Device layer rollup commit.

[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 1.26 2004/05/19 22:52:58 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 <vm/vm_page2.h>

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

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

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

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 void vfs_backgroundwritedone(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 vmiodirenable = TRUE;
int runningbufspace;
struct lwkt_token buftimetoken;  /* Interlock on setting prio and timo */

static vm_offset_t bogus_offset;

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;

SYSCTL_INT(_vfs, OID_AUTO, numdirtybuffers, CTLFLAG_RD,
        &numdirtybuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, lodirtybuffers, CTLFLAG_RW,
        &lodirtybuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, hidirtybuffers, CTLFLAG_RW,
        &hidirtybuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, numfreebuffers, CTLFLAG_RD,
        &numfreebuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, lofreebuffers, CTLFLAG_RW,
        &lofreebuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, hifreebuffers, CTLFLAG_RW,
        &hifreebuffers, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD,
        &runningbufspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, lorunningspace, CTLFLAG_RW,
        &lorunningspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, hirunningspace, CTLFLAG_RW,
        &hirunningspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RD,
        &maxbufspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, hibufspace, CTLFLAG_RD,
        &hibufspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, lobufspace, CTLFLAG_RD,
        &lobufspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, bufspace, CTLFLAG_RD,
        &bufspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RW,
        &maxbufmallocspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD,
        &bufmallocspace, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RW,
        &getnewbufcalls, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RW,
        &getnewbufrestarts, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, vmiodirenable, CTLFLAG_RW,
        &vmiodirenable, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RW,
        &bufdefragcnt, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RW,
        &buffreekvacnt, 0, "");
SYSCTL_INT(_vfs, OID_AUTO, bufreusecnt, CTLFLAG_RW,
        &bufreusecnt, 0, "");

/*
 * Disable background writes for now.  There appear to be races in the 
 * flags tests and locking operations as well as races in the completion
 * code modifying the original bp (origbp) without holding a lock, assuming
 * splbio protection when there might not be splbio protection.
 */
static int dobkgrdwrite = 0;
SYSCTL_INT(_debug, OID_AUTO, dobkgrdwrite, CTLFLAG_RW, &dobkgrdwrite, 0,
        "Do background writes (honoring the BV_BKGRDWRITE flag)?");

static int bufhashmask;
static int bufhashshift;
static LIST_HEAD(bufhashhdr, buf) *bufhashtbl, invalhash;
struct bqueues bufqueues[BUFFER_QUEUES] = { { 0 } };
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 */

/*
 * Buffer hash table code.  Note that the logical block scans linearly, which
 * gives us some L1 cache locality.
 */

static __inline 
struct bufhashhdr *
bufhash(struct vnode *vnp, daddr_t bn)
{
        u_int64_t hashkey64;
        int hashkey; 
        
        /*
         * A variation on the Fibonacci hash that Knuth credits to
         * R. W. Floyd, see Knuth's _Art of Computer Programming,
         * Volume 3 / Sorting and Searching_
         *
         * We reduce the argument to 32 bits before doing the hash to
         * avoid the need for a slow 64x64 multiply on 32 bit platforms.
         *
         * sizeof(struct vnode) is 168 on i386, so toss some of the lower
         * bits of the vnode address to reduce the key range, which
         * improves the distribution of keys across buckets.
         *
         * The file system cylinder group blocks are very heavily
         * used.  They are located at invervals of fbg, which is
         * on the order of 89 to 94 * 2^10, depending on other
         * filesystem parameters, for a 16k block size.  Smaller block
         * sizes will reduce fpg approximately proportionally.  This
         * will cause the cylinder group index to be hashed using the
         * lower bits of the hash multiplier, which will not distribute
         * the keys as uniformly in a classic Fibonacci hash where a
         * relatively small number of the upper bits of the result
         * are used.  Using 2^16 as a close-enough approximation to
         * fpg, split the hash multiplier in half, with the upper 16
         * bits being the inverse of the golden ratio, and the lower
         * 16 bits being a fraction between 1/3 and 3/7 (closer to
         * 3/7 in this case), that gives good experimental results.
         */
        hashkey64 = ((u_int64_t)(uintptr_t)vnp >> 3) + (u_int64_t)bn;
        hashkey = (((u_int32_t)(hashkey64 + (hashkey64 >> 32)) * 0x9E376DB1u) >>
            bufhashshift) & bufhashmask;
        return(&bufhashtbl[hashkey]);
}

/*
 *      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() - in-progress I/O accounting.
 *
 */
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)
{
        while (runningbufspace > hirunningspace) {
                int s;

                s = splbio();   /* fix race against interrupt/biodone() */
                ++runningbufreq;
                tsleep(&runningbufreq, 0, "wdrain", 0);
                splx(s);
        }
}

/*
 *      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;
        }
}

static __inline__
void
bd_wakeup(int dirtybuflevel)
{
        if (bd_request == 0 && numdirtybuffers >= dirtybuflevel) {
                bd_request = 1;
                wakeup(&bd_request);
        }
}

/*
 * bd_speedup - speedup the buffer cache flushing code
 */

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

/*
 * Initialize buffer headers and related structures. 
 */

caddr_t
bufhashinit(caddr_t vaddr)
{
        /* first, make a null hash table */
        bufhashshift = 29;
        for (bufhashmask = 8; bufhashmask < nbuf / 4; bufhashmask <<= 1)
                bufhashshift--;
        bufhashtbl = (void *)vaddr;
        vaddr = vaddr + sizeof(*bufhashtbl) * bufhashmask;
        --bufhashmask;
        return(vaddr);
}

void
bufinit(void)
{
        struct buf *bp;
        int i;

        TAILQ_INIT(&bswlist);
        LIST_INIT(&invalhash);
        lwkt_token_init(&buftimetoken);

        for (i = 0; i <= bufhashmask; i++)
                LIST_INIT(&bufhashtbl[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_dev = NODEV;
                bp->b_qindex = QUEUE_EMPTY;
                bp->b_xflags = 0;
                LIST_INIT(&bp->b_dep);
                BUF_LOCKINIT(bp);
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_EMPTY], bp, b_freelist);
                LIST_INSERT_HEAD(&invalhash, bp, b_hash);
        }

        /*
         * 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++;

