MFV: nc(1) from OpenBSD 4.8.

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

/*-
 * Copyright (c) 2004 Poul-Henning Kamp
 * 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, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * 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/cdefs.h>
__FBSDID("$FreeBSD$");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/bio.h>
#include <sys/conf.h>
#include <sys/buf.h>
#include <sys/devicestat.h>
#include <sys/eventhandler.h>
#include <sys/fail.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mount.h>
#include <sys/mutex.h>
#include <sys/kernel.h>
#include <sys/kthread.h>
#include <sys/proc.h>
#include <sys/resourcevar.h>
#include <sys/sysctl.h>
#include <sys/vmmeter.h>
#include <sys/vnode.h>
#include <geom/geom.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 "opt_compat.h"
#include "opt_directio.h"
#include "opt_swap.h"

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

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

struct  buf_ops buf_ops_bio = {
        .bop_name       =       "buf_ops_bio",
        .bop_write      =       bufwrite,
        .bop_strategy   =       bufstrategy,
        .bop_sync       =       bufsync,
        .bop_bdflush    =       bufbdflush,
};

/*
 * XXX buf is global because kern_shutdown.c and ffs_checkoverlap has
 * carnal knowledge of buffers.  This knowledge should be moved to vfs_bio.c.
 */
struct buf *buf;                /* buffer header pool */

static struct proc *bufdaemonproc;

static int inmem(struct vnode *vp, daddr_t blkno);
static void vm_hold_free_pages(struct buf *bp, int newbsize);
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, vm_page_t m);
static void vfs_page_set_validclean(struct buf *bp, vm_ooffset_t off,
                vm_page_t m);
static void vfs_drain_busy_pages(struct buf *bp);
static void vfs_clean_pages_dirty_buf(struct buf *bp);
static void vfs_setdirty_locked_object(struct buf *bp);
static void vfs_vmio_release(struct buf *bp);
static int vfs_bio_clcheck(struct vnode *vp, int size,
                daddr_t lblkno, daddr_t blkno);
static int buf_do_flush(struct vnode *vp);
static int flushbufqueues(struct vnode *, int, int);
static void buf_daemon(void);
static void bremfreel(struct buf *bp);
#if defined(COMPAT_FREEBSD4) || defined(COMPAT_FREEBSD5) || \
    defined(COMPAT_FREEBSD6) || defined(COMPAT_FREEBSD7)
static int sysctl_bufspace(SYSCTL_HANDLER_ARGS);
#endif

int vmiodirenable = TRUE;
SYSCTL_INT(_vfs, OID_AUTO, vmiodirenable, CTLFLAG_RW, &vmiodirenable, 0,
    "Use the VM system for directory writes");
long runningbufspace;
SYSCTL_LONG(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD, &runningbufspace, 0,
    "Amount of presently outstanding async buffer io");
static long bufspace;
#if defined(COMPAT_FREEBSD4) || defined(COMPAT_FREEBSD5) || \
    defined(COMPAT_FREEBSD6) || defined(COMPAT_FREEBSD7)
SYSCTL_PROC(_vfs, OID_AUTO, bufspace, CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RD,
    &bufspace, 0, sysctl_bufspace, "L", "Virtual memory used for buffers");
#else
SYSCTL_LONG(_vfs, OID_AUTO, bufspace, CTLFLAG_RD, &bufspace, 0,
    "Virtual memory used for buffers");
#endif
static long maxbufspace;
SYSCTL_LONG(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RD, &maxbufspace, 0,
    "Maximum allowed value of bufspace (including buf_daemon)");
static long bufmallocspace;
SYSCTL_LONG(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD, &bufmallocspace, 0,
    "Amount of malloced memory for buffers");
static long maxbufmallocspace;
SYSCTL_LONG(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RW, &maxbufmallocspace, 0,
    "Maximum amount of malloced memory for buffers");
static long lobufspace;
SYSCTL_LONG(_vfs, OID_AUTO, lobufspace, CTLFLAG_RD, &lobufspace, 0,
    "Minimum amount of buffers we want to have");
long hibufspace;
SYSCTL_LONG(_vfs, OID_AUTO, hibufspace, CTLFLAG_RD, &hibufspace, 0,
    "Maximum allowed value of bufspace (excluding buf_daemon)");
static int bufreusecnt;
SYSCTL_INT(_vfs, OID_AUTO, bufreusecnt, CTLFLAG_RW, &bufreusecnt, 0,
    "Number of times we have reused a buffer");
static int buffreekvacnt;
SYSCTL_INT(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RW, &buffreekvacnt, 0,
    "Number of times we have freed the KVA space from some buffer");
static int bufdefragcnt;
SYSCTL_INT(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RW, &bufdefragcnt, 0,
    "Number of times we have had to repeat buffer allocation to defragment");
static long lorunningspace;
SYSCTL_LONG(_vfs, OID_AUTO, lorunningspace, CTLFLAG_RW, &lorunningspace, 0,
    "Minimum preferred space used for in-progress I/O");
static long hirunningspace;
SYSCTL_LONG(_vfs, OID_AUTO, hirunningspace, CTLFLAG_RW, &hirunningspace, 0,
    "Maximum amount of space to use for in-progress I/O");
int dirtybufferflushes;
SYSCTL_INT(_vfs, OID_AUTO, dirtybufferflushes, CTLFLAG_RW, &dirtybufferflushes,
    0, "Number of bdwrite to bawrite conversions to limit dirty buffers");
int bdwriteskip;
SYSCTL_INT(_vfs, OID_AUTO, bdwriteskip, CTLFLAG_RW, &bdwriteskip,
    0, "Number of buffers supplied to bdwrite with snapshot deadlock risk");
int altbufferflushes;
SYSCTL_INT(_vfs, OID_AUTO, altbufferflushes, CTLFLAG_RW, &altbufferflushes,
    0, "Number of fsync flushes to limit dirty buffers");
static int recursiveflushes;
SYSCTL_INT(_vfs, OID_AUTO, recursiveflushes, CTLFLAG_RW, &recursiveflushes,
    0, "Number of flushes skipped due to being recursive");
static int numdirtybuffers;
SYSCTL_INT(_vfs, OID_AUTO, numdirtybuffers, CTLFLAG_RD, &numdirtybuffers, 0,
    "Number of buffers that are dirty (has unwritten changes) at the moment");
static int lodirtybuffers;
SYSCTL_INT(_vfs, OID_AUTO, lodirtybuffers, CTLFLAG_RW, &lodirtybuffers, 0,
    "How many buffers we want to have free before bufdaemon can sleep");
static int hidirtybuffers;
SYSCTL_INT(_vfs, OID_AUTO, hidirtybuffers, CTLFLAG_RW, &hidirtybuffers, 0,
    "When the number of dirty buffers is considered severe");
int dirtybufthresh;
SYSCTL_INT(_vfs, OID_AUTO, dirtybufthresh, CTLFLAG_RW, &dirtybufthresh,
    0, "Number of bdwrite to bawrite conversions to clear dirty buffers");
static int numfreebuffers;
SYSCTL_INT(_vfs, OID_AUTO, numfreebuffers, CTLFLAG_RD, &numfreebuffers, 0,
    "Number of free buffers");
static int lofreebuffers;
SYSCTL_INT(_vfs, OID_AUTO, lofreebuffers, CTLFLAG_RW, &lofreebuffers, 0,
   "XXX Unused");
static int hifreebuffers;
SYSCTL_INT(_vfs, OID_AUTO, hifreebuffers, CTLFLAG_RW, &hifreebuffers, 0,
   "XXX Complicatedly unused");
static int getnewbufcalls;
SYSCTL_INT(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RW, &getnewbufcalls, 0,
   "Number of calls to getnewbuf");
static int getnewbufrestarts;
SYSCTL_INT(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RW, &getnewbufrestarts, 0,
    "Number of times getnewbuf has had to restart a buffer aquisition");
static int flushbufqtarget = 100;
SYSCTL_INT(_vfs, OID_AUTO, flushbufqtarget, CTLFLAG_RW, &flushbufqtarget, 0,
    "Amount of work to do in flushbufqueues when helping bufdaemon");
static long notbufdflashes;
SYSCTL_LONG(_vfs, OID_AUTO, notbufdflashes, CTLFLAG_RD, &notbufdflashes, 0,
    "Number of dirty buffer flushes done by the bufdaemon helpers");

/*
 * Wakeup point for bufdaemon, as well as indicator of whether it is already
 * active.  Set to 1 when the bufdaemon is already "on" the queue, 0 when it
 * is idling.
 */
static int bd_request;

/*
 * Request for the buf daemon to write more buffers than is indicated by
 * lodirtybuf.  This may be necessary to push out excess dependencies or
 * defragment the address space where a simple count of the number of dirty
 * buffers is insufficient to characterize the demand for flushing them.
 */
static int bd_speedupreq;

/*
 * This lock synchronizes access to bd_request.
 */
static struct mtx bdlock;

/*
 * 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;

/*
 * Synchronization (sleep/wakeup) variable for active buffer space requests.
 * Set when wait starts, cleared prior to wakeup().
 * Used in runningbufwakeup() and waitrunningbufspace().
 */
static int runningbufreq;

/*
 * This lock protects the runningbufreq and synchronizes runningbufwakeup and
 * waitrunningbufspace().
 */
static struct mtx rbreqlock;

/* 
 * Synchronization (sleep/wakeup) variable for buffer requests.
 * Can contain the VFS_BIO_NEED flags defined below; setting/clearing is done
 * by and/or.
 * Used in numdirtywakeup(), bufspacewakeup(), bufcountwakeup(), bwillwrite(),
 * getnewbuf(), and getblk().
 */
static int needsbuffer;

/*
 * Lock that protects needsbuffer and the sleeps/wakeups surrounding it.
 */
static struct mtx nblock;

/*
 * Definitions for the buffer free lists.
 */
#define BUFFER_QUEUES   6       /* number of free buffer queues */

#define QUEUE_NONE      0       /* on no queue */
#define QUEUE_CLEAN     1       /* non-B_DELWRI buffers */
#define QUEUE_DIRTY     2       /* B_DELWRI buffers */
#define QUEUE_DIRTY_GIANT 3     /* B_DELWRI buffers that need giant */
#define QUEUE_EMPTYKVA  4       /* empty buffer headers w/KVA assignment */
#define QUEUE_EMPTY     5       /* empty buffer headers */
#define QUEUE_SENTINEL  1024    /* not an queue index, but mark for sentinel */

/* Queues for free buffers with various properties */
static TAILQ_HEAD(bqueues, buf) bufqueues[BUFFER_QUEUES] = { { 0 } };

/* Lock for the bufqueues */
static struct mtx bqlock;

/*
 * Single global constant for BUF_WMESG, to avoid getting multiple references.
 * buf_wmesg is referred from macros.
 */
const char *buf_wmesg = BUF_WMESG;

#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 */

#if defined(COMPAT_FREEBSD4) || defined(COMPAT_FREEBSD5) || \
    defined(COMPAT_FREEBSD6) || defined(COMPAT_FREEBSD7)
static int
sysctl_bufspace(SYSCTL_HANDLER_ARGS)
{
        long lvalue;
        int ivalue;

        if (sizeof(int) == sizeof(long) || req->oldlen >= sizeof(long))
                return (sysctl_handle_long(oidp, arg1, arg2, req));
        lvalue = *(long *)arg1;
        if (lvalue > INT_MAX)
                /* On overflow, still write out a long to trigger ENOMEM. */
                return (sysctl_handle_long(oidp, &lvalue, 0, req));
        ivalue = lvalue;
        return (sysctl_handle_int(oidp, &ivalue, 0, req));
}
#endif

