FreeBSD/Linux Kernel Cross Reference
sys/kern/vfs_bio.c

     /*-
      * Copyright (c) 2004 Poul-Henning Kamp
      * Copyright (c) 1994,1997 John S. Dyson
      * Copyright (c) 2013 The FreeBSD Foundation
      * All rights reserved.
      *
      * Portions of this software were developed by Konstantin Belousov
      * under sponsorship from the FreeBSD Foundation.
      *
     * 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: releng/11.0/sys/kern/vfs_bio.c 298817 2016-04-29 21:54:28Z pfg $");
    
    #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/racct.h>
    #include <sys/resourcevar.h>
    #include <sys/rwlock.h>
    #include <sys/smp.h>
    #include <sys/sysctl.h>
    #include <sys/sysproto.h>
    #include <sys/vmem.h>
    #include <sys/vmmeter.h>
    #include <sys/vnode.h>
    #include <sys/watchdog.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 <vm/swap_pager.h>
    #include "opt_compat.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,
    };
    
    static struct buf *buf;         /* buffer header pool */
   extern struct buf *swbuf;       /* Swap buffer header pool. */
   caddr_t unmapped_buf;
   
   /* Used below and for softdep flushing threads in ufs/ffs/ffs_softdep.c */
   struct proc *bufdaemonproc;
   struct proc *bufspacedaemonproc;
   
   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_clean_pages_dirty_buf(struct buf *bp);
   static void vfs_setdirty_locked_object(struct buf *bp);
   static void vfs_vmio_invalidate(struct buf *bp);
   static void vfs_vmio_truncate(struct buf *bp, int npages);
   static void vfs_vmio_extend(struct buf *bp, int npages, int size);
   static int vfs_bio_clcheck(struct vnode *vp, int size,
                   daddr_t lblkno, daddr_t blkno);
   static int buf_flush(struct vnode *vp, int);
   static int buf_recycle(bool);
   static int buf_scan(bool);
   static int flushbufqueues(struct vnode *, int, int);
   static void buf_daemon(void);
   static void bremfreel(struct buf *bp);
   static __inline void bd_wakeup(void);
   static int sysctl_runningspace(SYSCTL_HANDLER_ARGS);
   static void bufkva_reclaim(vmem_t *, int);
   static void bufkva_free(struct buf *);
   static int buf_import(void *, void **, int, int);
   static void buf_release(void *, void **, int);
   
   #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,
       "Physical memory used for buffers");
   #endif
   static long bufkvaspace;
   SYSCTL_LONG(_vfs, OID_AUTO, bufkvaspace, CTLFLAG_RD, &bufkvaspace, 0,
       "Kernel virtual memory used for buffers");
   static long maxbufspace;
   SYSCTL_LONG(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RW, &maxbufspace, 0,
       "Maximum allowed value of bufspace (including metadata)");
   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_RW, &lobufspace, 0,
       "Minimum amount of buffers we want to have");
   long hibufspace;
   SYSCTL_LONG(_vfs, OID_AUTO, hibufspace, CTLFLAG_RW, &hibufspace, 0,
       "Maximum allowed value of bufspace (excluding metadata)");
   long bufspacethresh;
   SYSCTL_LONG(_vfs, OID_AUTO, bufspacethresh, CTLFLAG_RW, &bufspacethresh,
       0, "Bufspace consumed before waking the daemon to free some");
   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_PROC(_vfs, OID_AUTO, lorunningspace, CTLTYPE_LONG | CTLFLAG_MPSAFE |
       CTLFLAG_RW, &lorunningspace, 0, sysctl_runningspace, "L",
       "Minimum preferred space used for in-progress I/O");
   static long hirunningspace;
   SYSCTL_PROC(_vfs, OID_AUTO, hirunningspace, CTLTYPE_LONG | CTLFLAG_MPSAFE |
       CTLFLAG_RW, &hirunningspace, 0, sysctl_runningspace, "L",
       "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,
      "Target number of free buffers");
   static int hifreebuffers;
   SYSCTL_INT(_vfs, OID_AUTO, hifreebuffers, CTLFLAG_RW, &hifreebuffers, 0,
      "Threshold for clean buffer recycling");
   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 acquisition");
   static int mappingrestarts;
   SYSCTL_INT(_vfs, OID_AUTO, mappingrestarts, CTLFLAG_RW, &mappingrestarts, 0,
       "Number of times getblk has had to restart a buffer mapping for "
       "unmapped buffer");
   static int numbufallocfails;
   SYSCTL_INT(_vfs, OID_AUTO, numbufallocfails, CTLFLAG_RW, &numbufallocfails, 0,
       "Number of times buffer allocations failed");
   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 notbufdflushes;
   SYSCTL_LONG(_vfs, OID_AUTO, notbufdflushes, CTLFLAG_RD, &notbufdflushes, 0,
       "Number of dirty buffer flushes done by the bufdaemon helpers");
   static long barrierwrites;
   SYSCTL_LONG(_vfs, OID_AUTO, barrierwrites, CTLFLAG_RW, &barrierwrites, 0,
       "Number of barrier writes");
   SYSCTL_INT(_vfs, OID_AUTO, unmapped_buf_allowed, CTLFLAG_RD,
       &unmapped_buf_allowed, 0,
       "Permit the use of the unmapped i/o");
   