}

/*
 * bfreekva() - free the kva allocation for a buffer.
 *
 *      Must be called at splbio() or higher 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 s = splbio();
        int old_qindex = bp->b_qindex;

        if (bp->b_qindex != QUEUE_NONE) {
                KASSERT(BUF_REFCNT(bp) == 1, ("bremfree: bp %p not locked",bp));
                TAILQ_REMOVE(&bufqueues[bp->b_qindex], bp, b_freelist);
                bp->b_qindex = QUEUE_NONE;
        } else {
                if (BUF_REFCNT(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 QUEUE_DIRTY:
                case QUEUE_CLEAN:
                case QUEUE_EMPTY:
                case QUEUE_EMPTYKVA:
                        --numfreebuffers;
                        break;
                default:
                        break;
                }
        }
        splx(s);
}


/*
 * 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, daddr_t blkno, int size, struct buf ** bpp)
{
        struct buf *bp;

        bp = getblk(vp, blkno, 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_READ;
                bp->b_flags &= ~(B_ERROR | B_INVAL);
                vfs_busy_pages(bp, 0);
                VOP_STRATEGY(vp, bp);
                return (biowait(bp));
        }
        return (0);
}

/*
 * 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, daddr_t blkno, int size, daddr_t * rablkno,
        int *rabsize, int cnt, struct buf ** bpp)
{
        struct buf *bp, *rabp;
        int i;
        int rv = 0, readwait = 0;

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

        /* if not found in cache, do some I/O */
        if ((bp->b_flags & B_CACHE) == 0) {
                bp->b_flags |= B_READ;
                bp->b_flags &= ~(B_ERROR | B_INVAL);
                vfs_busy_pages(bp, 0);
                VOP_STRATEGY(vp, bp);
                ++readwait;
        }

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

                if ((rabp->b_flags & B_CACHE) == 0) {
                        rabp->b_flags |= B_READ | B_ASYNC;
                        rabp->b_flags &= ~(B_ERROR | B_INVAL);
                        vfs_busy_pages(rabp, 0);
                        BUF_KERNPROC(rabp);
                        VOP_STRATEGY(vp, rabp);
                } else {
                        brelse(rabp);
                }
        }

        if (readwait) {
                rv = biowait(bp);
        }
        return (rv);
}

/*
 * 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, s;
        struct buf *newbp;

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

        oldflags = bp->b_flags;

        if (BUF_REFCNT(bp) == 0)
                panic("bwrite: buffer is not busy???");
        s = splbio();
        /*
         * If a background write is already in progress, delay
         * writing this block if it is asynchronous. Otherwise
         * wait for the background write to complete.
         */
        if (bp->b_xflags & BX_BKGRDINPROG) {
                if (bp->b_flags & B_ASYNC) {
                        splx(s);
                        bdwrite(bp);
                        return (0);
                }
                bp->b_xflags |= BX_BKGRDWAIT;
                tsleep(&bp->b_xflags, 0, "biord", 0);
                if (bp->b_xflags & BX_BKGRDINPROG)
                        panic("bwrite: still writing");
        }

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

        /*
         * If this buffer is marked for background writing and we
         * do not have to wait for it, make a copy and write the
         * copy so as to leave this buffer ready for further use.
         *
         * This optimization eats a lot of memory.  If we have a page
         * or buffer shortfull we can't do it.
         */
        if (dobkgrdwrite &&
            (bp->b_xflags & BX_BKGRDWRITE) &&
            (bp->b_flags & B_ASYNC) &&
            !vm_page_count_severe() &&
            !buf_dirty_count_severe()) {
                if (bp->b_flags & B_CALL)
                        panic("bwrite: need chained iodone");

                /* get a new block */
                newbp = geteblk(bp->b_bufsize);

                /* set it to be identical to the old block */
                memcpy(newbp->b_data, bp->b_data, bp->b_bufsize);
                bgetvp(bp->b_vp, newbp);
                newbp->b_lblkno = bp->b_lblkno;
                newbp->b_blkno = bp->b_blkno;
                newbp->b_offset = bp->b_offset;
                newbp->b_iodone = vfs_backgroundwritedone;
                newbp->b_flags |= B_ASYNC | B_CALL;
                newbp->b_flags &= ~B_INVAL;

                /* move over the dependencies */
                if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_movedeps)
                        (*bioops.io_movedeps)(bp, newbp);

                /*
                 * Initiate write on the copy, release the original to
                 * the B_LOCKED queue so that it cannot go away until
                 * the background write completes. If not locked it could go
                 * away and then be reconstituted while it was being written.
                 * If the reconstituted buffer were written, we could end up
                 * with two background copies being written at the same time.
                 */
                bp->b_xflags |= BX_BKGRDINPROG;
                bp->b_flags |= B_LOCKED;
                bqrelse(bp);
                bp = newbp;
        }

        bp->b_flags &= ~(B_READ | B_DONE | B_ERROR);
        bp->b_flags |= B_WRITEINPROG | B_CACHE;

        bp->b_vp->v_numoutput++;
        vfs_busy_pages(bp, 1);

        /*
         * Normal bwrites pipeline writes
         */
        bp->b_runningbufspace = bp->b_bufsize;
        runningbufspace += bp->b_runningbufspace;

        splx(s);
        if (oldflags & B_ASYNC)
                BUF_KERNPROC(bp);
        VOP_STRATEGY(bp->b_vp, bp);

        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);
}

/*
 * Complete a background write started from bwrite.
 */
static void
vfs_backgroundwritedone(bp)
        struct buf *bp;
{
        struct buf *origbp;