#ifdef DIRECTIO
extern void ffs_rawread_setup(void);
#endif /* DIRECTIO */
/*
 *      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) {
                mtx_lock(&nblock);
                if (needsbuffer & VFS_BIO_NEED_DIRTYFLUSH) {
                        needsbuffer &= ~VFS_BIO_NEED_DIRTYFLUSH;
                        wakeup(&needsbuffer);
                }
                mtx_unlock(&nblock);
        }
}

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

/*
 * runningbufwakeup() - in-progress I/O accounting.
 *
 */
void
runningbufwakeup(struct buf *bp)
{

        if (bp->b_runningbufspace) {
                atomic_subtract_long(&runningbufspace, bp->b_runningbufspace);
                bp->b_runningbufspace = 0;
                mtx_lock(&rbreqlock);
                if (runningbufreq && runningbufspace <= lorunningspace) {
                        runningbufreq = 0;
                        wakeup(&runningbufreq);
                }
                mtx_unlock(&rbreqlock);
        }
}

/*
 *      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(struct buf *bp) 
{
        int old;

        KASSERT((bp->b_vflags & BV_INFREECNT) == 0,
            ("buf %p already counted as free", bp));
        if (bp->b_bufobj != NULL)
                mtx_assert(BO_MTX(bp->b_bufobj), MA_OWNED);
        bp->b_vflags |= BV_INFREECNT;
        old = atomic_fetchadd_int(&numfreebuffers, 1);
        KASSERT(old >= 0 && old < nbuf,
            ("numfreebuffers climbed to %d", old + 1));
        mtx_lock(&nblock);
        if (needsbuffer) {
                needsbuffer &= ~VFS_BIO_NEED_ANY;
                if (numfreebuffers >= hifreebuffers)
                        needsbuffer &= ~VFS_BIO_NEED_FREE;
                wakeup(&needsbuffer);
        }
        mtx_unlock(&nblock);
}

/*
 *      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.
 */
void
waitrunningbufspace(void)
{

        mtx_lock(&rbreqlock);
        while (runningbufspace > hirunningspace) {
                ++runningbufreq;
                msleep(&runningbufreq, &rbreqlock, PVM, "wdrain", 0);
        }
        mtx_unlock(&rbreqlock);
}


/*
 *      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)
{

        VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED);
        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;
        }
}

/* Wake up the buffer daemon if necessary */
static __inline
void
bd_wakeup(int dirtybuflevel)
{

        mtx_lock(&bdlock);
        if (bd_request == 0 && numdirtybuffers >= dirtybuflevel) {
                bd_request = 1;
                wakeup(&bd_request);
        }
        mtx_unlock(&bdlock);
}

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

void
bd_speedup(void)
{
        int needwake;

        mtx_lock(&bdlock);
        needwake = 0;
        if (bd_speedupreq == 0 || bd_request == 0)
                needwake = 1;
        bd_speedupreq = 1;
        bd_request = 1;
        if (needwake)
                wakeup(&bd_request);
        mtx_unlock(&bdlock);
}

/*
 * Calculating buffer cache scaling values and reserve space for buffer
 * headers.  This is called during low level kernel initialization and
 * may be called more then once.  We CANNOT write to the memory area
 * being reserved at this time.
 */
caddr_t
kern_vfs_bio_buffer_alloc(caddr_t v, long physmem_est)
{
        int tuned_nbuf;
        long maxbuf;

        /*
         * physmem_est is in pages.  Convert it to kilobytes (assumes
         * PAGE_SIZE is >= 1K)
         */
        physmem_est = physmem_est * (PAGE_SIZE / 1024);

        /*
         * The nominal buffer size (and minimum KVA allocation) is BKVASIZE.
         * For the first 64MB of ram nominally allocate sufficient buffers to
         * cover 1/4 of our ram.  Beyond the first 64MB allocate additional
         * buffers to cover 1/10 of our ram over 64MB.  When auto-sizing
         * the buffer cache we limit the eventual kva reservation to
         * maxbcache bytes.
         *
         * factor represents the 1/4 x ram conversion.
         */
        if (nbuf == 0) {
                int factor = 4 * BKVASIZE / 1024;

                nbuf = 50;
                if (physmem_est > 4096)
                        nbuf += min((physmem_est - 4096) / factor,
                            65536 / factor);
                if (physmem_est > 65536)
                        nbuf += (physmem_est - 65536) * 2 / (factor * 5);

                if (maxbcache && nbuf > maxbcache / BKVASIZE)
                        nbuf = maxbcache / BKVASIZE;
                tuned_nbuf = 1;
        } else
                tuned_nbuf = 0;

        /* XXX Avoid unsigned long overflows later on with maxbufspace. */
        maxbuf = (LONG_MAX / 3) / BKVASIZE;
        if (nbuf > maxbuf) {
                if (!tuned_nbuf)
                        printf("Warning: nbufs lowered from %d to %ld\n", nbuf,
                            maxbuf);
                nbuf = maxbuf;
        }

        /*
         * swbufs are used as temporary holders for I/O, such as paging I/O.
         * We have no less then 16 and no more then 256.
         */
        nswbuf = max(min(nbuf/4, 256), 16);
#ifdef NSWBUF_MIN
        if (nswbuf < NSWBUF_MIN)
                nswbuf = NSWBUF_MIN;
#endif
#ifdef DIRECTIO
        ffs_rawread_setup();
#endif

        /*
         * Reserve space for the buffer cache buffers
         */
        swbuf = (void *)v;
        v = (caddr_t)(swbuf + nswbuf);
        buf = (void *)v;
        v = (caddr_t)(buf + nbuf);

        return(v);
}

/* Initialize the buffer subsystem.  Called before use of any buffers. */
void
bufinit(void)
{
        struct buf *bp;
        int i;

        mtx_init(&bqlock, "buf queue lock", NULL, MTX_DEF);
        mtx_init(&rbreqlock, "runningbufspace lock", NULL, MTX_DEF);
        mtx_init(&nblock, "needsbuffer lock", NULL, MTX_DEF);
        mtx_init(&bdlock, "buffer daemon lock", NULL, MTX_DEF);

        /* 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_rcred = NOCRED;
                bp->b_wcred = NOCRED;
                bp->b_qindex = QUEUE_EMPTY;
                bp->b_vflags = BV_INFREECNT;    /* buf is counted as free */
                bp->b_xflags = 0;
                LIST_INIT(&bp->b_dep);
                BUF_LOCKINIT(bp);
                TAILQ_INSERT_TAIL(&bufqueues[QUEUE_EMPTY], bp, b_freelist);
        }

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

        /*
         * Note: The 16 MB upper limit for hirunningspace was chosen
         * arbitrarily and may need further tuning. It corresponds to
         * 128 outstanding write IO requests (if IO size is 128 KiB),
         * which fits with many RAID controllers' tagged queuing limits.
         * The lower 1 MB limit is the historical upper limit for
         * hirunningspace.
         */
        hirunningspace = lmax(lmin(roundup(hibufspace / 64, MAXBSIZE),
            16 * 1024 * 1024), 1024 * 1024);
        lorunningspace = roundup(hirunningspace / 2, MAXBSIZE);

/*
 * 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;
        dirtybufthresh = hidirtybuffers * 9 / 10;
        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 ((long)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;

        bogus_page = vm_page_alloc(NULL, 0, VM_ALLOC_NOOBJ |
            VM_ALLOC_NORMAL | VM_ALLOC_WIRED);
}

/*
 * bfreekva() - free the kva allocation for a buffer.
 *
 *      Since this call frees up buffer space, we call bufspacewakeup().
 */
static void
bfreekva(struct buf *bp)
{

        if (bp->b_kvasize) {
                atomic_add_int(&buffreekvacnt, 1);
                atomic_subtract_long(&bufspace, bp->b_kvasize);
                vm_map_remove(buffer_map, (vm_offset_t) bp->b_kvabase,
                    (vm_offset_t) bp->b_kvabase + bp->b_kvasize);
                bp->b_kvasize = 0;
                bufspacewakeup();
        }
}

/*
 *      bremfree:
 *
 *      Mark the buffer for removal from the appropriate free list in brelse.
 *      
 */
void
bremfree(struct buf *bp)
{
        int old;

        CTR3(KTR_BUF, "bremfree(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT((bp->b_flags & B_REMFREE) == 0,
            ("bremfree: buffer %p already marked for delayed removal.", bp));
        KASSERT(bp->b_qindex != QUEUE_NONE,
            ("bremfree: buffer %p not on a queue.", bp));
        BUF_ASSERT_HELD(bp);

        bp->b_flags |= B_REMFREE;
        /* Fixup numfreebuffers count.  */
        if ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0) {
                KASSERT((bp->b_vflags & BV_INFREECNT) != 0,
                    ("buf %p not counted in numfreebuffers", bp));
                if (bp->b_bufobj != NULL)
                        mtx_assert(BO_MTX(bp->b_bufobj), MA_OWNED);
                bp->b_vflags &= ~BV_INFREECNT;
                old = atomic_fetchadd_int(&numfreebuffers, -1);
                KASSERT(old > 0, ("numfreebuffers dropped to %d", old - 1));
        }
}

/*
 *      bremfreef:
 *
 *      Force an immediate removal from a free list.  Used only in nfs when
 *      it abuses the b_freelist pointer.
 */
void
bremfreef(struct buf *bp)
{
        mtx_lock(&bqlock);
        bremfreel(bp);
        mtx_unlock(&bqlock);
}

/*
 *      bremfreel:
 *
 *      Removes a buffer from the free list, must be called with the
 *      bqlock held.
 */
static void
bremfreel(struct buf *bp)
{
        int old;

        CTR3(KTR_BUF, "bremfreel(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_qindex != QUEUE_NONE,
            ("bremfreel: buffer %p not on a queue.", bp));
        BUF_ASSERT_HELD(bp);
        mtx_assert(&bqlock, MA_OWNED);

        TAILQ_REMOVE(&bufqueues[bp->b_qindex], bp, b_freelist);
        bp->b_qindex = QUEUE_NONE;
        /*
         * If this was a delayed bremfree() we only need to remove the buffer
         * from the queue and return the stats are already done.
         */
        if (bp->b_flags & B_REMFREE) {
                bp->b_flags &= ~B_REMFREE;
                return;
        }
        /*
         * Fixup numfreebuffers count.  If the buffer is invalid or not
         * delayed-write, the buffer was free and we must decrement
         * numfreebuffers.
         */
        if ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0) {
                KASSERT((bp->b_vflags & BV_INFREECNT) != 0,
                    ("buf %p not counted in numfreebuffers", bp));
                if (bp->b_bufobj != NULL)
                        mtx_assert(BO_MTX(bp->b_bufobj), MA_OWNED);
                bp->b_vflags &= ~BV_INFREECNT;
                old = atomic_fetchadd_int(&numfreebuffers, -1);
                KASSERT(old > 0, ("numfreebuffers dropped to %d", old - 1));
        }
}


/*
 * Get a buffer with the specified data.  Look in the cache first.  We
 * must clear BIO_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() ).  This is really just a special case of breadn().
 */
int
bread(struct vnode * vp, daddr_t blkno, int size, struct ucred * cred,
    struct buf **bpp)
{

        return (breadn(vp, blkno, size, 0, 0, 0, cred, bpp));
}

/*
 * Attempt to initiate asynchronous I/O on read-ahead blocks.  We must
 * clear BIO_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.
 */
void
breada(struct vnode * vp, daddr_t * rablkno, int * rabsize,
    int cnt, struct ucred * cred)
{
        struct buf *rabp;
        int i;