   /*
    * This lock synchronizes access to bd_request.
    */
   static struct mtx_padalign bdlock;
   
   /*
    * This lock protects the runningbufreq and synchronizes runningbufwakeup and
    * waitrunningbufspace().
    */
   static struct mtx_padalign rbreqlock;
   
   /*
    * Lock that protects needsbuffer and the sleeps/wakeups surrounding it.
    */
   static struct rwlock_padalign nblock;
   
   /*
    * Lock that protects bdirtywait.
    */
   static struct mtx_padalign bdirtylock;
   
   /*
    * 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/wakeup point for the bufspace daemon.
    */
   static int bufspace_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;
   
   /*
    * 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;
   
   /* 
    * 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(), bufspace_wakeup(), bwillwrite(),
    * getnewbuf(), and getblk().
    */
   static volatile int needsbuffer;
   
   /*
    * Synchronization for bwillwrite() waiters.
    */
   static int bdirtywait;
   
   /*
    * Definitions for the buffer free lists.
    */
   #define QUEUE_NONE      0       /* on no queue */
   #define QUEUE_EMPTY     1       /* empty buffer headers */
   #define QUEUE_DIRTY     2       /* B_DELWRI buffers */
   #define QUEUE_CLEAN     3       /* non-B_DELWRI buffers */
   #define QUEUE_SENTINEL  1024    /* not an queue index, but mark for sentinel */
   
   /* Maximum number of clean buffer queues. */
   #define CLEAN_QUEUES    16
   
   /* Configured number of clean queues. */
   static int clean_queues;
   
   /* Maximum number of buffer queues. */
   #define BUFFER_QUEUES   (QUEUE_CLEAN + CLEAN_QUEUES)
   
   /* Queues for free buffers with various properties */
   static TAILQ_HEAD(bqueues, buf) bufqueues[BUFFER_QUEUES] = { { 0 } };
   #ifdef INVARIANTS
   static int bq_len[BUFFER_QUEUES];
   #endif
   
   /*
    * Lock for each bufqueue
    */
   static struct mtx_padalign bqlocks[BUFFER_QUEUES];
   
   /*
    * per-cpu empty buffer cache.
    */
   uma_zone_t buf_zone;
   
   /*
    * Single global constant for BUF_WMESG, to avoid getting multiple references.
    * buf_wmesg is referred from macros.
    */
   const char *buf_wmesg = BUF_WMESG;
   
   static int
   sysctl_runningspace(SYSCTL_HANDLER_ARGS)
   {
           long value;
           int error;
   
           value = *(long *)arg1;
           error = sysctl_handle_long(oidp, &value, 0, req);
           if (error != 0 || req->newptr == NULL)
                   return (error);
           mtx_lock(&rbreqlock);
           if (arg1 == &hirunningspace) {
                   if (value < lorunningspace)
                           error = EINVAL;
                   else
                           hirunningspace = value;
           } else {
                   KASSERT(arg1 == &lorunningspace,
                       ("%s: unknown arg1", __func__));
                   if (value > hirunningspace)
                           error = EINVAL;
                   else
                           lorunningspace = value;
           }
           mtx_unlock(&rbreqlock);
           return (error);
   }
   
   #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
   
   static int
   bqcleanq(void)
   {
           static int nextq;
   
           return ((atomic_fetchadd_int(&nextq, 1) % clean_queues) + QUEUE_CLEAN);
   }
   
   static int
   bqisclean(int qindex)
   {
   
           return (qindex >= QUEUE_CLEAN && qindex < QUEUE_CLEAN + CLEAN_QUEUES);
   }
   
   /*
    *      bqlock:
    *
    *      Return the appropriate queue lock based on the index.
    */
   static inline struct mtx *
   bqlock(int qindex)
   {
   
           return (struct mtx *)&bqlocks[qindex];
   }
   
   /*
    *      bdirtywakeup:
    *
    *      Wakeup any bwillwrite() waiters.
    */
   static void
   bdirtywakeup(void)
   {
           mtx_lock(&bdirtylock);
           if (bdirtywait) {
                   bdirtywait = 0;
                   wakeup(&bdirtywait);
           }
           mtx_unlock(&bdirtylock);
   }
   
   /*
    *      bdirtysub:
    *
    *      Decrement the numdirtybuffers count by one and wakeup any
    *      threads blocked in bwillwrite().
    */
   static void
   bdirtysub(void)
   {
   
           if (atomic_fetchadd_int(&numdirtybuffers, -1) ==
               (lodirtybuffers + hidirtybuffers) / 2)
                   bdirtywakeup();
   }
   
