
                        } else if (m->valid == VM_PAGE_BITS_ALL) {
                                bp->b_xio.xio_pages[i] = bogus_page;
                                bogus++;
                        }
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                }
                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 (p != NULL) {
                if (bp->b_cmd == BUF_CMD_READ)
                        p->p_stats->p_ru.ru_inblock++;
                else
                        p->p_stats->p_ru.ru_oublock++;
        }
}

/*
 * vfs_clean_pages:
 *      
 *      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 that 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)
{
        int i;

        if (bp->b_flags & B_VMIO) {
                vm_ooffset_t foff;

                foff = bp->b_loffset;
                KASSERT(foff != NOOFFSET, ("vfs_clean_pages: no buffer offset"));
                for (i = 0; i < bp->b_xio.xio_npages; i++) {
                        vm_page_t m = bp->b_xio.xio_pages[i];
                        vm_ooffset_t noff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                        vm_ooffset_t eoff = noff;

                        if (eoff > bp->b_loffset + bp->b_bufsize)
                                eoff = bp->b_loffset + bp->b_bufsize;
                        vfs_page_set_valid(bp, foff, i, m);
                        /* vm_page_clear_dirty(m, foff & PAGE_MASK, eoff - foff); */
                        foff = noff;
                }
        }
}

/*
 * vfs_bio_set_validclean:
 *
 *      Set the range within the buffer to valid and clean.  The range is 
 *      relative to the beginning of the buffer, b_loffset.  Note that
 *      b_loffset itself may be offset from the beginning of the first page.
 */

void   
vfs_bio_set_validclean(struct buf *bp, int base, int size)
{
        if (bp->b_flags & B_VMIO) {
                int i;
                int n;

                /*
                 * Fixup base to be relative to beginning of first page.
                 * Set initial n to be the maximum number of bytes in the
                 * first page that can be validated.
                 */

                base += (bp->b_loffset & PAGE_MASK);
                n = PAGE_SIZE - (base & PAGE_MASK);

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

                        if (n > size)
                                n = size;

                        vm_page_set_validclean(m, base & PAGE_MASK, n);
                        base += n;
                        size -= n;
                        n = PAGE_SIZE;
                }
        }
}

/*
 * 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_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;
                        }
                }
                ea = 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.
 *
 */
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;

        for (pg = from; pg < to; pg += PAGE_SIZE, index++) {

tryagain:

                /*
                 * Note: must allocate system pages since blocking here
                 * could intefere with paging I/O, no matter which
                 * process we are.
                 */
                p = vm_page_alloc(kernel_object,
                        ((pg - VM_MIN_KERNEL_ADDRESS) >> PAGE_SHIFT),
                        VM_ALLOC_NORMAL | VM_ALLOC_SYSTEM);
                if (!p) {
                        vm_pageout_deficit += (to - from) >> PAGE_SHIFT;
                        vm_wait();
                        goto tryagain;
                }
                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);
        }
        bp->b_xio.xio_npages = index;
}

/*
 * 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.
 */
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);
        newnpages = index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;

        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) {
                                printf("vm_hold_free_pages: doffset: %lld, loffset: %lld\n",
                                        bp->b_bio2.bio_offset, bp->b_loffset);
                        }
                        bp->b_xio.xio_pages[index] = NULL;
                        pmap_kremove(pg);
                        vm_page_busy(p);
                        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_paddr_t pa;
        int pidx;
        int i;
        int vmprot;
        struct vm_page *m;

        /* 
         * 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);

        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.
                 */
retry:
                if (addr >= udata)
                        i = vm_fault_quick(addr, vmprot);
                else
                        i = vm_fault_quick(udata, vmprot);
                if (i < 0) {
                        for (i = 0; i < pidx; ++i) {
                            vm_page_unhold(bp->b_xio.xio_pages[i]);
                            bp->b_xio.xio_pages[i] = NULL;
                        }
                        return(-1);
                }

                /*
                 * Extract from current process's address map.  Since the
                 * fault succeeded, an empty page indicates a race.
                 */
                pa = pmap_extract(&curproc->p_vmspace->vm_pmap, (vm_offset_t)addr);
                if (pa == 0) {
                        printf("vmapbuf: warning, race against user address during I/O");
                        goto retry;
                }
                m = PHYS_TO_VM_PAGE(pa);
                vm_page_hold(m);
                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);
}

/*
 * 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;
                crit_enter();
                TAILQ_FOREACH(bp, dp, b_freelist) {
                        counts[bp->b_bufsize/PAGE_SIZE]++;
                        count++;
                }
                crit_exit();
                printf("%s: total-%d", bname[i], count);
                for (j = 0; j <= MAXBSIZE/PAGE_SIZE; j++)
                        if (counts[j] != 0)
                                printf(", %d-%d", j * PAGE_SIZE, counts[j]);
                printf("\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, bp->b_bio2.bio_offset, (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 */