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

                if ((rabp->b_flags & B_CACHE) == 0) {
                        if (!TD_IS_IDLETHREAD(curthread))
                                curthread->td_ru.ru_inblock++;
                        rabp->b_flags |= B_ASYNC;
                        rabp->b_flags &= ~B_INVAL;
                        rabp->b_ioflags &= ~BIO_ERROR;
                        rabp->b_iocmd = BIO_READ;
                        if (rabp->b_rcred == NOCRED && cred != NOCRED)
                                rabp->b_rcred = crhold(cred);
                        vfs_busy_pages(rabp, 0);
                        BUF_KERNPROC(rabp);
                        rabp->b_iooffset = dbtob(rabp->b_blkno);
                        bstrategy(rabp);
                } else {
                        brelse(rabp);
                }
        }
}

/*
 * Operates like bread, but also starts asynchronous I/O on
 * read-ahead blocks.
 */
int
breadn(struct vnode * vp, daddr_t blkno, int size,
    daddr_t * rablkno, int *rabsize,
    int cnt, struct ucred * cred, struct buf **bpp)
{
        struct buf *bp;
        int rv = 0, readwait = 0;

        CTR3(KTR_BUF, "breadn(%p, %jd, %d)", vp, blkno, size);
        *bpp = bp = getblk(vp, blkno, size, 0, 0, 0);

        /* if not found in cache, do some I/O */
        if ((bp->b_flags & B_CACHE) == 0) {
                if (!TD_IS_IDLETHREAD(curthread))
                        curthread->td_ru.ru_inblock++;
                bp->b_iocmd = BIO_READ;
                bp->b_flags &= ~B_INVAL;
                bp->b_ioflags &= ~BIO_ERROR;
                if (bp->b_rcred == NOCRED && cred != NOCRED)
                        bp->b_rcred = crhold(cred);
                vfs_busy_pages(bp, 0);
                bp->b_iooffset = dbtob(bp->b_blkno);
                bstrategy(bp);
                ++readwait;
        }

        breada(vp, rablkno, rabsize, cnt, cred);

        if (readwait) {
                rv = bufwait(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
bufwrite(struct buf *bp)
{
        int oldflags;
        struct vnode *vp;
        int vp_md;

        CTR3(KTR_BUF, "bufwrite(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        if (bp->b_flags & B_INVAL) {
                brelse(bp);
                return (0);
        }

        oldflags = bp->b_flags;

        BUF_ASSERT_HELD(bp);

        if (bp->b_pin_count > 0)
                bunpin_wait(bp);

        KASSERT(!(bp->b_vflags & BV_BKGRDINPROG),
            ("FFS background buffer should not get here %p", bp));

        vp = bp->b_vp;
        if (vp)
                vp_md = vp->v_vflag & VV_MD;
        else
                vp_md = 0;

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

        bp->b_flags &= ~B_DONE;
        bp->b_ioflags &= ~BIO_ERROR;
        bp->b_flags |= B_CACHE;
        bp->b_iocmd = BIO_WRITE;

        bufobj_wref(bp->b_bufobj);
        vfs_busy_pages(bp, 1);

        /*
         * Normal bwrites pipeline writes
         */
        bp->b_runningbufspace = bp->b_bufsize;
        atomic_add_long(&runningbufspace, bp->b_runningbufspace);

        if (!TD_IS_IDLETHREAD(curthread))
                curthread->td_ru.ru_oublock++;
        if (oldflags & B_ASYNC)
                BUF_KERNPROC(bp);
        bp->b_iooffset = dbtob(bp->b_blkno);
        bstrategy(bp);

        if ((oldflags & B_ASYNC) == 0) {
                int rtval = bufwait(bp);
                brelse(bp);
                return (rtval);
        } else {
                /*
                 * don't allow the async write to saturate the I/O
                 * system.  We will not deadlock here because
                 * we are blocking waiting for I/O that is already in-progress
                 * to complete. We do not block here if it is the update
                 * or syncer daemon trying to clean up as that can lead
                 * to deadlock.
                 */
                if ((curthread->td_pflags & TDP_NORUNNINGBUF) == 0 && !vp_md)
                        waitrunningbufspace();
        }

        return (0);
}

void
bufbdflush(struct bufobj *bo, struct buf *bp)
{
        struct buf *nbp;

        if (bo->bo_dirty.bv_cnt > dirtybufthresh + 10) {
                (void) VOP_FSYNC(bp->b_vp, MNT_NOWAIT, curthread);
                altbufferflushes++;
        } else if (bo->bo_dirty.bv_cnt > dirtybufthresh) {
                BO_LOCK(bo);
                /*
                 * Try to find a buffer to flush.
                 */
                TAILQ_FOREACH(nbp, &bo->bo_dirty.bv_hd, b_bobufs) {
                        if ((nbp->b_vflags & BV_BKGRDINPROG) ||
                            BUF_LOCK(nbp,
                                     LK_EXCLUSIVE | LK_NOWAIT, NULL))
                                continue;
                        if (bp == nbp)
                                panic("bdwrite: found ourselves");
                        BO_UNLOCK(bo);
                        /* Don't countdeps with the bo lock held. */
                        if (buf_countdeps(nbp, 0)) {
                                BO_LOCK(bo);
                                BUF_UNLOCK(nbp);
                                continue;
                        }
                        if (nbp->b_flags & B_CLUSTEROK) {
                                vfs_bio_awrite(nbp);
                        } else {
                                bremfree(nbp);
                                bawrite(nbp);
                        }
                        dirtybufferflushes++;
                        break;
                }
                if (nbp == NULL)
                        BO_UNLOCK(bo);
        }
}

/*
 * 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)
{
        struct thread *td = curthread;
        struct vnode *vp;
        struct bufobj *bo;

        CTR3(KTR_BUF, "bdwrite(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        BUF_ASSERT_HELD(bp);

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

        /*
         * If we have too many dirty buffers, don't create any more.
         * If we are wildly over our limit, then force a complete
         * cleanup. Otherwise, just keep the situation from getting
         * out of control. Note that we have to avoid a recursive
         * disaster and not try to clean up after our own cleanup!
         */
        vp = bp->b_vp;
        bo = bp->b_bufobj;
        if ((td->td_pflags & (TDP_COWINPROGRESS|TDP_INBDFLUSH)) == 0) {
                td->td_pflags |= TDP_INBDFLUSH;
                BO_BDFLUSH(bo, bp);
                td->td_pflags &= ~TDP_INBDFLUSH;
        } else
                recursiveflushes++;

        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 (vp->v_type != VCHR && bp->b_lblkno == bp->b_blkno) {
                VOP_BMAP(vp, bp->b_lblkno, NULL, &bp->b_blkno, NULL, NULL);
        }

        /*
         * Set the *dirty* buffer range based upon the VM system dirty
         * pages.
         *
         * Mark the buffer pages as clean.  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_dirty_buf(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 BIO_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.
 *
 *      The buffer must be on QUEUE_NONE.
 */
void
bdirty(struct buf *bp)
{

        CTR3(KTR_BUF, "bdirty(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        KASSERT(bp->b_flags & B_REMFREE || bp->b_qindex == QUEUE_NONE,
            ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
        BUF_ASSERT_HELD(bp);
        bp->b_flags &= ~(B_RELBUF);
        bp->b_iocmd = BIO_WRITE;

        if ((bp->b_flags & B_DELWRI) == 0) {
                bp->b_flags |= /* XXX B_DONE | */ B_DELWRI;
                reassignbuf(bp);
                atomic_add_int(&numdirtybuffers, 1);
                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.
 *      
 *      The buffer must be on QUEUE_NONE.
 */

void
bundirty(struct buf *bp)
{

        CTR3(KTR_BUF, "bundirty(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(bp->b_bufobj != NULL, ("No b_bufobj %p", bp));
        KASSERT(bp->b_flags & B_REMFREE || bp->b_qindex == QUEUE_NONE,
            ("bundirty: buffer %p still on queue %d", bp, bp->b_qindex));
        BUF_ASSERT_HELD(bp);

        if (bp->b_flags & B_DELWRI) {
                bp->b_flags &= ~B_DELWRI;
                reassignbuf(bp);
                atomic_subtract_int(&numdirtybuffers, 1);
                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) bwrite(bp);
}

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

void
bwillwrite(void)
{

        if (numdirtybuffers >= hidirtybuffers) {
                mtx_lock(&nblock);
                while (numdirtybuffers >= hidirtybuffers) {
                        bd_wakeup(1);
                        needsbuffer |= VFS_BIO_NEED_DIRTYFLUSH;
                        msleep(&needsbuffer, &nblock,
                            (PRIBIO + 4), "flswai", 0);
                }
                mtx_unlock(&nblock);
        }
}

/*
 * Return true if we have too many dirty buffers.
 */
int
buf_dirty_count_severe(void)
{

        return(numdirtybuffers >= hidirtybuffers);
}

static __noinline int
buf_vm_page_count_severe(void)
{

        KFAIL_POINT_CODE(DEBUG_FP, buf_pressure, return 1);

        return vm_page_count_severe();
}

/*
 *      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)
{
        CTR3(KTR_BUF, "brelse(%p) vp %p flags %X",
            bp, bp->b_vp, bp->b_flags);
        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
            ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));

        if (bp->b_flags & B_MANAGED) {
                bqrelse(bp);
                return;
        }

        if (bp->b_iocmd == BIO_WRITE && (bp->b_ioflags & BIO_ERROR) &&
            bp->b_error == EIO && !(bp->b_flags & B_INVAL)) {
                /*
                 * Failed write, redirty.  Must clear BIO_ERROR to prevent
                 * pages from being scrapped.  If the error is anything
                 * other than an I/O error (EIO), assume that retrying
                 * is futile.
                 */
                bp->b_ioflags &= ~BIO_ERROR;
                bdirty(bp);
        } else if ((bp->b_flags & (B_NOCACHE | B_INVAL)) ||
            (bp->b_ioflags & BIO_ERROR) || (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_EMPTY(&bp->b_dep))
                        buf_deallocate(bp);
                if (bp->b_flags & B_DELWRI) {
                        atomic_subtract_int(&numdirtybuffers, 1);
                        numdirtywakeup(lodirtybuffers);
                }
                bp->b_flags &= ~(B_DELWRI | B_CACHE);
                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 (buf_vm_page_count_severe()) {
                /*
                 * The locking of the BO_LOCK is not necessary since
                 * BKGRDINPROG cannot be set while we hold the buf
                 * lock, it can only be cleared if it is already
                 * pending.
                 */
                if (bp->b_vp) {
                        if (!(bp->b_vflags & BV_BKGRDINPROG))
                                bp->b_flags |= B_RELBUF;
                } else
                        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 BIO_ERROR or B_NOCACHE is set, pages in the VM object will be
         * invalidated.  BIO_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_mount != NULL &&
                 (bp->b_vp->v_mount->mnt_vfc->vfc_flags & VFCF_NETWORK) != 0 &&
                 !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;

                obj = bp->b_bufobj->bo_object;

                /*
                 * 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;
                VM_OBJECT_LOCK(obj);
                for (i = 0; i < bp->b_npages; i++) {
                        int had_bogus = 0;

                        m = bp->b_pages[i];

                        /*
                         * If we hit a bogus page, fixup *all* the bogus pages
                         * now.
                         */
                        if (m == bogus_page) {
                                poff = OFF_TO_IDX(bp->b_offset);
                                had_bogus = 1;

                                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\n");
                                                }
                                                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];
                        }
                        if ((bp->b_flags & B_NOCACHE) ||
                            (bp->b_ioflags & BIO_ERROR &&
                             bp->b_iocmd == BIO_READ)) {
                                int poffset = foff & PAGE_MASK;
                                int presid = resid > (PAGE_SIZE - poffset) ?
                                        (PAGE_SIZE - poffset) : resid;