        /*
         * Find the original buffer that we are writing.
         */
        if ((origbp = gbincore(bp->b_vp, bp->b_lblkno)) == NULL)
                panic("backgroundwritedone: lost buffer");
        /*
         * Process dependencies then return any unfinished ones.
         */
        if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_complete)
                (*bioops.io_complete)(bp);
        if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_movedeps)
                (*bioops.io_movedeps)(bp, origbp);
        /*
         * Clear the BX_BKGRDINPROG flag in the original buffer
         * and awaken it if it is waiting for the write to complete.
         * If BX_BKGRDINPROG is not set in the original buffer it must
         * have been released and re-instantiated - which is not legal.
         */
        KASSERT((origbp->b_xflags & BX_BKGRDINPROG), ("backgroundwritedone: lost buffer2"));
        origbp->b_xflags &= ~BX_BKGRDINPROG;
        if (origbp->b_xflags & BX_BKGRDWAIT) {
                origbp->b_xflags &= ~BX_BKGRDWAIT;
                wakeup(&origbp->b_xflags);
        }
        /*
         * Clear the B_LOCKED flag and remove it from the locked
         * queue if it currently resides there.
         */
        origbp->b_flags &= ~B_LOCKED;
        if (BUF_LOCK(origbp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
                bremfree(origbp);
                bqrelse(origbp);
        }
        /*
         * This buffer is marked B_NOCACHE, so when it is released
         * by biodone, it will be tossed. We mark it with B_READ
         * to avoid biodone doing a second vwakeup.
         */
        bp->b_flags |= B_NOCACHE | B_READ;
        bp->b_flags &= ~(B_CACHE | B_CALL | B_DONE);
        bp->b_iodone = 0;
        biodone(bp);
}

/*
 * 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_REFCNT(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_lblkno == bp->b_blkno) {
                VOP_BMAP(bp->b_vp, bp->b_lblkno, NULL, &bp->b_blkno, 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.  We must clear B_READ and
 *      B_RELBUF, and we must set B_DELWRI.  We reassign the buffer to 
 *      itself to properly update it in the dirty/clean lists.  We mark it
 *      B_DONE to ensure that any asynchronization of the buffer properly
 *      clears B_DONE ( else a panic will occur later ).  
 *
 *      bdirty() is kinda like bdwrite() - we have to clear B_INVAL which
 *      might have been set pre-getblk().  Unlike bwrite/bdwrite, bdirty()
 *      should only be called if the buffer is known-good.
 *
 *      Since the buffer is not on a queue, we do not update the numfreebuffers
 *      count.
 *
 *      Must be called at splbio().
 *      The buffer must be on QUEUE_NONE.
 */
void
bdirty(bp)
        struct buf *bp;
{
        KASSERT(bp->b_qindex == QUEUE_NONE, ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
        bp->b_flags &= ~(B_READ|B_RELBUF);

        if ((bp->b_flags & B_DELWRI) == 0) {
                bp->b_flags |= B_DONE | B_DELWRI;
                reassignbuf(bp, bp->b_vp);
                ++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 at splbio().
 *      The buffer must be on QUEUE_NONE.
 */

void
bundirty(bp)
        struct buf *bp;
{
        KASSERT(bp->b_qindex == QUEUE_NONE, ("bundirty: buffer %p still on queue %d", bp, bp->b_qindex));

        if (bp->b_flags & B_DELWRI) {
                bp->b_flags &= ~B_DELWRI;
                reassignbuf(bp, bp->b_vp);
                --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;
        (void) VOP_BWRITE(bp->b_vp, 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 (VOP_BWRITE(bp->b_vp, 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) {
                int s;

                s = splbio();
                while (numdirtybuffers >= hidirtybuffers) {
                        bd_wakeup(1);
                        needsbuffer |= VFS_BIO_NEED_DIRTYFLUSH;
                        tsleep(&needsbuffer, 0, "flswai", 0);
                }
                splx(s);
        }
}

/*
 * 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)
{
        int s;

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

        s = splbio();

        if (bp->b_flags & B_LOCKED)
                bp->b_flags &= ~B_ERROR;

        if ((bp->b_flags & (B_READ | 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 | B_FREEBUF)) ||
            (bp->b_bufsize <= 0)) {
                /*
                 * 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 | B_FREEBUF);
                if ((bp->b_flags & B_VMIO) == 0) {
                        if (bp->b_bufsize)
                                allocbuf(bp, 0);
                        if (bp->b_vp)
                                brelvp(bp);
                }
        }

        /*
         * 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_xflags & BX_BKGRDINPROG))
                bp->b_flags |= B_RELBUF;

        /*
         * VMIO buffer rundown.  It is not very necessary to keep a VMIO buffer
         * constituted, not even NFS buffers now.  Two flags effect this.  If
         * B_INVAL, the struct buf is invalidated but the VM object is kept
         * around ( i.e. so it is trivial to reconstitute the buffer later ).
         *
         * If B_ERROR or B_NOCACHE is set, pages in the VM object will be
         * invalidated.  B_ERROR cannot be set for a failed write unless the
         * buffer is also B_INVAL because it hits the re-dirtying code above.
         *
         * Normally we can do this whether a buffer is B_DELWRI or not.  If
         * the buffer is an NFS buffer, it is tracking piecemeal writes or
         * the commit state and we cannot afford to lose the buffer. If the
         * buffer has a background write in progress, we need to keep it
         * around to prevent it from being reconstituted and starting a second
         * background write.
         */
        if ((bp->b_flags & B_VMIO)
            && !(bp->b_vp->v_tag == VT_NFS &&
                 !vn_isdisk(bp->b_vp, NULL) &&
                 (bp->b_flags & B_DELWRI))
            ) {

                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_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_offset;

                for (i = 0; i < bp->b_npages; i++) {
                        m = bp->b_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 splvm() protection is necessary.
                         */
                        if (m == bogus_page) {
                                VOP_GETVOBJECT(vp, &obj);
                                poff = OFF_TO_IDX(bp->b_offset);

                                for (j = i; j < bp->b_npages; j++) {
                                        vm_page_t mtmp;

                                        mtmp = bp->b_pages[j];
                                        if (mtmp == bogus_page) {
                                                mtmp = vm_page_lookup(obj, poff + j);
                                                if (!mtmp) {
                                                        panic("brelse: page missing");
                                                }
                                                bp->b_pages[j] = mtmp;
                                        }
                                }

                                if ((bp->b_flags & B_INVAL) == 0) {
                                        pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_npages);
                                }
                                m = bp->b_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 if (bp->b_flags & B_VMIO) {

                if (bp->b_flags & (B_INVAL | B_RELBUF))
                        vfs_vmio_release(bp);

        }
                        
        if (bp->b_qindex != QUEUE_NONE)
                panic("brelse: free buffer onto another queue???");
        if (BUF_REFCNT(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);
                splx(s);
                return;
        }