   /*
    *      bdirtyadd:
    *
    *      Increment the numdirtybuffers count by one and wakeup the buf 
    *      daemon if needed.
    */
   static void
   bdirtyadd(void)
   {
   
           /*
            * Only do the wakeup once as we cross the boundary.  The
            * buf daemon will keep running until the condition clears.
            */
           if (atomic_fetchadd_int(&numdirtybuffers, 1) ==
               (lodirtybuffers + hidirtybuffers) / 2)
                   bd_wakeup();
   }
   
   /*
    *      bufspace_wakeup:
    *
    *      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 void
   bufspace_wakeup(void)
   {
   
           /*
            * If someone is waiting for bufspace, wake them up.
            *
            * Since needsbuffer is set prior to doing an additional queue
            * scan it is safe to check for the flag prior to acquiring the
            * lock.  The thread that is preparing to scan again before
            * blocking would discover the buf we released.
            */
           if (needsbuffer) {
                   rw_rlock(&nblock);
                   if (atomic_cmpset_int(&needsbuffer, 1, 0) == 1)
                           wakeup(__DEVOLATILE(void *, &needsbuffer));
                   rw_runlock(&nblock);
           }
   }
   
   /*
    *      bufspace_daemonwakeup:
    *
    *      Wakeup the daemon responsible for freeing clean bufs.
    */
   static void
   bufspace_daemonwakeup(void)
   {
           rw_rlock(&nblock);
           if (bufspace_request == 0) {
                   bufspace_request = 1;
                   wakeup(&bufspace_request);
           }
           rw_runlock(&nblock);
   }
   
   /*
    *      bufspace_adjust:
    *
    *      Adjust the reported bufspace for a KVA managed buffer, possibly
    *      waking any waiters.
    */
   static void
   bufspace_adjust(struct buf *bp, int bufsize)
   {
           long space;
           int diff;
   
           KASSERT((bp->b_flags & B_MALLOC) == 0,
               ("bufspace_adjust: malloc buf %p", bp));
           diff = bufsize - bp->b_bufsize;
           if (diff < 0) {
                   atomic_subtract_long(&bufspace, -diff);
                   bufspace_wakeup();
           } else {
                   space = atomic_fetchadd_long(&bufspace, diff);
                   /* Wake up the daemon on the transition. */
                   if (space < bufspacethresh && space + diff >= bufspacethresh)
                           bufspace_daemonwakeup();
           }
           bp->b_bufsize = bufsize;
   }
   
   /*
    *      bufspace_reserve:
    *
    *      Reserve bufspace before calling allocbuf().  metadata has a
    *      different space limit than data.
    */
   static int
   bufspace_reserve(int size, bool metadata)
   {
           long limit;
           long space;
   
           if (metadata)
                   limit = maxbufspace;
           else
                   limit = hibufspace;
           do {
                   space = bufspace;
                   if (space + size > limit)
                           return (ENOSPC);
           } while (atomic_cmpset_long(&bufspace, space, space + size) == 0);
   
           /* Wake up the daemon on the transition. */
           if (space < bufspacethresh && space + size >= bufspacethresh)
                   bufspace_daemonwakeup();
   
           return (0);
   }
   
   /*
    *      bufspace_release:
    *
    *      Release reserved bufspace after bufspace_adjust() has consumed it.
    */
   static void
   bufspace_release(int size)
   {
           atomic_subtract_long(&bufspace, size);
           bufspace_wakeup();
   }
   
   /*
    *      bufspace_wait:
    *
    *      Wait for bufspace, acting as the buf daemon if a locked vnode is
    *      supplied.  needsbuffer must be set in a safe fashion prior to
    *      polling for space.  The operation must be re-tried on return.
    */
   static void
   bufspace_wait(struct vnode *vp, int gbflags, int slpflag, int slptimeo)
   {
           struct thread *td;
           int error, fl, norunbuf;
   
           if ((gbflags & GB_NOWAIT_BD) != 0)
                   return;
   
           td = curthread;
           rw_wlock(&nblock);
           while (needsbuffer != 0) {
                   if (vp != NULL && vp->v_type != VCHR &&
                       (td->td_pflags & TDP_BUFNEED) == 0) {
                           rw_wunlock(&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.  The getnewbuf() function
                            * may be called while the thread owns lock
                            * for another dirty buffer for the same
                            * vnode, which makes it impossible to use
                            * VOP_FSYNC() there, due to the buffer lock
                            * recursion.
                            */
                           td->td_pflags |= TDP_BUFNEED | TDP_NORUNNINGBUF;
                           fl = buf_flush(vp, flushbufqtarget);
                           td->td_pflags &= norunbuf;
                           rw_wlock(&nblock);
                           if (fl != 0)
                                   continue;
                           if (needsbuffer == 0)
                                   break;
                   }
                   error = rw_sleep(__DEVOLATILE(void *, &needsbuffer), &nblock,
                       (PRIBIO + 4) | slpflag, "newbuf", slptimeo);
                   if (error != 0)
                           break;
           }
           rw_wunlock(&nblock);
   }
   