                                KASSERT(presid >= 0, ("brelse: extra page"));
                                vm_page_set_invalid(m, poffset, presid);
                                if (had_bogus)
                                        printf("avoided corruption bug in bogus_page/brelse code\n");
                        }
                        resid -= PAGE_SIZE - (foff & PAGE_MASK);
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                }
                VM_OBJECT_UNLOCK(obj);
                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);
                }

        } else if ((bp->b_flags & (B_INVAL | B_RELBUF)) != 0) {
                if (bp->b_bufsize != 0)
                        allocbuf(bp, 0);
                if (bp->b_vp != NULL)
                        brelvp(bp);
        }
                        
        if (BUF_LOCKRECURSED(bp)) {
                /* do not release to free list */
                BUF_UNLOCK(bp);
                return;
        }

        /* enqueue */
        mtx_lock(&bqlock);
        /* Handle delayed bremfree() processing. */
        if (bp->b_flags & B_REMFREE) {
                struct bufobj *bo;

                bo = bp->b_bufobj;
                if (bo != NULL)
                        BO_LOCK(bo);
                bremfreel(bp);
                if (bo != NULL)
                        BO_UNLOCK(bo);
        }
        if (bp->b_qindex != QUEUE_NONE)
                panic("brelse: free buffer onto another queue???");

        /*
         * If the buffer has junk contents signal it and eventually
         * clean up B_DELWRI and diassociate the vnode so that gbincore()
         * doesn't find it.
         */
        if (bp->b_bufsize == 0 || (bp->b_ioflags & BIO_ERROR) != 0 ||
            (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF)) != 0)
                bp->b_flags |= B_INVAL;
        if (bp->b_flags & B_INVAL) {
                if (bp->b_flags & B_DELWRI)
                        bundirty(bp);
                if (bp->b_vp)
                        brelvp(bp);
        }

        /* buffers with no memory */
        if (bp->b_bufsize == 0) {
                bp->b_xflags &= ~(BX_BKGRDWRITE | BX_ALTDATA);
                if (bp->b_vflags & BV_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);
        /* buffers with junk contents */
        } else if (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF) ||
            (bp->b_ioflags & BIO_ERROR)) {
                bp->b_xflags &= ~(BX_BKGRDWRITE | BX_ALTDATA);
                if (bp->b_vflags & BV_BKGRDINPROG)
                        panic("losing buffer 2");
                bp->b_qindex = QUEUE_CLEAN;
                TAILQ_INSERT_HEAD(&bufqueues[QUEUE_CLEAN], bp, b_freelist);
        /* remaining buffers */
        } else {
                if ((bp->b_flags & (B_DELWRI|B_NEEDSGIANT)) ==
                    (B_DELWRI|B_NEEDSGIANT))
                        bp->b_qindex = QUEUE_DIRTY_GIANT;
                else if (bp->b_flags & B_DELWRI)
                        bp->b_qindex = QUEUE_DIRTY;
                else
                        bp->b_qindex = QUEUE_CLEAN;
                if (bp->b_flags & B_AGE)
                        TAILQ_INSERT_HEAD(&bufqueues[bp->b_qindex], bp, b_freelist);
                else
                        TAILQ_INSERT_TAIL(&bufqueues[bp->b_qindex], bp, b_freelist);
        }
        mtx_unlock(&bqlock);

        /*
         * 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_DELWRI)) {
                struct bufobj *bo;

                bo = bp->b_bufobj;
                if (bo != NULL)
                        BO_LOCK(bo);
                bufcountwakeup(bp);
                if (bo != NULL)
                        BO_UNLOCK(bo);
        }

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

        bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF | B_DIRECT);
        if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
                panic("brelse: not dirty");
        /* unlock */
        BUF_UNLOCK(bp);
}

/*
 * 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)
{
        struct bufobj *bo;

        CTR3(KTR_BUF, "bqrelse(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
            ("bqrelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));

        if (BUF_LOCKRECURSED(bp)) {
                /* do not release to free list */
                BUF_UNLOCK(bp);
                return;
        }

        bo = bp->b_bufobj;
        if (bp->b_flags & B_MANAGED) {
                if (bp->b_flags & B_REMFREE) {
                        mtx_lock(&bqlock);
                        if (bo != NULL)
                                BO_LOCK(bo);
                        bremfreel(bp);
                        if (bo != NULL)
                                BO_UNLOCK(bo);
                        mtx_unlock(&bqlock);
                }
                bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
                BUF_UNLOCK(bp);
                return;
        }

        mtx_lock(&bqlock);
        /* Handle delayed bremfree() processing. */
        if (bp->b_flags & B_REMFREE) {
                if (bo != NULL)
                        BO_LOCK(bo);
                bremfreel(bp);
                if (bo != NULL)
                        BO_UNLOCK(bo);
        }
        if (bp->b_qindex != QUEUE_NONE)
                panic("bqrelse: free buffer onto another queue???");
        /* buffers with stale but valid contents */
        if (bp->b_flags & B_DELWRI) {
                if (bp->b_flags & B_NEEDSGIANT)
                        bp->b_qindex = QUEUE_DIRTY_GIANT;
                else
                        bp->b_qindex = QUEUE_DIRTY;
                TAILQ_INSERT_TAIL(&bufqueues[bp->b_qindex], bp, b_freelist);
        } else {
                /*
                 * The locking of the BO_LOCK for checking of the
                 * BV_BKGRDINPROG is not necessary since the
                 * BV_BKGRDINPROG cannot be set while we hold the buf
                 * lock, it can only be cleared if it is already
                 * pending.
                 */
                if (!buf_vm_page_count_severe() || (bp->b_vflags & BV_BKGRDINPROG)) {
                        bp->b_qindex = QUEUE_CLEAN;
                        TAILQ_INSERT_TAIL(&bufqueues[QUEUE_CLEAN], bp,
                            b_freelist);
                } else {
                        /*
                         * 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*.
                         */
                        mtx_unlock(&bqlock);
                        brelse(bp);
                        return;
                }
        }
        mtx_unlock(&bqlock);

        if ((bp->b_flags & B_INVAL) || !(bp->b_flags & B_DELWRI)) {
                if (bo != NULL)
                        BO_LOCK(bo);
                bufcountwakeup(bp);
                if (bo != NULL)
                        BO_UNLOCK(bo);
        }

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

        bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
        if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
                panic("bqrelse: not dirty");
        /* unlock */
        BUF_UNLOCK(bp);
}

/* Give pages used by the bp back to the VM system (where possible) */
static void
vfs_vmio_release(struct buf *bp)
{
        int i;
        vm_page_t m;

        VM_OBJECT_LOCK(bp->b_bufobj->bo_object);
        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_lock(m);
                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->oflags & VPO_BUSY) == 0 && m->busy == 0 &&
                    m->wire_count == 0) {
                        /*
                         * 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_free(m);
                        } else if (bp->b_flags & B_DIRECT) {
                                vm_page_try_to_free(m);
                        } else if (buf_vm_page_count_severe()) {
                                vm_page_try_to_cache(m);
                        }
                }
                vm_page_unlock(m);
        }
        VM_OBJECT_UNLOCK(bp->b_bufobj->bo_object);
        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 at a particular lbn is available for a clustered
 * write.
 */
static int
vfs_bio_clcheck(struct vnode *vp, int size, daddr_t lblkno, daddr_t blkno)
{
        struct buf *bpa;
        int match;

        match = 0;

        /* If the buf isn't in core skip it */
        if ((bpa = gbincore(&vp->v_bufobj, lblkno)) == NULL)
                return (0);

        /* If the buf is busy we don't want to wait for it */
        if (BUF_LOCK(bpa, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                return (0);

        /* Only cluster with valid clusterable delayed write buffers */
        if ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) !=
            (B_DELWRI | B_CLUSTEROK))
                goto done;

        if (bpa->b_bufsize != size)
                goto done;

        /*
         * Check to see if it is in the expected place on disk and that the
         * block has been mapped.
         */
        if ((bpa->b_blkno != bpa->b_lblkno) && (bpa->b_blkno == blkno))
                match = 1;
done:
        BUF_UNLOCK(bpa);
        return (match);
}

/*
 *      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)
{
        struct bufobj *bo;
        int i;
        int j;
        daddr_t lblkno = bp->b_lblkno;
        struct vnode *vp = bp->b_vp;
        int ncl;
        int nwritten;
        int size;
        int maxcl;

        bo = &vp->v_bufobj;
        /*
         * 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;

                BO_LOCK(bo);
                for (i = 1; i < maxcl; i++)
                        if (vfs_bio_clcheck(vp, size, lblkno + i,
                            bp->b_blkno + ((i * size) >> DEV_BSHIFT)) == 0)
                                break;

                for (j = 1; i + j <= maxcl && j <= lblkno; j++) 
                        if (vfs_bio_clcheck(vp, size, lblkno - j,
                            bp->b_blkno - ((j * size) >> DEV_BSHIFT)) == 0)
                                break;
                BO_UNLOCK(bo);
                --j;
                ncl = i + j;
                /*
                 * this is a possible cluster write
                 */
                if (ncl != 1) {
                        BUF_UNLOCK(bp);
                        nwritten = cluster_wbuild(vp, size, lblkno - j, ncl);
                        return nwritten;
                }
        }
        bremfree(bp);
        bp->b_flags |= B_ASYNC;
        /*
         * default (old) behavior, writing out only one block
         *
         * XXX returns b_bufsize instead of b_bcount for nwritten?
         */
        nwritten = bp->b_bufsize;
        (void) bwrite(bp);

        return nwritten;
}

/*
 *      getnewbuf:
 *
 *      Find and initialize a new buffer header, freeing up existing buffers 
 *      in the bufqueues as necessary.  The new buffer is returned locked.
 *
 *      Important:  B_INVAL is not set.  If the caller wishes to throw the
 *      buffer away, the caller must set B_INVAL prior to calling brelse().
 *
 *      We block if:
 *              We have insufficient buffer headers
 *              We have insufficient buffer space
 *              buffer_map is too fragmented ( space reservation fails )
 *              If we have to flush dirty buffers ( but we try to avoid this )
 *
 *      To avoid VFS layer recursion we do not flush dirty buffers ourselves.
 *      Instead we ask the buf daemon to do it for us.  We attempt to
 *      avoid piecemeal wakeups of the pageout daemon.
 */

static struct buf *
getnewbuf(struct vnode *vp, int slpflag, int slptimeo, int size, int maxsize,
    int gbflags)
{
        struct thread *td;
        struct buf *bp;
        struct buf *nbp;
        int defrag = 0;
        int nqindex;
        static int flushingbufs;

        td = curthread;
        /*
         * 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.
         */
        atomic_add_int(&getnewbufcalls, 1);
        atomic_subtract_int(&getnewbufrestarts, 1);
restart:
        atomic_add_int(&getnewbufrestarts, 1);

        /*
         * 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.
         */
        mtx_lock(&bqlock);
        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;
                                /* FALLTHROUGH */
                        case QUEUE_EMPTYKVA:
                                nqindex = QUEUE_CLEAN;
                                if ((nbp = TAILQ_FIRST(&bufqueues[QUEUE_CLEAN])))
                                        break;
                                /* FALLTHROUGH */
                        case QUEUE_CLEAN:
                                /*
                                 * nbp is NULL. 
                                 */
                                break;
                        }
                }
                /*
                 * 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, NULL) != 0)
                        continue;
                if (bp->b_vp) {
                        BO_LOCK(bp->b_bufobj);
                        if (bp->b_vflags & BV_BKGRDINPROG) {
                                BO_UNLOCK(bp->b_bufobj);
                                BUF_UNLOCK(bp);
                                continue;
                        }
                        BO_UNLOCK(bp->b_bufobj);
                }
                CTR6(KTR_BUF,
                    "getnewbuf(%p) vp %p flags %X kvasize %d bufsize %d "
                    "queue %d (recycling)", bp, bp->b_vp, bp->b_flags,
                    bp->b_kvasize, bp->b_bufsize, qindex);