        /* enqueue */

        /* buffers with no memory */
        if (bp->b_bufsize == 0) {
                bp->b_flags |= B_INVAL;
                bp->b_xflags &= ~BX_BKGRDWRITE;
                if (bp->b_xflags & BX_BKGRDINPROG)
                        panic("losing buffer 1");
                if (bp->b_kvasize) {
                        bp->b_qindex = QUEUE_EMPTYKVA;
                } else {
                        bp->b_qindex = QUEUE_EMPTY;
                }
                TAILQ_INSERT_HEAD(&bufqueues[bp->b_qindex], bp, b_freelist);
                LIST_REMOVE(bp, b_hash);
                LIST_INSERT_HEAD(&invalhash, bp, b_hash);
                bp->b_dev = NODEV;
        /* buffers with junk contents */
        } else if (bp->b_flags & (B_ERROR | B_INVAL | B_NOCACHE | B_RELBUF)) {
                bp->b_flags |= B_INVAL;
                bp->b_xflags &= ~BX_BKGRDWRITE;
                if (bp->b_xflags & BX_BKGRDINPROG)
                        panic("losing buffer 2");
                bp->b_qindex = QUEUE_CLEAN;
                TAILQ_INSERT_HEAD(&bufqueues[QUEUE_CLEAN], bp, b_freelist);
                LIST_REMOVE(bp, b_hash);
                LIST_INSERT_HEAD(&invalhash, bp, b_hash);
                bp->b_dev = NODEV;

        /* buffers that are locked */
        } else if (bp->b_flags & B_LOCKED) {
                bp->b_qindex = QUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_LOCKED], bp, b_freelist);

        /* remaining buffers */
        } else {
                switch(bp->b_flags & (B_DELWRI|B_AGE)) {
                case B_DELWRI | B_AGE:
                    bp->b_qindex = QUEUE_DIRTY;
                    TAILQ_INSERT_HEAD(&bufqueues[QUEUE_DIRTY], bp, b_freelist);
                    break;
                case B_DELWRI:
                    bp->b_qindex = QUEUE_DIRTY;
                    TAILQ_INSERT_TAIL(&bufqueues[QUEUE_DIRTY], bp, b_freelist);
                    break;
                case B_AGE:
                    bp->b_qindex = QUEUE_CLEAN;
                    TAILQ_INSERT_HEAD(&bufqueues[QUEUE_CLEAN], bp, b_freelist);
                    break;
                default:
                    bp->b_qindex = QUEUE_CLEAN;
                    TAILQ_INSERT_TAIL(&bufqueues[QUEUE_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();

        /* unlock */
        BUF_UNLOCK(bp);
        bp->b_flags &= ~(B_ORDERED | B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF |
                        B_DIRECT | B_NOWDRAIN);
        splx(s);
}

/*
 * 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)
{
        int s;

        s = splbio();

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

        if (bp->b_qindex != QUEUE_NONE)
                panic("bqrelse: free buffer onto another queue???");
        if (BUF_REFCNT(bp) > 1) {
                /* do not release to free list */
                panic("bqrelse: multiple refs");
                BUF_UNLOCK(bp);
                splx(s);
                return;
        }
        if (bp->b_flags & B_LOCKED) {
                bp->b_flags &= ~B_ERROR;
                bp->b_qindex = QUEUE_LOCKED;
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_LOCKED], bp, b_freelist);
                /* buffers with stale but valid contents */
        } else if (bp->b_flags & B_DELWRI) {
                bp->b_qindex = QUEUE_DIRTY;
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_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*.
                 */
                splx(s);
                brelse(bp);
                return;
        } else {
                bp->b_qindex = QUEUE_CLEAN;
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_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();

        /* unlock */
        BUF_UNLOCK(bp);
        bp->b_flags &= ~(B_ORDERED | B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
        splx(s);
}

static void
vfs_vmio_release(bp)
        struct buf *bp;
{
        int i, s;
        vm_page_t m;

        s = splvm();
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                bp->b_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);
                        }
                }
        }
        splx(s);
        pmap_qremove(trunc_page((vm_offset_t) bp->b_data), bp->b_npages);
        if (bp->b_bufsize) {
                bufspacewakeup();
                bp->b_bufsize = 0;
        }
        bp->b_npages = 0;
        bp->b_flags &= ~B_VMIO;
        if (bp->b_vp)
                brelvp(bp);
}

/*
 * Check to see if a block is currently memory resident.
 */
struct buf *
gbincore(struct vnode * vp, daddr_t blkno)
{
        struct buf *bp;
        struct bufhashhdr *bh;

        bh = bufhash(vp, blkno);

        /* Search hash chain */
        LIST_FOREACH(bp, bh, b_hash) {
                /* hit */
                if (bp->b_vp == vp && bp->b_lblkno == blkno &&
                    (bp->b_flags & B_INVAL) == 0) {
                        break;
                }
        }
        return (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.
 */
int
vfs_bio_awrite(struct buf * bp)
{
        int i;
        int j;
        daddr_t lblkno = bp->b_lblkno;
        struct vnode *vp = bp->b_vp;
        int s;
        int ncl;
        struct buf *bpa;
        int nwritten;
        int size;
        int maxcl;

        s = splbio();
        /*
         * 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.
         */
        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;
                maxcl = MAXPHYS / size;

                for (i = 1; i < maxcl; i++) {
                        if ((bpa = gbincore(vp, lblkno + 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_blkno == bpa->b_lblkno) ||
                                    (bpa->b_blkno !=
                                     bp->b_blkno + ((i * size) >> DEV_BSHIFT)))
                                        break;
                        } else {
                                break;
                        }
                }
                for (j = 1; i + j <= maxcl && j <= lblkno; j++) {
                        if ((bpa = gbincore(vp, lblkno - 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_blkno == bpa->b_lblkno) ||
                                    (bpa->b_blkno !=
                                     bp->b_blkno - ((j * size) >> DEV_BSHIFT)))
                                        break;
                        } else {
                                break;
                        }
                }
                --j;
                ncl = i + j;
                /*
                 * this is a possible cluster write
                 */
                if (ncl != 1) {
                        nwritten = cluster_wbuild(vp, size, lblkno - j, ncl);
                        splx(s);
                        return nwritten;
                }
        }