   
   /*
    *      bufspace_daemon:
    *
    *      buffer space management daemon.  Tries to maintain some marginal
    *      amount of free buffer space so that requesting processes neither
    *      block nor work to reclaim buffers.
    */
   static void
   bufspace_daemon(void)
   {
           for (;;) {
                   kproc_suspend_check(bufspacedaemonproc);
   
                   /*
                    * Free buffers from the clean queue until we meet our
                    * targets.
                    *
                    * Theory of operation:  The buffer cache is most efficient
                    * when some free buffer headers and space are always
                    * available to getnewbuf().  This daemon attempts to prevent
                    * the excessive blocking and synchronization associated
                    * with shortfall.  It goes through three phases according
                    * demand:
                    *
                    * 1)   The daemon wakes up voluntarily once per-second
                    *      during idle periods when the counters are below
                    *      the wakeup thresholds (bufspacethresh, lofreebuffers).
                    *
                    * 2)   The daemon wakes up as we cross the thresholds
                    *      ahead of any potential blocking.  This may bounce
                    *      slightly according to the rate of consumption and
                    *      release.
                    *
                    * 3)   The daemon and consumers are starved for working
                    *      clean buffers.  This is the 'bufspace' sleep below
                    *      which will inefficiently trade bufs with bqrelse
                    *      until we return to condition 2.
                    */
                   while (bufspace > lobufspace ||
                       numfreebuffers < hifreebuffers) {
                           if (buf_recycle(false) != 0) {
                                   atomic_set_int(&needsbuffer, 1);
                                   if (buf_recycle(false) != 0) {
                                           rw_wlock(&nblock);
                                           if (needsbuffer)
                                                   rw_sleep(__DEVOLATILE(void *,
                                                       &needsbuffer), &nblock,
                                                       PRIBIO|PDROP, "bufspace",
                                                       hz/10);
                                           else
                                                   rw_wunlock(&nblock);
                                   }
                           }
                           maybe_yield();
                   }
   
                   /*
                    * Re-check our limits under the exclusive nblock.
                    */
                   rw_wlock(&nblock);
                   if (bufspace < bufspacethresh &&
                       numfreebuffers > lofreebuffers) {
                           bufspace_request = 0;
                           rw_sleep(&bufspace_request, &nblock, PRIBIO|PDROP,
                               "-", hz);
                   } else
                           rw_wunlock(&nblock);
           }
   }
   
   static struct kproc_desc bufspace_kp = {
           "bufspacedaemon",
           bufspace_daemon,
           &bufspacedaemonproc
   };
   SYSINIT(bufspacedaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start,
       &bufspace_kp);
   
   /*
    *      bufmallocadjust:
    *
    *      Adjust the reported bufspace for a malloc managed buffer, possibly
    *      waking any waiters.
    */
   static void
   bufmallocadjust(struct buf *bp, int bufsize)
   {
           int diff;
   
           KASSERT((bp->b_flags & B_MALLOC) != 0,
               ("bufmallocadjust: non-malloc buf %p", bp));
           diff = bufsize - bp->b_bufsize;
           if (diff < 0)
                   atomic_subtract_long(&bufmallocspace, -diff);
           else
                   atomic_add_long(&bufmallocspace, diff);
           bp->b_bufsize = bufsize;
   }
   
   /*
    *      runningwakeup:
    *
    *      Wake up processes that are waiting on asynchronous writes to fall
    *      below lorunningspace.
    */
   static void
   runningwakeup(void)
   {
   
           mtx_lock(&rbreqlock);
           if (runningbufreq) {
                   runningbufreq = 0;
                   wakeup(&runningbufreq);
           }
           mtx_unlock(&rbreqlock);
   }
   
   /*
    *      runningbufwakeup:
    *
    *      Decrement the outstanding write count according.
    */
   void
   runningbufwakeup(struct buf *bp)
   {
           long space, bspace;
   
           bspace = bp->b_runningbufspace;
           if (bspace == 0)
                   return;
           space = atomic_fetchadd_long(&runningbufspace, -bspace);
           KASSERT(space >= bspace, ("runningbufspace underflow %ld %ld",
               space, bspace));
           bp->b_runningbufspace = 0;
           /*
            * Only acquire the lock and wakeup on the transition from exceeding
            * the threshold to falling below it.
            */
           if (space < lorunningspace)
                   return;
           if (space - bspace > lorunningspace)
                   return;
           runningwakeup();
   }
   