                /*
                 * 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 (bp->b_bufobj != NULL)
                        BO_LOCK(bp->b_bufobj);
                bremfreel(bp);
                if (bp->b_bufobj != NULL)
                        BO_UNLOCK(bp->b_bufobj);
                mtx_unlock(&bqlock);

                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 (bp->b_rcred != NOCRED) {
                        crfree(bp->b_rcred);
                        bp->b_rcred = NOCRED;
                }
                if (bp->b_wcred != NOCRED) {
                        crfree(bp->b_wcred);
                        bp->b_wcred = NOCRED;
                }
                if (!LIST_EMPTY(&bp->b_dep))
                        buf_deallocate(bp);
                if (bp->b_vflags & BV_BKGRDINPROG)
                        panic("losing buffer 3");
                KASSERT(bp->b_vp == NULL,
                    ("bp: %p still has vnode %p.  qindex: %d",
                    bp, bp->b_vp, qindex));
                KASSERT((bp->b_xflags & (BX_VNCLEAN|BX_VNDIRTY)) == 0,
                   ("bp: %p still on a buffer list. xflags %X",
                    bp, bp->b_xflags));

                if (bp->b_bufsize)
                        allocbuf(bp, 0);

                bp->b_flags = 0;
                bp->b_ioflags = 0;
                bp->b_xflags = 0;
                KASSERT((bp->b_vflags & BV_INFREECNT) == 0,
                    ("buf %p still counted as free?", bp));
                bp->b_vflags = 0;
                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;
                bp->b_bufobj = NULL;
                bp->b_pin_count = 0;
                bp->b_fsprivate1 = NULL;
                bp->b_fsprivate2 = NULL;
                bp->b_fsprivate3 = NULL;

                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;
                }

                /*
                 * Notify any waiters for the buffer lock about
                 * identity change by freeing the buffer.
                 */
                if (qindex == QUEUE_CLEAN && BUF_LOCKWAITERS(bp)) {
                        bp->b_flags |= B_INVAL;
                        bfreekva(bp);
                        brelse(bp);
                        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, norunbuf;
                char *waitmsg;
                int fl;

                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;
                }
                mtx_lock(&nblock);
                needsbuffer |= flags;
                mtx_unlock(&nblock);
                mtx_unlock(&bqlock);

                bd_speedup();   /* heeeelp */
                if (gbflags & GB_NOWAIT_BD)
                        return (NULL);

                mtx_lock(&nblock);
                while (needsbuffer & flags) {
                        if (vp != NULL && (td->td_pflags & TDP_BUFNEED) == 0) {
                                mtx_unlock(&nblock);
                                /*
                                 * getblk() is called with a vnode
                                 * locked, and some majority of the
                                 * dirty buffers may as well belong to
                                 * the vnode. Flushing the buffers
                                 * there would make a progress that
                                 * cannot be achieved by the
                                 * buf_daemon, that cannot lock the
                                 * vnode.
                                 */
                                norunbuf = ~(TDP_BUFNEED | TDP_NORUNNINGBUF) |
                                    (td->td_pflags & TDP_NORUNNINGBUF);
                                /* play bufdaemon */
                                td->td_pflags |= TDP_BUFNEED | TDP_NORUNNINGBUF;
                                fl = buf_do_flush(vp);
                                td->td_pflags &= norunbuf;
                                mtx_lock(&nblock);
                                if (fl != 0)
                                        continue;
                                if ((needsbuffer & flags) == 0)
                                        break;
                        }
                        if (msleep(&needsbuffer, &nblock,
                            (PRIBIO + 4) | slpflag, waitmsg, slptimeo)) {
                                mtx_unlock(&nblock);
                                return (NULL);
                        }
                }
                mtx_unlock(&nblock);
        } 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;

                        bfreekva(bp);

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

                                bp->b_kvabase = (caddr_t) addr;
                                bp->b_kvasize = maxsize;
                                atomic_add_long(&bufspace, bp->b_kvasize);
                                atomic_add_int(&bufreusecnt, 1);
                        }
                        vm_map_unlock(buffer_map);
                }
                bp->b_saveaddr = bp->b_kvabase;
                bp->b_data = bp->b_saveaddr;
        }
        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 kproc_desc buf_kp = {
        "bufdaemon",
        buf_daemon,
        &bufdaemonproc
};
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start, &buf_kp);

static int
buf_do_flush(struct vnode *vp)
{
        int flushed;

        flushed = flushbufqueues(vp, QUEUE_DIRTY, 0);
        /* The list empty check here is slightly racy */
        if (!TAILQ_EMPTY(&bufqueues[QUEUE_DIRTY_GIANT])) {
                mtx_lock(&Giant);
                flushed += flushbufqueues(vp, QUEUE_DIRTY_GIANT, 0);
                mtx_unlock(&Giant);
        }
        if (flushed == 0) {
                /*
                 * Could not find any buffers without rollback
                 * dependencies, so just write the first one
                 * in the hopes of eventually making progress.
                 */
                flushbufqueues(vp, QUEUE_DIRTY, 1);
                if (!TAILQ_EMPTY(
                            &bufqueues[QUEUE_DIRTY_GIANT])) {
                        mtx_lock(&Giant);
                        flushbufqueues(vp, QUEUE_DIRTY_GIANT, 1);
                        mtx_unlock(&Giant);
                }
        }
        return (flushed);
}

static void
buf_daemon()
{
        int lodirtysave;

        /*
         * This process needs to be suspended prior to shutdown sync.
         */
        EVENTHANDLER_REGISTER(shutdown_pre_sync, kproc_shutdown, bufdaemonproc,
            SHUTDOWN_PRI_LAST);

        /*
         * This process is allowed to take the buffer cache to the limit
         */
        curthread->td_pflags |= TDP_NORUNNINGBUF | TDP_BUFNEED;
        mtx_lock(&bdlock);
        for (;;) {
                bd_request = 0;
                mtx_unlock(&bdlock);

                kproc_suspend_check(bufdaemonproc);
                lodirtysave = lodirtybuffers;
                if (bd_speedupreq) {
                        lodirtybuffers = numdirtybuffers / 2;
                        bd_speedupreq = 0;
                }
                /*
                 * 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 (buf_do_flush(NULL) == 0)
                                break;
                        uio_yield();
                }
                lodirtybuffers = lodirtysave;

                /*
                 * Only clear bd_request if we have reached our low water
                 * mark.  The buf_daemon normally waits 1 second 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.
                 */
                mtx_lock(&bdlock);
                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;
                        msleep(&bd_request, &bdlock, PVM, "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)
                         */
                        msleep(&bd_request, &bdlock, PVM, "qsleep", hz / 10);
                }
        }
}

/*
 *      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 flushwithdeps = 0;
SYSCTL_INT(_vfs, OID_AUTO, flushwithdeps, CTLFLAG_RW, &flushwithdeps,
    0, "Number of buffers flushed with dependecies that require rollbacks");

static int
flushbufqueues(struct vnode *lvp, int queue, int flushdeps)
{
        struct buf *sentinel;
        struct vnode *vp;
        struct mount *mp;
        struct buf *bp;
        int hasdeps;
        int flushed;
        int target;

        if (lvp == NULL) {
                target = numdirtybuffers - lodirtybuffers;
                if (flushdeps && target > 2)
                        target /= 2;
        } else
                target = flushbufqtarget;
        flushed = 0;
        bp = NULL;
        sentinel = malloc(sizeof(struct buf), M_TEMP, M_WAITOK | M_ZERO);
        sentinel->b_qindex = QUEUE_SENTINEL;
        mtx_lock(&bqlock);
        TAILQ_INSERT_HEAD(&bufqueues[queue], sentinel, b_freelist);
        while (flushed != target) {
                bp = TAILQ_NEXT(sentinel, b_freelist);
                if (bp != NULL) {
                        TAILQ_REMOVE(&bufqueues[queue], sentinel, b_freelist);
                        TAILQ_INSERT_AFTER(&bufqueues[queue], bp, sentinel,
                            b_freelist);
                } else
                        break;
                /*
                 * Skip sentinels inserted by other invocations of the
                 * flushbufqueues(), taking care to not reorder them.
                 */
                if (bp->b_qindex == QUEUE_SENTINEL)
                        continue;
                /*
                 * Only flush the buffers that belong to the
                 * vnode locked by the curthread.
                 */
                if (lvp != NULL && bp->b_vp != lvp)
                        continue;
                if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                        continue;
                if (bp->b_pin_count > 0) {
                        BUF_UNLOCK(bp);
                        continue;
                }
                BO_LOCK(bp->b_bufobj);
                if ((bp->b_vflags & BV_BKGRDINPROG) != 0 ||
                    (bp->b_flags & B_DELWRI) == 0) {
                        BO_UNLOCK(bp->b_bufobj);
                        BUF_UNLOCK(bp);
                        continue;
                }
                BO_UNLOCK(bp->b_bufobj);
                if (bp->b_flags & B_INVAL) {
                        bremfreel(bp);
                        mtx_unlock(&bqlock);
                        brelse(bp);
                        flushed++;
                        numdirtywakeup((lodirtybuffers + hidirtybuffers) / 2);
                        mtx_lock(&bqlock);
                        continue;
                }

                if (!LIST_EMPTY(&bp->b_dep) && buf_countdeps(bp, 0)) {
                        if (flushdeps == 0) {
                                BUF_UNLOCK(bp);
                                continue;
                        }
                        hasdeps = 1;
                } else
                        hasdeps = 0;
                /*
                 * We must hold the lock on a vnode before writing
                 * one of its buffers. Otherwise we may confuse, or
                 * in the case of a snapshot vnode, deadlock the
                 * system.
                 *
                 * The lock order here is the reverse of the normal
                 * of vnode followed by buf lock.  This is ok because
                 * the NOWAIT will prevent deadlock.
                 */
                vp = bp->b_vp;
                if (vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                        BUF_UNLOCK(bp);
                        continue;
                }
                if (vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT | LK_CANRECURSE) == 0) {
                        mtx_unlock(&bqlock);
                        CTR3(KTR_BUF, "flushbufqueue(%p) vp %p flags %X",
                            bp, bp->b_vp, bp->b_flags);
                        if (curproc == bufdaemonproc)
                                vfs_bio_awrite(bp);
                        else {
                                bremfree(bp);
                                bwrite(bp);
                                notbufdflashes++;
                        }
                        vn_finished_write(mp);
                        VOP_UNLOCK(vp, 0);
                        flushwithdeps += hasdeps;
                        flushed++;

                        /*
                         * Sleeping on runningbufspace while holding
                         * vnode lock leads to deadlock.
                         */
                        if (curproc == bufdaemonproc)
                                waitrunningbufspace();
                        numdirtywakeup((lodirtybuffers + hidirtybuffers) / 2);
                        mtx_lock(&bqlock);
                        continue;
                }
                vn_finished_write(mp);
                BUF_UNLOCK(bp);
        }
        TAILQ_REMOVE(&bufqueues[queue], sentinel, b_freelist);
        mtx_unlock(&bqlock);
        free(sentinel, M_TEMP);
        return (flushed);
}

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

        BO_LOCK(bo);
        bp = gbincore(bo, blkno);
        BO_UNLOCK(bo);
        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.
 */

static 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;

        ASSERT_VOP_LOCKED(vp, "inmem");

        if (incore(&vp->v_bufobj, blkno))
                return 1;
        if (vp->v_mount == NULL)
                return 0;
        obj = vp->v_object;
        if (obj == NULL)
                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;