        BUF_LOCK(bp, LK_EXCLUSIVE);
        bremfree(bp);
        bp->b_flags |= B_ASYNC;

        splx(s);
        /*
         * default (old) behavior, writing out only one block
         *
         * XXX returns b_bufsize instead of b_bcount for nwritten?
         */
        nwritten = bp->b_bufsize;
        (void) VOP_BWRITE(bp->b_vp, 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 = QUEUE_EMPTYKVA;
        nbp = TAILQ_FIRST(&bufqueues[QUEUE_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 = QUEUE_CLEAN;
                        nbp = TAILQ_FIRST(&bufqueues[QUEUE_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 = QUEUE_EMPTY;
                        nbp = TAILQ_FIRST(&bufqueues[QUEUE_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 QUEUE_EMPTY:
                                nqindex = QUEUE_EMPTYKVA;
                                if ((nbp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTYKVA])))
                                        break;
                                /* fall through */
                        case QUEUE_EMPTYKVA:
                                nqindex = QUEUE_CLEAN;
                                if ((nbp = TAILQ_FIRST(&bufqueues[QUEUE_CLEAN])))
                                        break;
                                /* fall through */
                        case QUEUE_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 QUEUE_EMPTY[KVA] bp's.
                 */

                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0)
                        panic("getnewbuf: locked buf");
                bremfree(bp);

                if (qindex == QUEUE_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.
                 */

                if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_deallocate)
                        (*bioops.io_deallocate)(bp);
                if (bp->b_xflags & BX_BKGRDINPROG)
                        panic("losing buffer 3");
                LIST_REMOVE(bp, b_hash);
                LIST_INSERT_HEAD(&invalhash, bp, b_hash);

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

                bp->b_flags = 0;
                bp->b_xflags = 0;
                bp->b_dev = NODEV;
                bp->b_vp = NULL;
                bp->b_blkno = bp->b_lblkno = 0;
                bp->b_offset = NOOFFSET;
                bp->b_iodone = 0;
                bp->b_error = 0;
                bp->b_resid = 0;
                bp->b_bcount = 0;
                bp->b_npages = 0;
                bp->b_dirtyoff = bp->b_dirtyend = 0;

                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 to 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()
{
        int s;

        /*
         * 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
         */
        s = splbio();

        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[QUEUE_DIRTY]);

        while (bp) {
                KASSERT((bp->b_flags & B_DELWRI), ("unexpected clean buffer %p", bp));
                if ((bp->b_flags & B_DELWRI) != 0 &&
                    (bp->b_xflags & BX_BKGRDINPROG) == 0) {
                        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[QUEUE_DIRTY],
                                    bp, b_freelist);
                                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_DIRTY],
                                    bp, b_freelist);
                                bp->b_flags |= B_DEFERRED;
                                bp = TAILQ_FIRST(&bufqueues[QUEUE_DIRTY]);
                                continue;
                        }
                        vfs_bio_awrite(bp);
                        ++r;
                        break;
                }
                bp = TAILQ_NEXT(bp, b_freelist);
        }
        return (r);
}

/*
 * Check to see if a block is currently memory resident.
 */
struct buf *
incore(struct vnode * vp, daddr_t blkno)
{
        struct buf *bp;

        int s = splbio();
        bp = gbincore(vp, blkno);
        splx(s);
        return (bp);
}

/*
 * Returns true if no I/O is needed to access the associated VM object.
 * This is like incore 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 splx().
 */
int
inmem(struct vnode * vp, daddr_t blkno)
{
        vm_object_t obj;
        vm_offset_t toff, tinc, size;
        vm_page_t m;
        vm_ooffset_t off;

        if (incore(vp, blkno))
                return 1;
        if (vp->v_mount == NULL)
                return 0;
        if (VOP_GETVOBJECT(vp, &obj) != 0 || (vp->v_flag & VOBJBUF) == 0)
                return 0;

        size = PAGE_SIZE;
        if (size > vp->v_mount->mnt_stat.f_iosize)
                size = vp->v_mount->mnt_stat.f_iosize;
        off = (vm_ooffset_t)blkno * (vm_ooffset_t)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(off + toff));
                if (!m)
                        return 0;
                tinc = size;
                if (tinc > PAGE_SIZE - ((toff + off) & PAGE_MASK))
                        tinc = PAGE_SIZE - ((toff + off) & PAGE_MASK);
                if (vm_page_is_valid(m,
                    (vm_offset_t) ((toff + off) & 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_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_npages; i++) {
                        vm_page_flag_clear(bp->b_pages[i], PG_ZERO);
                        vm_page_test_dirty(bp->b_pages[i]);
                }

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

                for (i = 0; i < bp->b_npages; i++) {
                        if (bp->b_pages[i]->dirty)
                                break;
                }
                boffset = (i << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);

                for (i = bp->b_npages - 1; i >= 0; --i) {
                        if (bp->b_pages[i]->dirty) {
                                break;
                        }
                }
                eoffset = ((i + 1) << PAGE_SHIFT) - (bp->b_offset & 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;
                }
        }
}

/*
 *      getblk:
 *
 *      Get a block given a specified block and offset into a file/device.
 *      The buffers B_DONE bit will be cleared on return, making it almost
 *      ready for an I/O initiation.  B_INVAL may or may not be set on 
 *      return.  The caller should clear B_INVAL prior to initiating a
 *      READ.
 *
 *      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 VOP_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, daddr_t blkno, int size, int slpflag, int slptimeo)
{
        struct buf *bp;
        int s;
        struct bufhashhdr *bh;

        if (size > MAXBSIZE)
                panic("getblk: size(%d) > MAXBSIZE(%d)", size, MAXBSIZE);

        s = splbio();
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 gbincore() 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 = gbincore(vp, blkno))) {
                /*
                 * Buffer is in-core.  If the buffer is not busy, it must
                 * be on a queue.
                 */

                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                        if (BUF_TIMELOCK(bp, LK_EXCLUSIVE | LK_SLEEPFAIL,
                            "getblk", slpflag, slptimeo) == ENOLCK)
                                goto loop;
                        splx(s);
                        return (struct buf *) NULL;
                }

                /*
                 * The buffer is locked.  B_CACHE is cleared if the buffer is 
                 * invalid.  Ohterwise, for a non-VMIO buffer, B_CACHE is set
                 * and for a VMIO buffer B_CACHE is adjusted according to the
                 * backing VM cache.
                 */
                if (bp->b_flags & B_INVAL)
                        bp->b_flags &= ~B_CACHE;
                else if ((bp->b_flags & (B_VMIO | B_INVAL)) == 0)
                        bp->b_flags |= B_CACHE;
                bremfree(bp);