   /*
    *      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.
    *
    *      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 = 1;
                   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_ASSERT_LOCKED(m->object);
           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(void)
   {
   
           mtx_lock(&bdlock);
           if (bd_request == 0) {
                   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);
   }
   
   #ifndef NSWBUF_MIN
   #define NSWBUF_MIN      16
   #endif
   
   #ifdef __i386__
   #define TRANSIENT_DENOM 5
   #else
   #define TRANSIENT_DENOM 10
   #endif
   
   /*
    * 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, maxbuf_sz, buf_sz, biotmap_sz;
   
           /*
            * 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 += min((physmem_est - 65536) * 2 / (factor * 5),
                               32 * 1024 * 1024 / (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;
           }
   
           /*
            * Ideal allocation size for the transient bio submap is 10%
            * of the maximal space buffer map.  This roughly corresponds
            * to the amount of the buffer mapped for typical UFS load.
            *
            * Clip the buffer map to reserve space for the transient
            * BIOs, if its extent is bigger than 90% (80% on i386) of the
            * maximum buffer map extent on the platform.
            *
            * The fall-back to the maxbuf in case of maxbcache unset,
            * allows to not trim the buffer KVA for the architectures
            * with ample KVA space.
            */
           if (bio_transient_maxcnt == 0 && unmapped_buf_allowed) {
                   maxbuf_sz = maxbcache != 0 ? maxbcache : maxbuf * BKVASIZE;
                   buf_sz = (long)nbuf * BKVASIZE;
                   if (buf_sz < maxbuf_sz / TRANSIENT_DENOM *
                       (TRANSIENT_DENOM - 1)) {
                           /*
                            * There is more KVA than memory.  Do not
                            * adjust buffer map size, and assign the rest
                            * of maxbuf to transient map.
                            */
                           biotmap_sz = maxbuf_sz - buf_sz;
                   } else {
                           /*
                            * Buffer map spans all KVA we could afford on
                            * this platform.  Give 10% (20% on i386) of
                            * the buffer map to the transient bio map.
                            */
                           biotmap_sz = buf_sz / TRANSIENT_DENOM;
                           buf_sz -= biotmap_sz;
                   }
                   if (biotmap_sz / INT_MAX > MAXPHYS)
                           bio_transient_maxcnt = INT_MAX;
                   else
                           bio_transient_maxcnt = biotmap_sz / MAXPHYS;
                   /*
                    * Artificially limit to 1024 simultaneous in-flight I/Os
                    * using the transient mapping.
                    */
                   if (bio_transient_maxcnt > 1024)
                           bio_transient_maxcnt = 1024;
                   if (tuned_nbuf)
                           nbuf = buf_sz / BKVASIZE;
           }
   
           /*
            * 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 = min(nbuf / 4, 256);
           TUNABLE_INT_FETCH("kern.nswbuf", &nswbuf);
           if (nswbuf < NSWBUF_MIN)
                   nswbuf = NSWBUF_MIN;
   
           /*
            * 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;
  
          CTASSERT(MAXBCACHEBUF >= MAXBSIZE);
          mtx_init(&bqlocks[QUEUE_DIRTY], "bufq dirty lock", NULL, MTX_DEF);
          mtx_init(&bqlocks[QUEUE_EMPTY], "bufq empty lock", NULL, MTX_DEF);
          for (i = QUEUE_CLEAN; i < QUEUE_CLEAN + CLEAN_QUEUES; i++)
                  mtx_init(&bqlocks[i], "bufq clean lock", NULL, MTX_DEF);
          mtx_init(&rbreqlock, "runningbufspace lock", NULL, MTX_DEF);
          rw_init(&nblock, "needsbuffer lock");
          mtx_init(&bdlock, "buffer daemon lock", NULL, MTX_DEF);
          mtx_init(&bdirtylock, "dirty buf lock", NULL, MTX_DEF);
  
          /* next, make a null set of free lists */
          for (i = 0; i < BUFFER_QUEUES; i++)
                  TAILQ_INIT(&bufqueues[i]);
  
          unmapped_buf = (caddr_t)kva_alloc(MAXPHYS);
  
          /* 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;
                  bp->b_rcred = NOCRED;
                  bp->b_wcred = NOCRED;
                  bp->b_qindex = QUEUE_EMPTY;
                  bp->b_xflags = 0;
                  bp->b_data = bp->b_kvabase = unmapped_buf;
                  LIST_INIT(&bp->b_dep);
                  BUF_LOCKINIT(bp);
                  TAILQ_INSERT_TAIL(&bufqueues[QUEUE_EMPTY], bp, b_freelist);
  #ifdef INVARIANTS
                  bq_len[QUEUE_EMPTY]++;
  #endif
          }
  