        VM_OBJECT_LOCK(obj);
        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)
                        goto notinmem;
                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)
                        goto notinmem;
        }
        VM_OBJECT_UNLOCK(obj);
        return 1;

notinmem:
        VM_OBJECT_UNLOCK(obj);
        return (0);
}

/*
 * Set 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.
 *
 * 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_dirty_buf(struct buf *bp)
{
        vm_ooffset_t foff, noff, eoff;
        vm_page_t m;
        int i;

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

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

        VM_OBJECT_LOCK(bp->b_bufobj->bo_object);
        vfs_drain_busy_pages(bp);
        vfs_setdirty_locked_object(bp);
        for (i = 0; i < bp->b_npages; i++) {
                noff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                eoff = noff;
                if (eoff > bp->b_offset + bp->b_bufsize)
                        eoff = bp->b_offset + bp->b_bufsize;
                m = bp->b_pages[i];
                vfs_page_set_validclean(bp, foff, m);
                /* vm_page_clear_dirty(m, foff & PAGE_MASK, eoff - foff); */
                foff = noff;
        }
        VM_OBJECT_UNLOCK(bp->b_bufobj->bo_object);
}

static void
vfs_setdirty_locked_object(struct buf *bp)
{
        vm_object_t object;
        int i;

        object = bp->b_bufobj->bo_object;
        VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);

        /*
         * We qualify the scan for modified pages on whether the
         * object has been flushed yet.
         */
        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_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 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 BIO_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,
    int flags)
{
        struct buf *bp;
        struct bufobj *bo;
        int error;

        CTR3(KTR_BUF, "getblk(%p, %ld, %d)", vp, (long)blkno, size);
        ASSERT_VOP_LOCKED(vp, "getblk");
        if (size > MAXBSIZE)
                panic("getblk: size(%d) > MAXBSIZE(%d)\n", size, MAXBSIZE);

        bo = &vp->v_bufobj;
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 if 0 sections (clean this up after its proven)
         */
        if (numfreebuffers == 0) {
                if (TD_IS_IDLETHREAD(curthread))
                        return NULL;
                mtx_lock(&nblock);
                needsbuffer |= VFS_BIO_NEED_ANY;
                mtx_unlock(&nblock);
        }

        BO_LOCK(bo);
        bp = gbincore(bo, blkno);
        if (bp != NULL) {
                int lockflags;
                /*
                 * Buffer is in-core.  If the buffer is not busy, it must
                 * be on a queue.
                 */
                lockflags = LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK;

                if (flags & GB_LOCK_NOWAIT)
                        lockflags |= LK_NOWAIT;

                error = BUF_TIMELOCK(bp, lockflags,
                    BO_MTX(bo), "getblk", slpflag, slptimeo);

                /*
                 * If we slept and got the lock we have to restart in case
                 * the buffer changed identities.
                 */
                if (error == ENOLCK)
                        goto loop;
                /* We timed out or were interrupted. */
                else if (error)
                        return (NULL);

                /*
                 * The buffer is locked.  B_CACHE is cleared if the buffer is 
                 * invalid.  Otherwise, 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;
                BO_LOCK(bo);
                bremfree(bp);
                BO_UNLOCK(bo);

                /*
                 * 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) {
                                        /*
                                         * If buffer is pinned and caller does
                                         * not want sleep  waiting for it to be
                                         * unpinned, bail out
                                         * */
                                        if (bp->b_pin_count > 0) {
                                                if (flags & GB_LOCK_NOWAIT) {
                                                        bqrelse(bp);
                                                        return (NULL);
                                                } else {
                                                        bunpin_wait(bp);
                                                }
                                        }
                                        bp->b_flags |= B_NOCACHE;
                                        bwrite(bp);
                                } else {
                                        if (LIST_EMPTY(&bp->b_dep)) {
                                                bp->b_flags |= B_RELBUF;
                                                brelse(bp);
                                        } else {
                                                bp->b_flags |= B_NOCACHE;
                                                bwrite(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;
                        bwrite(bp);
                        goto loop;
                }
                bp->b_flags &= ~B_DONE;
        } else {
                int bsize, maxsize, vmio;
                off_t offset;

                /*
                 * 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).
                 */
                BO_UNLOCK(bo);
                /*
                 * If the user does not want us to create the buffer, bail out
                 * here.
                 */
                if (flags & GB_NOCREAT)
                        return NULL;
                bsize = vn_isdisk(vp, NULL) ? DEV_BSIZE : bo->bo_bsize;
                offset = blkno * bsize;
                vmio = vp->v_object != NULL;
                maxsize = vmio ? size + (offset & PAGE_MASK) : size;
                maxsize = imax(maxsize, bsize);

                bp = getnewbuf(vp, slpflag, slptimeo, size, maxsize, flags);
                if (bp == NULL) {
                        if (slpflag || slptimeo)
                                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.
                 *
                 * Note: this must occur before we associate the buffer
                 * with the vp especially considering limitations in
                 * the splay tree implementation when dealing with duplicate
                 * lblkno's.
                 */
                BO_LOCK(bo);
                if (gbincore(bo, blkno)) {
                        BO_UNLOCK(bo);
                        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);
                BO_UNLOCK(bo);

                /*
                 * 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: VMIO on vnode type %d\n",
                                        vp->v_type);
#endif
                        KASSERT(vp->v_object == bp->b_bufobj->bo_object,
                            ("ARGH! different b_bufobj->bo_object %p %p %p\n",
                            bp, vp->v_object, bp->b_bufobj->bo_object));
                } else {
                        bp->b_flags &= ~B_VMIO;
                        KASSERT(bp->b_bufobj->bo_object == NULL,
                            ("ARGH! has b_bufobj->bo_object %p %p\n",
                            bp, bp->b_bufobj->bo_object));
                }

                allocbuf(bp, size);
                bp->b_flags &= ~B_DONE;
        }
        CTR4(KTR_BUF, "getblk(%p, %ld, %d) = %p", vp, (long)blkno, size, bp);
        BUF_ASSERT_HELD(bp);
        KASSERT(bp->b_bufobj == bo,
            ("bp %p wrong b_bufobj %p should be %p", bp, bp->b_bufobj, bo));
        return (bp);
}

/*
 * Get an empty, disassociated buffer of given size.  The buffer is initially
 * set to B_INVAL.
 */
struct buf *
geteblk(int size, int flags)
{
        struct buf *bp;
        int maxsize;

        maxsize = (size + BKVAMASK) & ~BKVAMASK;
        while ((bp = getnewbuf(NULL, 0, 0, size, maxsize, flags)) == NULL) {
                if ((flags & GB_NOWAIT_BD) &&
                    (curthread->td_pflags & TDP_BUFNEED) != 0)
                        return (NULL);
        }
        allocbuf(bp, size);
        bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
        BUF_ASSERT_HELD(bp);
        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.
 */

int
allocbuf(struct buf *bp, int size)
{
        int newbsize, mbsize;
        int i;

        BUF_ASSERT_HELD(bp);

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

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

                if (newbsize < bp->b_bufsize) {
                        /*
                         * malloced buffers are not shrunk
                         */
                        if (bp->b_flags & B_MALLOC) {
                                if (newbsize) {
                                        bp->b_bcount = size;
                                } else {
                                        free(bp->b_data, M_BIOBUF);
                                        if (bp->b_bufsize) {
                                                atomic_subtract_long(
                                                    &bufmallocspace,
                                                    bp->b_bufsize);
                                                bufspacewakeup();
                                                bp->b_bufsize = 0;
                                        }
                                        bp->b_saveaddr = bp->b_kvabase;
                                        bp->b_data = bp->b_saveaddr;
                                        bp->b_bcount = 0;
                                        bp->b_flags &= ~B_MALLOC;
                                }
                                return 1;
                        }               
                        vm_hold_free_pages(bp, newbsize);
                } else if (newbsize > bp->b_bufsize) {
                        /*
                         * We only use malloced memory on the first allocation.
                         * and revert to page-allocated memory when the buffer
                         * grows.
                         */
                        /*
                         * There is a potential smp race here that could lead
                         * to bufmallocspace slightly passing the max.  It
                         * is probably extremely rare and not worth worrying
                         * over.
                         */
                        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;
                                atomic_add_long(&bufmallocspace, mbsize);
                                return 1;
                        }
                        origbuf = NULL;
                        origbufsize = 0;
                        /*
                         * If the buffer is growing on its other-than-first allocation,
                         * then we revert to the page-allocation scheme.
                         */
                        if (bp->b_flags & B_MALLOC) {
                                origbuf = bp->b_data;
                                origbufsize = bp->b_bufsize;
                                bp->b_data = bp->b_kvabase;
                                if (bp->b_bufsize) {
                                        atomic_subtract_long(&bufmallocspace,
                                            bp->b_bufsize);
                                        bufspacewakeup();
                                        bp->b_bufsize = 0;
                                }
                                bp->b_flags &= ~B_MALLOC;
                                newbsize = round_page(newbsize);
                        }
                        vm_hold_load_pages(
                            bp,
                            (vm_offset_t) bp->b_data + bp->b_bufsize,
                            (vm_offset_t) bp->b_data + newbsize);
                        if (origbuf) {
                                bcopy(origbuf, bp->b_data, origbufsize);
                                free(origbuf, M_BIOBUF);
                        }
                }
        } else {
                int desiredpages;

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

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

                if (newbsize < bp->b_bufsize) {
                        /*
                         * DEV_BSIZE aligned new buffer size is less then the
                         * DEV_BSIZE aligned existing buffer size.  Figure out
                         * if we have to remove any pages.
                         */
                        if (desiredpages < bp->b_npages) {
                                vm_page_t m;

                                VM_OBJECT_LOCK(bp->b_bufobj->bo_object);
                                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_if_busy(m, TRUE,
                                            "biodep"))
                                                continue;

                                        bp->b_pages[i] = NULL;
                                        vm_page_lock(m);
                                        vm_page_unwire(m, 0);
                                        vm_page_unlock(m);
                                }
                                VM_OBJECT_UNLOCK(bp->b_bufobj->bo_object);
                                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.
                         */
                        vm_object_t obj;
                        vm_offset_t toff;
                        vm_offset_t tinc;

                        /*
                         * Step 1, bring in the VM pages from the object, 
                         * allocating them if necessary.  We must clear
                         * B_CACHE if these pages are not valid for the 
                         * range covered by the buffer.
                         */

                        obj = bp->b_bufobj->bo_object;

                        VM_OBJECT_LOCK(obj);
                        while (bp->b_npages < desiredpages) {
                                vm_page_t m;

                                /*
                                 * We must allocate system pages since blocking
                                 * here could intefere with paging I/O, no
                                 * matter which process we are.
                                 *
                                 * We can only test VPO_BUSY here.  Blocking on
                                 * m->busy might lead to a deadlock:
                                 *  vm_fault->getpages->cluster_read->allocbuf
                                 * Thus, we specify VM_ALLOC_IGN_SBUSY.
                                 */
                                m = vm_page_grab(obj, OFF_TO_IDX(bp->b_offset) +
                                    bp->b_npages, VM_ALLOC_NOBUSY |
                                    VM_ALLOC_SYSTEM | VM_ALLOC_WIRED |
                                    VM_ALLOC_RETRY | VM_ALLOC_IGN_SBUSY |
                                    VM_ALLOC_COUNT(desiredpages - bp->b_npages));
                                if (m->valid == 0)
                                        bp->b_flags &= ~B_CACHE;
                                bp->b_pages[bp->b_npages] = m;
                                ++bp->b_npages;
                        }

                        /*
                         * 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;
                        }
                        VM_OBJECT_UNLOCK(obj);

                        /*
                         * 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;
}

void
biodone(struct bio *bp)
{
        struct mtx *mtxp;
        void (*done)(struct bio *);

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        bp->bio_flags |= BIO_DONE;
        done = bp->bio_done;
        if (done == NULL)
                wakeup(bp);
        mtx_unlock(mtxp);
        if (done != NULL)
                done(bp);
}