                /*
                 * check for size inconsistancies for non-VMIO case.
                 */

                if (bp->b_bcount != size) {
                        if ((bp->b_flags & B_VMIO) == 0 ||
                            (size > bp->b_kvasize)) {
                                if (bp->b_flags & B_DELWRI) {
                                        bp->b_flags |= B_NOCACHE;
                                        VOP_BWRITE(bp->b_vp, bp);
                                } else {
                                        if ((bp->b_flags & B_VMIO) &&
                                           (LIST_FIRST(&bp->b_dep) == NULL)) {
                                                bp->b_flags |= B_RELBUF;
                                                brelse(bp);
                                        } else {
                                                bp->b_flags |= B_NOCACHE;
                                                VOP_BWRITE(bp->b_vp, bp);
                                        }
                                }
                                goto loop;
                        }
                }

                /*
                 * If the size is inconsistant in the VMIO case, we can resize
                 * the buffer.  This might lead to B_CACHE getting set or
                 * cleared.  If the size has not changed, B_CACHE remains
                 * unchanged from its previous state.
                 */

                if (bp->b_bcount != size)
                        allocbuf(bp, size);

                KASSERT(bp->b_offset != 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;
                        VOP_BWRITE(bp->b_vp, bp);
                        goto loop;
                }

                splx(s);
                bp->b_flags &= ~B_DONE;
        } 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).
                 */
                int bsize, maxsize, vmio;
                off_t offset;

                if (vn_isdisk(vp, NULL))
                        bsize = DEV_BSIZE;
                else if (vp->v_mountedhere)
                        bsize = vp->v_mountedhere->mnt_stat.f_iosize;
                else if (vp->v_mount)
                        bsize = vp->v_mount->mnt_stat.f_iosize;
                else
                        bsize = size;

                offset = (off_t)blkno * bsize;
                vmio = (VOP_GETVOBJECT(vp, NULL) == 0) && (vp->v_flag & VOBJBUF);
                maxsize = vmio ? size + (offset & PAGE_MASK) : size;
                maxsize = imax(maxsize, bsize);

                if ((bp = getnewbuf(slpflag, slptimeo, size, maxsize)) == NULL) {
                        if (slpflag || slptimeo) {
                                splx(s);
                                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 at splbio() from the
                 * point of the duplicate buffer creation through to here,
                 * and we've locked the buffer.
                 */
                if (gbincore(vp, blkno)) {
                        bp->b_flags |= B_INVAL;
                        brelse(bp);
                        goto loop;
                }

                /*
                 * Insert the buffer into the hash, so that it can
                 * be found by incore.
                 */
                bp->b_blkno = bp->b_lblkno = blkno;
                bp->b_offset = offset;

                bgetvp(vp, bp);
                LIST_REMOVE(bp, b_hash);
                bh = bufhash(vp, blkno);
                LIST_INSERT_HEAD(bh, bp, b_hash);

                /*
                 * set B_VMIO bit.  allocbuf() the buffer bigger.  Since the
                 * buffer size starts out as 0, B_CACHE will be set by
                 * allocbuf() for the VMIO case prior to it testing the
                 * backing store for validity.
                 */

                if (vmio) {
                        bp->b_flags |= B_VMIO;
#if defined(VFS_BIO_DEBUG)
                        if (vn_canvmio(vp) != TRUE)
                                printf("getblk: vmioing file type %d???\n", vp->v_type);
#endif
                } else {
                        bp->b_flags &= ~B_VMIO;
                }

                allocbuf(bp, size);

                splx(s);
                bp->b_flags &= ~B_DONE;
        }
        return (bp);
}

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

        maxsize = (size + BKVAMASK) & ~BKVAMASK;

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


/*
 * 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 at splbio() 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 !defined(NO_B_MALLOC)
                if (bp->b_flags & B_MALLOC)
                        newbsize = mbsize;
                else
#endif
                        newbsize = round_page(size);

                if (newbsize < bp->b_bufsize) {
#if !defined(NO_B_MALLOC)
                        /*
                         * 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;
                        }               
#endif
                        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) {
#if !defined(NO_B_MALLOC)
                        /*
                         * 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;
                        }
#endif
                        origbuf = NULL;
                        origbufsize = 0;
#if !defined(NO_B_MALLOC)
                        /*
                         * 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);
                        }
#endif
                        vm_hold_load_pages(
                            bp,
                            (vm_offset_t) bp->b_data + bp->b_bufsize,
                            (vm_offset_t) bp->b_data + newbsize);
#if !defined(NO_B_MALLOC)
                        if (origbuf) {
                                bcopy(origbuf, bp->b_data, origbufsize);
                                free(origbuf, M_BIOBUF);
                        }
#endif
                }
        } else {
                vm_page_t m;
                int desiredpages;

                newbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                desiredpages = (size == 0) ? 0 :
                        num_pages((bp->b_offset & PAGE_MASK) + newbsize);

#if !defined(NO_B_MALLOC)
                if (bp->b_flags & B_MALLOC)
                        panic("allocbuf: VMIO buffer can't be malloced");
#endif
                /*
                 * 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_npages) {
                                for (i = desiredpages; i < bp->b_npages; i++) {
                                        /*
                                         * the page is not freed here -- it
                                         * is the responsibility of 
                                         * vnode_pager_setsize
                                         */
                                        m = bp->b_pages[i];
                                        KASSERT(m != bogus_page,
                                            ("allocbuf: bogus page found"));
                                        while (vm_page_sleep_busy(m, TRUE, "biodep"))
                                                ;

                                        bp->b_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_npages - desiredpages));
                                bp->b_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;
                        int s;

                        /*
                         * 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.
                         *
                         * spl 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;
                        VOP_GETVOBJECT(vp, &obj);

                        s = splbio();
                        while (bp->b_npages < desiredpages) {
                                vm_page_t m;
                                vm_pindex_t pi;

                                pi = OFF_TO_IDX(bp->b_offset) + bp->b_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_npages;
                                        } else {
                                                vm_page_wire(m);
                                                vm_page_wakeup(m);
                                                bp->b_flags &= ~B_CACHE;
                                                bp->b_pages[bp->b_npages] = m;
                                                ++bp->b_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_pages[bp->b_npages] = m;
                                ++bp->b_npages;
                        }
                        splx(s);