          /*
           * 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 metadata.  hibufspace is the nominal maximum
           * used by most other requests.  The differential is required to 
           * ensure that metadata deadlocks don't occur.
           *
           * maxbufspace is based on BKVASIZE.  Allocating buffers larger then
           * this may result in KVM fragmentation which is not handled optimally
           * by the system. XXX This is less true with vmem.  We could use
           * PAGE_SIZE.
           */
          maxbufspace = (long)nbuf * BKVASIZE;
          hibufspace = lmax(3 * maxbufspace / 4, maxbufspace - MAXBCACHEBUF * 10);
          lobufspace = (hibufspace / 20) * 19; /* 95% */
          bufspacethresh = lobufspace + (hibufspace - lobufspace) / 2;
  
          /*
           * Note: The 16 MiB 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 MiB limit is the historical upper limit for
           * hirunningspace.
           */
          hirunningspace = lmax(lmin(roundup(hibufspace / 64, MAXBCACHEBUF),
              16 * 1024 * 1024), 1024 * 1024);
          lorunningspace = roundup((hirunningspace * 2) / 3, MAXBCACHEBUF);
  
          /*
           * 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 occurring 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 buffers.
           */
          while ((long)hidirtybuffers * BKVASIZE > 3 * hibufspace / 4) {
                  hidirtybuffers >>= 1;
          }
          lodirtybuffers = hidirtybuffers / 2;
  
          /*
           * lofreebuffers should be sufficient to avoid stalling waiting on
           * buf headers under heavy utilization.  The bufs in per-cpu caches
           * are counted as free but will be unavailable to threads executing
           * on other cpus.
           *
           * hifreebuffers is the free target for the bufspace daemon.  This
           * should be set appropriately to limit work per-iteration.
           */
          lofreebuffers = MIN((nbuf / 25) + (20 * mp_ncpus), 128 * mp_ncpus);
          hifreebuffers = (3 * lofreebuffers) / 2;
          numfreebuffers = nbuf;
  
          bogus_page = vm_page_alloc(NULL, 0, VM_ALLOC_NOOBJ |
              VM_ALLOC_NORMAL | VM_ALLOC_WIRED);
  
          /* Setup the kva and free list allocators. */
          vmem_set_reclaim(buffer_arena, bufkva_reclaim);
          buf_zone = uma_zcache_create("buf free cache", sizeof(struct buf),
              NULL, NULL, NULL, NULL, buf_import, buf_release, NULL, 0);
  
          /*
           * Size the clean queue according to the amount of buffer space.
           * One queue per-256mb up to the max.  More queues gives better
           * concurrency but less accurate LRU.
           */
          clean_queues = MIN(howmany(maxbufspace, 256*1024*1024), CLEAN_QUEUES);
  
  }
  
  #ifdef INVARIANTS
  static inline void
  vfs_buf_check_mapped(struct buf *bp)
  {
  
          KASSERT(bp->b_kvabase != unmapped_buf,
              ("mapped buf: b_kvabase was not updated %p", bp));
          KASSERT(bp->b_data != unmapped_buf,
              ("mapped buf: b_data was not updated %p", bp));
          KASSERT(bp->b_data < unmapped_buf || bp->b_data >= unmapped_buf +
              MAXPHYS, ("b_data + b_offset unmapped %p", bp));
  }
  
  static inline void
  vfs_buf_check_unmapped(struct buf *bp)
  {
  
          KASSERT(bp->b_data == unmapped_buf,
              ("unmapped buf: corrupted b_data %p", bp));
  }
  
  #define BUF_CHECK_MAPPED(bp) vfs_buf_check_mapped(bp)
  #define BUF_CHECK_UNMAPPED(bp) vfs_buf_check_unmapped(bp)
  #else
  #define BUF_CHECK_MAPPED(bp) do {} while (0)
  #define BUF_CHECK_UNMAPPED(bp) do {} while (0)
  #endif
  
  static int
  isbufbusy(struct buf *bp)
  {
          if (((bp->b_flags & B_INVAL) == 0 && BUF_ISLOCKED(bp)) ||
              ((bp->b_flags & (B_DELWRI | B_INVAL)) == B_DELWRI))
                  return (1);
          return (0);
  }
  
  /*
   * Shutdown the system cleanly to prepare for reboot, halt, or power off.
   */
  void
  bufshutdown(int show_busybufs)
  {
          static int first_buf_printf = 1;
          struct buf *bp;
          int iter, nbusy, pbusy;
  #ifndef PREEMPTION
          int subiter;
  #endif
  
          /* 
           * Sync filesystems for shutdown
           */
          wdog_kern_pat(WD_LASTVAL);
          sys_sync(curthread, NULL);
  