/*
 * Wait for a BIO to finish.
 *
 * XXX: resort to a timeout for now.  The optimal locking (if any) for this
 * case is not yet clear.
 */
int
biowait(struct bio *bp, const char *wchan)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        while ((bp->bio_flags & BIO_DONE) == 0)
                msleep(bp, mtxp, PRIBIO, wchan, hz / 10);
        mtx_unlock(mtxp);
        if (bp->bio_error != 0)
                return (bp->bio_error);
        if (!(bp->bio_flags & BIO_ERROR))
                return (0);
        return (EIO);
}

void
biofinish(struct bio *bp, struct devstat *stat, int error)
{
        
        if (error) {
                bp->bio_error = error;
                bp->bio_flags |= BIO_ERROR;
        }
        if (stat != NULL)
                devstat_end_transaction_bio(stat, bp);
        biodone(bp);
}

/*
 *      bufwait:
 *
 *      Wait for buffer I/O completion, returning error status.  The buffer
 *      is left locked and B_DONE on return.  B_EINTR is converted into an EINTR
 *      error and cleared.
 */
int
bufwait(struct buf *bp)
{
        if (bp->b_iocmd == BIO_READ)
                bwait(bp, PRIBIO, "biord");
        else
                bwait(bp, PRIBIO, "biowr");
        if (bp->b_flags & B_EINTR) {
                bp->b_flags &= ~B_EINTR;
                return (EINTR);
        }
        if (bp->b_ioflags & BIO_ERROR) {
                return (bp->b_error ? bp->b_error : EIO);
        } else {
                return (0);
        }
}

 /*
  * Call back function from struct bio back up to struct buf.
  */
static void
bufdonebio(struct bio *bip)
{
        struct buf *bp;

        bp = bip->bio_caller2;
        bp->b_resid = bp->b_bcount - bip->bio_completed;
        bp->b_resid = bip->bio_resid;   /* XXX: remove */
        bp->b_ioflags = bip->bio_flags;
        bp->b_error = bip->bio_error;
        if (bp->b_error)
                bp->b_ioflags |= BIO_ERROR;
        bufdone(bp);
        g_destroy_bio(bip);
}

void
dev_strategy(struct cdev *dev, struct buf *bp)
{
        struct cdevsw *csw;
        struct bio *bip;
        int ref;

        if ((!bp->b_iocmd) || (bp->b_iocmd & (bp->b_iocmd - 1)))
                panic("b_iocmd botch");
        for (;;) {
                bip = g_new_bio();
                if (bip != NULL)
                        break;
                /* Try again later */
                tsleep(&bp, PRIBIO, "dev_strat", hz/10);
        }
        bip->bio_cmd = bp->b_iocmd;
        bip->bio_offset = bp->b_iooffset;
        bip->bio_length = bp->b_bcount;
        bip->bio_bcount = bp->b_bcount; /* XXX: remove */
        bip->bio_data = bp->b_data;
        bip->bio_done = bufdonebio;
        bip->bio_caller2 = bp;
        bip->bio_dev = dev;
        KASSERT(dev->si_refcount > 0,
            ("dev_strategy on un-referenced struct cdev *(%s)",
            devtoname(dev)));
        csw = dev_refthread(dev, &ref);
        if (csw == NULL) {
                g_destroy_bio(bip);
                bp->b_error = ENXIO;
                bp->b_ioflags = BIO_ERROR;
                bufdone(bp);
                return;
        }
        (*csw->d_strategy)(bip);
        dev_relthread(dev, ref);
}

/*
 *      bufdone:
 *
 *      Finish I/O on a buffer, optionally calling a completion function.
 *      This is usually called from an interrupt so process blocking is
 *      not allowed.
 *
 *      biodone is also responsible for setting B_CACHE in a B_VMIO bp.
 *      In a non-VMIO bp, B_CACHE will be set on the next getblk() 
 *      assuming B_INVAL is clear.
 *
 *      For the VMIO case, we set B_CACHE if the op was a read and no
 *      read error occured, or if the op was a write.  B_CACHE is never
 *      set if the buffer is invalid or otherwise uncacheable.
 *
 *      biodone does not mess with B_INVAL, allowing the I/O routine or the
 *      initiator to leave B_INVAL set to brelse the buffer out of existance
 *      in the biodone routine.
 */
void
bufdone(struct buf *bp)
{
        struct bufobj *dropobj;
        void    (*biodone)(struct buf *);

        CTR3(KTR_BUF, "bufdone(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
        dropobj = NULL;

        KASSERT(!(bp->b_flags & B_DONE), ("biodone: bp %p already done", bp));
        BUF_ASSERT_HELD(bp);

        runningbufwakeup(bp);
        if (bp->b_iocmd == BIO_WRITE)
                dropobj = bp->b_bufobj;
        /* call optional completion function if requested */
        if (bp->b_iodone != NULL) {
                biodone = bp->b_iodone;
                bp->b_iodone = NULL;
                (*biodone) (bp);
                if (dropobj)
                        bufobj_wdrop(dropobj);
                return;
        }

        bufdone_finish(bp);

        if (dropobj)
                bufobj_wdrop(dropobj);
}

void
bufdone_finish(struct buf *bp)
{
        BUF_ASSERT_HELD(bp);

        if (!LIST_EMPTY(&bp->b_dep))
                buf_complete(bp);

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

                obj = bp->b_bufobj->bo_object;

#if defined(VFS_BIO_DEBUG)
                mp_fixme("usecount and vflag accessed without locks.");
                if (vp->v_usecount == 0) {
                        panic("biodone: zero vnode ref count");
                }

                KASSERT(vp->v_object != NULL,
                        ("biodone: vnode %p has no vm_object", vp));
#endif

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

                VM_OBJECT_LOCK(obj);
#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_iocmd == BIO_READ &&
                    !(bp->b_flags & (B_INVAL|B_NOCACHE)) &&
                    !(bp->b_ioflags & BIO_ERROR)) {
                        bp->b_flags |= B_CACHE;
                }
                bogus = 0;
                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
                         */
                        m = bp->b_pages[i];
                        if (m == bogus_page) {
                                bogus = bogusflag = 1;
                                m = vm_page_lookup(obj, OFF_TO_IDX(foff));
                                if (m == NULL)
                                        panic("biodone: page disappeared!");
                                bp->b_pages[i] = m;
                        }
#if defined(VFS_BIO_DEBUG)
                        if (OFF_TO_IDX(foff) != m->pindex) {
                                printf(
"biodone: foff(%jd)/m->pindex(%ju) mismatch\n",
                                    (intmax_t)foff, (uintmax_t)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_iocmd == BIO_READ) && !bogusflag && resid > 0) {
                                KASSERT((m->dirty & vm_page_bits(foff &
                                    PAGE_MASK, resid)) == 0, ("bufdone_finish:"
                                    " page %p has unexpected dirty bits", m));
                                vfs_page_set_valid(bp, foff, m);
                        }

                        /*
                         * 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: %jd, lblkno: %jd, flags: 0x%x, npages: %d\n",
                                            (intmax_t)bp->b_vp->v_mount->mnt_stat.f_iosize,
                                            (intmax_t) bp->b_lblkno,
                                            bp->b_flags, bp->b_npages);
                                else
                                        printf(" VDEV, lblkno: %jd, flags: 0x%x, npages: %d\n",
                                            (intmax_t) bp->b_lblkno,
                                            bp->b_flags, bp->b_npages);
                                printf(" valid: 0x%lx, dirty: 0x%lx, wired: %d\n",
                                    (u_long)m->valid, (u_long)m->dirty,
                                    m->wire_count);
                                panic("biodone: page busy < 0\n");
                        }
                        vm_page_io_finish(m);
                        vm_object_pip_subtract(obj, 1);
                        foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                        iosize -= resid;
                }
                vm_object_pip_wakeupn(obj, 0);
                VM_OBJECT_UNLOCK(obj);
                if (bogus)
                        pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                            bp->b_pages, bp->b_npages);
        }

        /*
         * 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_RELBUF)) || (bp->b_ioflags & BIO_ERROR))
                        brelse(bp);
                else
                        bqrelse(bp);
        } else
                bdone(bp);
}

/*
 * 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;
        vm_object_t obj;
        vm_page_t m;

        runningbufwakeup(bp);
        if (!(bp->b_flags & B_VMIO))
                return;

        obj = bp->b_bufobj->bo_object;
        VM_OBJECT_LOCK(obj);
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                if (m == bogus_page) {
                        m = vm_page_lookup(obj, OFF_TO_IDX(bp->b_offset) + i);
                        if (!m)
                                panic("vfs_unbusy_pages: page missing\n");
                        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_io_finish(m);
        }
        vm_object_pip_wakeupn(obj, 0);
        VM_OBJECT_UNLOCK(obj);
}

/*
 * 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, vm_page_t m)
{
        vm_ooffset_t eoff;

        /*
         * Compute the end offset, eoff, such that [off, eoff) does not span a
         * page boundary and eoff is not greater than the end of the buffer.
         * The end of the buffer, in this case, is our file EOF, not the
         * allocation size of the buffer.
         */
        eoff = (off + PAGE_SIZE) & ~(vm_ooffset_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 > off)
                vm_page_set_valid(m, off & PAGE_MASK, eoff - off);
}

/*
 * vfs_page_set_validclean:
 *
 *      Set the valid bits and clear the dirty bits in a page based on the
 *      supplied offset.   The range is restricted to the buffer's size.
 */
static void
vfs_page_set_validclean(struct buf *bp, vm_ooffset_t off, 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)
                );
        }
}

/*
 * Ensure that all buffer pages are not busied by VPO_BUSY flag. If
 * any page is busy, drain the flag.
 */
static void
vfs_drain_busy_pages(struct buf *bp)
{
        vm_page_t m;
        int i, last_busied;

        VM_OBJECT_LOCK_ASSERT(bp->b_bufobj->bo_object, MA_OWNED);
        last_busied = 0;
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];
                if ((m->oflags & VPO_BUSY) != 0) {
                        for (; last_busied < i; last_busied++)
                                vm_page_busy(bp->b_pages[last_busied]);
                        while ((m->oflags & VPO_BUSY) != 0)
                                vm_page_sleep(m, "vbpage");
                }
        }
        for (i = 0; i < last_busied; i++)
                vm_page_wakeup(bp->b_pages[i]);
}

/*
 * 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 VPO_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 BIO_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;
        vm_object_t obj;
        vm_ooffset_t foff;
        vm_page_t m;

        if (!(bp->b_flags & B_VMIO))
                return;

        obj = bp->b_bufobj->bo_object;
        foff = bp->b_offset;
        KASSERT(bp->b_offset != NOOFFSET,
            ("vfs_busy_pages: no buffer offset"));
        VM_OBJECT_LOCK(obj);
        vfs_drain_busy_pages(bp);
        if (bp->b_bufsize != 0)
                vfs_setdirty_locked_object(bp);
        bogus = 0;
        for (i = 0; i < bp->b_npages; i++) {
                m = bp->b_pages[i];

                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.
                 */
                if (clear_modify) {
                        pmap_remove_write(m);
                        vfs_page_set_validclean(bp, foff, 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;
        }
        VM_OBJECT_UNLOCK(obj);
        if (bogus)
                pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                    bp->b_pages, bp->b_npages);
}

/*
 *      vfs_bio_set_valid:
 *
 *      Set the range within the buffer to valid.  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_valid(struct buf *bp, int base, int size)
{
        int i, n;
        vm_page_t m;

        if (!(bp->b_flags & B_VMIO))
                return;

        /*
         * 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);