                        /*
                         * 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 ), new 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_offset + 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_offset & PAGE_MASK) + toff) >> 
                                    PAGE_SHIFT;

                                vfs_buf_test_cache(
                                    bp, 
                                    bp->b_offset,
                                    toff, 
                                    tinc, 
                                    bp->b_pages[pi]
                                );
                                toff += tinc;
                                tinc = PAGE_SIZE;
                        }

                        /*
                         * Step 3, fixup the KVM pmap.  Remember that
                         * bp->b_data is relative to bp->b_offset, but 
                         * bp->b_offset 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_pages, 
                            bp->b_npages
                        );
                        bp->b_data = (caddr_t)((vm_offset_t)bp->b_data | 
                            (vm_offset_t)(bp->b_offset & 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 and B_DONE on return.  B_EINTR is converted into a EINTR
 *      error and cleared.
 */
int
biowait(struct buf * bp)
{
        int s;

        s = splbio();
        while ((bp->b_flags & B_DONE) == 0) {
#if defined(NO_SCHEDULE_MODS)
                tsleep(bp, 0, "biowait", 0);
#else
                if (bp->b_flags & B_READ)
                        tsleep(bp, 0, "biord", 0);
                else
                        tsleep(bp, 0, "biowr", 0);
#endif
        }
        splx(s);
        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);
        }
}

/*
 *      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.
 *
 *      b_dev is required to be reinitialized prior to the top level strategy
 *      call in a device stack.  To avoid improper reuse, biodone() sets
 *      b_dev to NODEV.
 */
void
biodone(struct buf * bp)
{
        int s, error;

        s = splbio();

        KASSERT(BUF_REFCNT(bp) > 0, ("biodone: bp %p not busy %d", bp, BUF_REFCNT(bp)));
        KASSERT(!(bp->b_flags & B_DONE), ("biodone: bp %p already done", bp));

        bp->b_flags |= B_DONE;
        bp->b_dev = NODEV;
        runningbufwakeup(bp);

        if (bp->b_flags & B_FREEBUF) {
                brelse(bp);
                splx(s);
                return;
        }

        if ((bp->b_flags & B_READ) == 0) {
                vwakeup(bp);
        }

        /* call optional completion function if requested */
        if (bp->b_flags & B_CALL) {
                bp->b_flags &= ~B_CALL;
                (*bp->b_iodone) (bp);
                splx(s);
                return;
        }
        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;

                error = VOP_GETVOBJECT(vp, &obj);

#if defined(VFS_BIO_DEBUG)
                if (vp->v_holdcnt == 0) {
                        panic("biodone: zero vnode hold count");
                }

                if (error) {
                        panic("biodone: missing VM object");
                }

                if ((vp->v_flag & VOBJBUF) == 0) {
                        panic("biodone: vnode is not setup for merged cache");
                }
#endif

                foff = bp->b_offset;
                KASSERT(bp->b_offset != NOOFFSET,
                    ("biodone: no buffer offset"));

                if (error) {
                        panic("biodone: no object");
                }
#if defined(VFS_BIO_DEBUG)
                if (obj->paging_in_progress < bp->b_npages) {
                        printf("biodone: paging in progress(%d) < bp->b_npages(%d)\n",
                            obj->paging_in_progress, bp->b_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 ((bp->b_flags & (B_READ|B_FREEBUF|B_INVAL|B_NOCACHE|B_ERROR)) == B_READ) {
                        bp->b_flags |= B_CACHE;
                }

                for (i = 0; i < bp->b_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_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_pages[i] = m;
                                pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_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 ((bp->b_flags & B_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, lblkno: %d, flags: 0x%lx, npages: %d\n",
                                            bp->b_vp->v_mount->mnt_stat.f_iosize,
                                            (int) bp->b_lblkno,
                                            bp->b_flags, bp->b_npages);
                                else
                                        printf(" VDEV, lblkno: %d, flags: 0x%lx, npages: %d\n",
                                            (int) bp->b_lblkno,
                                            bp->b_flags, bp->b_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);
        }
        splx(s);
}

/*
 * 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;

                VOP_GETVOBJECT(vp, &obj);

                for (i = 0; i < bp->b_npages; i++) {
                        vm_page_t m = bp->b_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
                         * spl protection particularly.
                         */
                        if (m == bogus_page) {
                                m = vm_page_lookup(obj, OFF_TO_IDX(bp->b_offset) + i);
                                if (!m) {
                                        panic("vfs_unbusy_pages: page missing");
                                }
                                bp->b_pages[i] = m;
                                pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_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_offset + bp->b_bcount)
                eoff = bp->b_offset + 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)
                );
        }
}

/*
 * 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 buf * bp, int clear_modify)
{
        int i, bogus;

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

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

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

                bogus = 0;
                for (i = 0; i < bp->b_npages; i++) {
                        vm_page_t m = bp->b_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 buffer for a read ( i.e
                         * clear_modify == 0 ), it is important to do
                         * bogus_page replacement for valid pages in 
                         * partially instantiated buffers.  Partially 
                         * instantiated buffers can, in turn, occur when
                         * reconstituting a buffer from its VM backing store
                         * base.  We only have to do this if B_CACHE is
                         * clear ( which causes the I/O to occur in the
                         * first place ).  The replacement prevents the read
                         * I/O from overwriting potentially dirty VM-backed
                         * pages.  XXX bogus page replacement is, uh, bogus.
                         * It may not work properly with small-block devices.
                         * We need to find a better way.
                         */

                        vm_page_protect(m, VM_PROT_NONE);
                        if (clear_modify)
                                vfs_page_set_valid(bp, foff, i, m);
                        else if (m->valid == VM_PAGE_BITS_ALL &&
                                (bp->b_flags & B_CACHE) == 0) {
                                bp->b_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_pages, bp->b_npages);
        }

        /*
         * This is the easiest place to put the process accounting for the I/O
         * for now.
         */
        {
                struct proc *p;

                if ((p = curthread->td_proc) != NULL) {
                        if (bp->b_flags & B_READ)
                                p->p_stats->p_ru.ru_inblock++;
                        else
                                p->p_stats->p_ru.ru_oublock++;
                }
        }
}