          /*
           * With soft updates, some buffers that are
           * written will be remarked as dirty until other
           * buffers are written.
           */
          for (iter = pbusy = 0; iter < 20; iter++) {
                  nbusy = 0;
                  for (bp = &buf[nbuf]; --bp >= buf; )
                          if (isbufbusy(bp))
                                  nbusy++;
                  if (nbusy == 0) {
                          if (first_buf_printf)
                                  printf("All buffers synced.");
                          break;
                  }
                  if (first_buf_printf) {
                          printf("Syncing disks, buffers remaining... ");
                          first_buf_printf = 0;
                  }
                  printf("%d ", nbusy);
                  if (nbusy < pbusy)
                          iter = 0;
                  pbusy = nbusy;
  
                  wdog_kern_pat(WD_LASTVAL);
                  sys_sync(curthread, NULL);
  
  #ifdef PREEMPTION
                  /*
                   * Drop Giant and spin for a while to allow
                   * interrupt threads to run.
                   */
                  DROP_GIANT();
                  DELAY(50000 * iter);
                  PICKUP_GIANT();
  #else
                  /*
                   * Drop Giant and context switch several times to
                   * allow interrupt threads to run.
                   */
                  DROP_GIANT();
                  for (subiter = 0; subiter < 50 * iter; subiter++) {
                          thread_lock(curthread);
                          mi_switch(SW_VOL, NULL);
                          thread_unlock(curthread);
                          DELAY(1000);
                  }
                  PICKUP_GIANT();
  #endif
          }
          printf("\n");
          /*
           * Count only busy local buffers to prevent forcing 
           * a fsck if we're just a client of a wedged NFS server
           */
          nbusy = 0;
          for (bp = &buf[nbuf]; --bp >= buf; ) {
                  if (isbufbusy(bp)) {
  #if 0
  /* XXX: This is bogus.  We should probably have a BO_REMOTE flag instead */
                          if (bp->b_dev == NULL) {
                                  TAILQ_REMOVE(&mountlist,
                                      bp->b_vp->v_mount, mnt_list);
                                  continue;
                          }
  #endif
                          nbusy++;
                          if (show_busybufs > 0) {
                                  printf(
              "%d: buf:%p, vnode:%p, flags:%0x, blkno:%jd, lblkno:%jd, buflock:",
                                      nbusy, bp, bp->b_vp, bp->b_flags,
                                      (intmax_t)bp->b_blkno,
                                      (intmax_t)bp->b_lblkno);
                                  BUF_LOCKPRINTINFO(bp);
                                  if (show_busybufs > 1)
                                          vn_printf(bp->b_vp,
                                              "vnode content: ");
                          }
                  }
          }
          if (nbusy) {
                  /*
                   * Failed to sync all blocks. Indicate this and don't
                   * unmount filesystems (thus forcing an fsck on reboot).
                   */
                  printf("Giving up on %d buffers\n", nbusy);
                  DELAY(5000000); /* 5 seconds */
          } else {
                  if (!first_buf_printf)
                          printf("Final sync complete\n");
                  /*
                   * Unmount filesystems
                   */
                  if (panicstr == NULL)
                          vfs_unmountall();
          }
          swapoff_all();
          DELAY(100000);          /* wait for console output to finish */
  }
  
  static void
  bpmap_qenter(struct buf *bp)
  {
  
          BUF_CHECK_MAPPED(bp);
  
          /*
           * 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));
  }
  
  /*
   *      binsfree:
   *
   *      Insert the buffer into the appropriate free list.
   */
  static void
  binsfree(struct buf *bp, int qindex)
  {
          struct mtx *olock, *nlock;
  
          if (qindex != QUEUE_EMPTY) {
                  BUF_ASSERT_XLOCKED(bp);
          }
  
          /*
           * Stick to the same clean queue for the lifetime of the buf to
           * limit locking below.  Otherwise pick ont sequentially.
           */
          if (qindex == QUEUE_CLEAN) {
                  if (bqisclean(bp->b_qindex))
                          qindex = bp->b_qindex;
                  else
                          qindex = bqcleanq();
          }
  
          /*
           * Handle delayed bremfree() processing.
           */
          nlock = bqlock(qindex);
          if (bp->b_flags & B_REMFREE) {
                  olock = bqlock(bp->b_qindex);
                  mtx_lock(olock);
                  bremfreel(bp);
                  if (olock != nlock) {
                          mtx_unlock(olock);
                          mtx_lock(nlock);
                  }
          } else
                  mtx_lock(nlock);
  
          if (bp->b_qindex != QUEUE_NONE)
                  panic("binsfree: free buffer onto another queue???");
  
          bp->b_qindex = qindex;
          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);
  #ifdef INVARIANTS
          bq_len[bp->b_qindex]++;
  #endif
          mtx_unlock(nlock);
  }
  
  /*
   * buf_free:
   *
   *      Free a buffer to the buf zone once it no longer has valid contents.
   */
  static void
  buf_free(struct buf *bp)
  {
  
          if (bp->b_flags & B_REMFREE)
                  bremfreef(bp);
          if (bp->b_vflags & BV_BKGRDINPROG)
                  panic("losing buffer 1");
          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);
          bufkva_free(bp);
          BUF_UNLOCK(bp);
          uma_zfree(buf_zone, bp);
          atomic_add_int(&numfreebuffers, 1);
          bufspace_wakeup();
  }
  