        VM_OBJECT_LOCK(bp->b_bufobj->bo_object);
        for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                m = bp->b_pages[i];
                if (n > size)
                        n = size;
                vm_page_set_valid(m, base & PAGE_MASK, n);
                base += n;
                size -= n;
                n = PAGE_SIZE;
        }
        VM_OBJECT_UNLOCK(bp->b_bufobj->bo_object);
}

/*
 *      vfs_bio_clrbuf:
 *
 *      If the specified buffer is a non-VMIO buffer, clear the entire
 *      buffer.  If the specified buffer is a VMIO buffer, clear and
 *      validate only the previously invalid portions of the buffer.
 *      This routine essentially fakes an I/O, so we need to clear
 *      BIO_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, j, mask;
        caddr_t sa, ea;

        if ((bp->b_flags & (B_VMIO | B_MALLOC)) != B_VMIO) {
                clrbuf(bp);
                return;
        }
        bp->b_flags &= ~B_INVAL;
        bp->b_ioflags &= ~BIO_ERROR;
        VM_OBJECT_LOCK(bp->b_bufobj->bo_object);
        if ((bp->b_npages == 1) && (bp->b_bufsize < PAGE_SIZE) &&
            (bp->b_offset & PAGE_MASK) == 0) {
                if (bp->b_pages[0] == bogus_page)
                        goto unlock;
                mask = (1 << (bp->b_bufsize / DEV_BSIZE)) - 1;
                VM_OBJECT_LOCK_ASSERT(bp->b_pages[0]->object, MA_OWNED);
                if ((bp->b_pages[0]->valid & mask) == mask)
                        goto unlock;
                if ((bp->b_pages[0]->valid & mask) == 0) {
                        bzero(bp->b_data, bp->b_bufsize);
                        bp->b_pages[0]->valid |= mask;
                        goto unlock;
                }
        }
        ea = sa = bp->b_data;
        for(i = 0; i < bp->b_npages; i++, sa = ea) {
                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);
                if (bp->b_pages[i] == bogus_page)
                        continue;
                j = ((vm_offset_t)sa & PAGE_MASK) / DEV_BSIZE;
                mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                VM_OBJECT_LOCK_ASSERT(bp->b_pages[i]->object, MA_OWNED);
                if ((bp->b_pages[i]->valid & mask) == mask)
                        continue;
                if ((bp->b_pages[i]->valid & mask) == 0)
                        bzero(sa, ea - sa);
                else {
                        for (; sa < ea; sa += DEV_BSIZE, j++) {
                                if ((bp->b_pages[i]->valid & (1 << j)) == 0)
                                        bzero(sa, DEV_BSIZE);
                        }
                }
                bp->b_pages[i]->valid |= mask;
        }
unlock:
        VM_OBJECT_UNLOCK(bp->b_bufobj->bo_object);
        bp->b_resid = 0;
}

/*
 * vm_hold_load_pages and vm_hold_free_pages get pages into
 * a buffers address space.  The pages are anonymous and are
 * not associated with a file object.
 */
static 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 interfere with paging I/O, no matter which
                 * process we are.
                 */
                p = vm_page_alloc(NULL, pg >> PAGE_SHIFT, VM_ALLOC_NOOBJ |
                    VM_ALLOC_SYSTEM | VM_ALLOC_WIRED |
                    VM_ALLOC_COUNT((to - pg) >> PAGE_SHIFT));
                if (!p) {
                        VM_WAIT;
                        goto tryagain;
                }
                pmap_qenter(pg, &p, 1);
                bp->b_pages[index] = p;
        }
        bp->b_npages = index;
}

/* Return pages associated with this buf to the vm system */
static void
vm_hold_free_pages(struct buf *bp, int newbsize)
{
        vm_offset_t from;
        vm_page_t p;
        int index, newnpages;

        from = round_page((vm_offset_t)bp->b_data + newbsize);
        newnpages = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
        if (bp->b_npages > newnpages)
                pmap_qremove(from, bp->b_npages - newnpages);
        for (index = newnpages; index < bp->b_npages; index++) {
                p = bp->b_pages[index];
                bp->b_pages[index] = NULL;
                if (p->busy != 0)
                        printf("vm_hold_free_pages: blkno: %jd, lblkno: %jd\n",
                            (intmax_t)bp->b_blkno, (intmax_t)bp->b_lblkno);
                p->wire_count--;
                vm_page_free(p);
                atomic_subtract_int(&cnt.v_wire_count, 1);
        }
        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.
 *
 * Note that even if the caller determines that the address space should
 * be valid, a race or a smaller-file mapped into a larger space may
 * actually cause vmapbuf() to fail, so all callers of vmapbuf() MUST
 * check the return value.
 */
int
vmapbuf(struct buf *bp)
{
        caddr_t addr, kva;
        vm_prot_t prot;
        int pidx, i;
        struct vm_page *m;
        struct pmap *pmap = &curproc->p_vmspace->vm_pmap;

        if (bp->b_bufsize < 0)
                return (-1);
        prot = VM_PROT_READ;
        if (bp->b_iocmd == BIO_READ)
                prot |= VM_PROT_WRITE;  /* Less backwards than it looks */
        for (addr = (caddr_t)trunc_page((vm_offset_t)bp->b_data), pidx = 0;
             addr < bp->b_data + bp->b_bufsize;
             addr += PAGE_SIZE, pidx++) {
                /*
                 * Do the vm_fault if needed; do the copy-on-write thing
                 * when reading stuff off device into memory.
                 *
                 * NOTE! Must use pmap_extract() because addr may be in
                 * the userland address space, and kextract is only guarenteed
                 * to work for the kernland address space (see: sparc64 port).
                 */
retry:
                if (vm_fault_quick(addr >= bp->b_data ? addr : bp->b_data,
                    prot) < 0) {
                        for (i = 0; i < pidx; ++i) {
                                vm_page_lock(bp->b_pages[i]);
                                vm_page_unhold(bp->b_pages[i]);
                                vm_page_unlock(bp->b_pages[i]);
                                bp->b_pages[i] = NULL;
                        }
                        return(-1);
                }
                m = pmap_extract_and_hold(pmap, (vm_offset_t)addr, prot);
                if (m == NULL)
                        goto retry;
                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(struct buf *bp)
{
        int pidx;
        int npages;

        npages = bp->b_npages;
        pmap_qremove(trunc_page((vm_offset_t)bp->b_data), npages);
        for (pidx = 0; pidx < npages; pidx++) {
                vm_page_lock(bp->b_pages[pidx]);
                vm_page_unhold(bp->b_pages[pidx]);
                vm_page_unlock(bp->b_pages[pidx]);
        }
        
        bp->b_data = bp->b_saveaddr;
}

void
bdone(struct buf *bp)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        bp->b_flags |= B_DONE;
        wakeup(bp);
        mtx_unlock(mtxp);
}

void
bwait(struct buf *bp, u_char pri, const char *wchan)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        while ((bp->b_flags & B_DONE) == 0)
                msleep(bp, mtxp, pri, wchan, 0);
        mtx_unlock(mtxp);
}

int
bufsync(struct bufobj *bo, int waitfor)
{

        return (VOP_FSYNC(bo->__bo_vnode, waitfor, curthread));
}

void
bufstrategy(struct bufobj *bo, struct buf *bp)
{
        int i = 0;
        struct vnode *vp;

        vp = bp->b_vp;
        KASSERT(vp == bo->bo_private, ("Inconsistent vnode bufstrategy"));
        KASSERT(vp->v_type != VCHR && vp->v_type != VBLK,
            ("Wrong vnode in bufstrategy(bp=%p, vp=%p)", bp, vp));
        i = VOP_STRATEGY(vp, bp);
        KASSERT(i == 0, ("VOP_STRATEGY failed bp=%p vp=%p", bp, bp->b_vp));
}

void
bufobj_wrefl(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wref"));
        ASSERT_BO_LOCKED(bo);
        bo->bo_numoutput++;
}

void
bufobj_wref(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wref"));
        BO_LOCK(bo);
        bo->bo_numoutput++;
        BO_UNLOCK(bo);
}

void
bufobj_wdrop(struct bufobj *bo)
{

        KASSERT(bo != NULL, ("NULL bo in bufobj_wdrop"));
        BO_LOCK(bo);
        KASSERT(bo->bo_numoutput > 0, ("bufobj_wdrop non-positive count"));
        if ((--bo->bo_numoutput == 0) && (bo->bo_flag & BO_WWAIT)) {
                bo->bo_flag &= ~BO_WWAIT;
                wakeup(&bo->bo_numoutput);
        }
        BO_UNLOCK(bo);
}

int
bufobj_wwait(struct bufobj *bo, int slpflag, int timeo)
{
        int error;

        KASSERT(bo != NULL, ("NULL bo in bufobj_wwait"));
        ASSERT_BO_LOCKED(bo);
        error = 0;
        while (bo->bo_numoutput) {
                bo->bo_flag |= BO_WWAIT;
                error = msleep(&bo->bo_numoutput, BO_MTX(bo),
                    slpflag | (PRIBIO + 1), "bo_wwait", timeo);
                if (error)
                        break;
        }
        return (error);
}

void
bpin(struct buf *bp)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        bp->b_pin_count++;
        mtx_unlock(mtxp);
}

void
bunpin(struct buf *bp)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        if (--bp->b_pin_count == 0)
                wakeup(bp);
        mtx_unlock(mtxp);
}

void
bunpin_wait(struct buf *bp)
{
        struct mtx *mtxp;

        mtxp = mtx_pool_find(mtxpool_sleep, bp);
        mtx_lock(mtxp);
        while (bp->b_pin_count > 0)
                msleep(bp, mtxp, PRIBIO, "bwunpin", 0);
        mtx_unlock(mtxp);
}

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

/* DDB command to show buffer data */
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("buf at %p\n", bp);
        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\n"
            "b_bufobj = (%p), b_data = %p, b_blkno = %jd, b_dep = %p\n",
            bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
            bp->b_bufobj, bp->b_data, (intmax_t)bp->b_blkno,
            bp->b_dep.lh_first);
        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");
        }
        db_printf(" ");
        lockmgr_printinfo(&bp->b_lock);
}

DB_SHOW_COMMAND(lockedbufs, lockedbufs)
{
        struct buf *bp;
        int i;

        for (i = 0; i < nbuf; i++) {
                bp = &buf[i];
                if (BUF_ISLOCKED(bp)) {
                        db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                        db_printf("\n");
                }
        }
}

DB_SHOW_COMMAND(vnodebufs, db_show_vnodebufs)
{
        struct vnode *vp;
        struct buf *bp;

        if (!have_addr) {
                db_printf("usage: show vnodebufs <addr>\n");
                return;
        }
        vp = (struct vnode *)addr;
        db_printf("Clean buffers:\n");
        TAILQ_FOREACH(bp, &vp->v_bufobj.bo_clean.bv_hd, b_bobufs) {
                db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                db_printf("\n");
        }
        db_printf("Dirty buffers:\n");
        TAILQ_FOREACH(bp, &vp->v_bufobj.bo_dirty.bv_hd, b_bobufs) {
                db_show_buffer((uintptr_t)bp, 1, 0, NULL);
                db_printf("\n");
        }
}

DB_COMMAND(countfreebufs, db_coundfreebufs)
{
        struct buf *bp;
        int i, used = 0, nfree = 0;

        if (have_addr) {
                db_printf("usage: countfreebufs\n");
                return;
        }

        for (i = 0; i < nbuf; i++) {
                bp = &buf[i];
                if ((bp->b_vflags & BV_INFREECNT) != 0)
                        nfree++;
                else
                        used++;
        }

        db_printf("Counted %d free, %d used (%d tot)\n", nfree, used,
            nfree + used);
        db_printf("numfreebuffers is %d\n", numfreebuffers);
}
#endif /* DDB */