/*
 * Tell the VM system that the pages associated with this buffer
 * are clean.  This is used for delayed writes where the data is
 * going to go to disk eventually without additional VM intevention.
 *
 * Note 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_offset;
                KASSERT(bp->b_offset != NOOFFSET,
                    ("vfs_clean_pages: no buffer offset"));
                for (i = 0; i < bp->b_npages; i++) {
                        vm_page_t m = bp->b_pages[i];
                        vm_ooffset_t noff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                        vm_ooffset_t eoff = noff;

                        if (eoff > bp->b_offset + bp->b_bufsize)
                                eoff = bp->b_offset + 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_offset.  Note that b_offset
 *      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_offset & PAGE_MASK);
                n = PAGE_SIZE - (base & PAGE_MASK);

                for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                        vm_page_t m = bp->b_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_npages == 1) && (bp->b_bufsize < PAGE_SIZE) &&
                    (bp->b_offset & PAGE_MASK) == 0) {
                        mask = (1 << (bp->b_bufsize / DEV_BSIZE)) - 1;
                        if ((bp->b_pages[0]->valid & mask) == mask) {
                                bp->b_resid = 0;
                                return;
                        }
                        if (((bp->b_pages[0]->flags & PG_ZERO) == 0) &&
                            ((bp->b_pages[0]->valid & mask) == 0)) {
                                bzero(bp->b_data, bp->b_bufsize);
                                bp->b_pages[0]->valid |= mask;
                                bp->b_resid = 0;
                                return;
                        }
                }
                ea = sa = bp->b_data;
                for(i=0;i<bp->b_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_pages[i]->valid & mask) == mask)
                                continue;
                        if ((bp->b_pages[i]->valid & mask) == 0) {
                                if ((bp->b_pages[i]->flags & PG_ZERO) == 0) {
                                        bzero(sa, ea - sa);
                                }
                        } else {
                                for (; sa < ea; sa += DEV_BSIZE, j++) {
                                        if (((bp->b_pages[i]->flags & PG_ZERO) == 0) &&
                                                (bp->b_pages[i]->valid & (1<<j)) == 0)
                                                bzero(sa, DEV_BSIZE);
                                }
                        }
                        bp->b_pages[i]->valid |= mask;
                        vm_page_flag_clear(bp->b_pages[i], PG_ZERO);
                }
                bp->b_resid = 0;
        } else {
                clrbuf(bp);
        }
}

/*
 * vm_hold_load_pages and vm_hold_unload pages get pages into
 * a buffers address space.  The pages are anonymous and are
 * not associated with a file object.
 */
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_pages[index] = p;
                vm_page_wakeup(p);
        }
        bp->b_npages = index;
}

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_pages[index];
                if (p && (index < bp->b_npages)) {
                        if (p->busy) {
                                printf("vm_hold_free_pages: blkno: %d, lblkno: %d\n",
                                        bp->b_blkno, bp->b_lblkno);
                        }
                        bp->b_pages[index] = NULL;
                        pmap_kremove(pg);
                        vm_page_busy(p);
                        vm_page_unwire(p, 0);
                        vm_page_free(p);
                }
        }
        bp->b_npages = newnpages;
}

/*
 * Map an IO request into kernel virtual address space.
 *
 * All requests are (re)mapped into kernel VA space.
 * Notice that we use b_bufsize for the size of the buffer
 * to be mapped.  b_bcount might be modified by the driver.
 */
int
vmapbuf(struct buf *bp)
{
        caddr_t addr, v, kva;
        vm_paddr_t pa;
        int pidx;
        int i;
        struct vm_page *m;

        if ((bp->b_flags & B_PHYS) == 0)
                panic("vmapbuf");
        if (bp->b_bufsize < 0)
                return (-1);
        for (v = bp->b_saveaddr,
                     addr = (caddr_t)trunc_page((vm_offset_t)bp->b_data),
                     pidx = 0;
             addr < bp->b_data + bp->b_bufsize;
             addr += PAGE_SIZE, v += PAGE_SIZE, pidx++) {
                /*
                 * Do the vm_fault if needed; do the copy-on-write thing
                 * when reading stuff off device into memory.
                 */
retry:
                i = vm_fault_quick((addr >= bp->b_data) ? addr : bp->b_data,
                        (bp->b_flags&B_READ)?(VM_PROT_READ|VM_PROT_WRITE):VM_PROT_READ);
                if (i < 0) {
                        for (i = 0; i < pidx; ++i) {
                            vm_page_unhold(bp->b_pages[i]);
                            bp->b_pages[i] = NULL;
                        }
                        return(-1);
                }

                /*
                 * WARNING!  If sparc support is MFCd in the future this will
                 * have to be changed from pmap_kextract() to pmap_extract()
                 * ala -current.
                 */
#ifdef __sparc64__
#error "If MFCing sparc support use pmap_extract"
#endif
                pa = pmap_kextract((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_pages[pidx] = m;
        }
        if (pidx > btoc(MAXPHYS))
                panic("vmapbuf: mapped more than MAXPHYS");
        pmap_qenter((vm_offset_t)bp->b_saveaddr, bp->b_pages, pidx);
        
        kva = bp->b_saveaddr;
        bp->b_npages = pidx;
        bp->b_saveaddr = bp->b_data;
        bp->b_data = kva + (((vm_offset_t) bp->b_data) & PAGE_MASK);
        return(0);
}

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

        if ((bp->b_flags & B_PHYS) == 0)
                panic("vunmapbuf");

        npages = bp->b_npages;
        pmap_qremove(trunc_page((vm_offset_t)bp->b_data),
                     npages);
        m = bp->b_pages;
        for (pidx = 0; pidx < npages; pidx++)
                vm_page_unhold(*m++);

        bp->b_data = bp->b_saveaddr;
}

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

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_error = %d, b_bufsize = %ld, b_bcount = %ld, "
                  "b_resid = %ld\nb_dev = (%d,%d), b_data = %p, "
                  "b_blkno = %d, b_pblkno = %d\n",
                  bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
                  major(bp->b_dev), minor(bp->b_dev),
                  bp->b_data, bp->b_blkno, bp->b_pblkno);
        if (bp->b_npages) {
                int i;
                db_printf("b_npages = %d, pages(OBJ, IDX, PA): ", bp->b_npages);
                for (i = 0; i < bp->b_npages; i++) {
                        vm_page_t m;
                        m = bp->b_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_npages)
                                db_printf(",");
                }
                db_printf("\n");
        }
}
#endif /* DDB */