  /*
   * buf_import:
   *
   *      Import bufs into the uma cache from the buf list.  The system still
   *      expects a static array of bufs and much of the synchronization
   *      around bufs assumes type stable storage.  As a result, UMA is used
   *      only as a per-cpu cache of bufs still maintained on a global list.
   */
  static int
  buf_import(void *arg, void **store, int cnt, int flags)
  {
          struct buf *bp;
          int i;
  
          mtx_lock(&bqlocks[QUEUE_EMPTY]);
          for (i = 0; i < cnt; i++) {
                  bp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTY]);
                  if (bp == NULL)
                          break;
                  bremfreel(bp);
                  store[i] = bp;
          }
          mtx_unlock(&bqlocks[QUEUE_EMPTY]);
  
          return (i);
  }
  
  /*
   * buf_release:
   *
   *      Release bufs from the uma cache back to the buffer queues.
   */
  static void
  buf_release(void *arg, void **store, int cnt)
  {
          int i;
  
          for (i = 0; i < cnt; i++)
                  binsfree(store[i], QUEUE_EMPTY);
  }
  
  /*
   * buf_alloc:
   *
   *      Allocate an empty buffer header.
   */
  static struct buf *
  buf_alloc(void)
  {
          struct buf *bp;
  
          bp = uma_zalloc(buf_zone, M_NOWAIT);
          if (bp == NULL) {
                  bufspace_daemonwakeup();
                  atomic_add_int(&numbufallocfails, 1);
                  return (NULL);
          }
  
          /*
           * Wake-up the bufspace daemon on transition.
           */
          if (atomic_fetchadd_int(&numfreebuffers, -1) == lofreebuffers)
                  bufspace_daemonwakeup();
  
          if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                  panic("getnewbuf_empty: Locked buf %p on free queue.", bp);
          
          KASSERT(bp->b_vp == NULL,
              ("bp: %p still has vnode %p.", bp, bp->b_vp));
          KASSERT((bp->b_flags & (B_DELWRI | B_NOREUSE)) == 0,
              ("invalid buffer %p flags %#x", bp, bp->b_flags));
          KASSERT((bp->b_xflags & (BX_VNCLEAN|BX_VNDIRTY)) == 0,
              ("bp: %p still on a buffer list. xflags %X", bp, bp->b_xflags));
          KASSERT(bp->b_npages == 0,
              ("bp: %p still has %d vm pages\n", bp, bp->b_npages));
          KASSERT(bp->b_kvasize == 0, ("bp: %p still has kva\n", bp));
          KASSERT(bp->b_bufsize == 0, ("bp: %p still has bufspace\n", bp));
  
          bp->b_flags = 0;
          bp->b_ioflags = 0;
          bp->b_xflags = 0;
          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_data = bp->b_kvabase = unmapped_buf;
          bp->b_fsprivate1 = NULL;
          bp->b_fsprivate2 = NULL;
          bp->b_fsprivate3 = NULL;
          LIST_INIT(&bp->b_dep);
  
          return (bp);
  }
  
  /*
   *      buf_qrecycle:
   *
   *      Free a buffer from the given bufqueue.  kva controls whether the
   *      freed buf must own some kva resources.  This is used for
   *      defragmenting.
   */
  static int
  buf_qrecycle(int qindex, bool kva)
  {
          struct buf *bp, *nbp;
  
          if (kva)
                  atomic_add_int(&bufdefragcnt, 1);
          nbp = NULL;
          mtx_lock(&bqlocks[qindex]);
          nbp = TAILQ_FIRST(&bufqueues[qindex]);
  
          /*
           * Run scan, possibly freeing data and/or kva mappings on the fly
           * depending.
           */
          while ((bp = nbp) != NULL) {
                  /*
                   * Calculate next bp (we can only use it if we do not
                   * release the bqlock).
                   */
                  nbp = TAILQ_NEXT(bp, b_freelist);
  
                  /*
                   * If we are defragging then we need a buffer with 
                   * some kva to reclaim.
                   */
                  if (kva && bp->b_kvasize == 0)
                          continue;
  
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
                          continue;
  
                  /*
                   * Skip buffers with background writes in progress.
                   */
                  if ((bp->b_vflags & BV_BKGRDINPROG) != 0) {
                          BUF_UNLOCK(bp);
                          continue;
                  }
  
                  KASSERT(bp->b_qindex == qindex,
                      ("getnewbuf: inconsistent queue %d bp %p", qindex, bp));
                  /*
                   * NOTE:  nbp is now entirely invalid.  We can only restart
                   * the scan from this point on.
                   */
                  bremfreel(bp);
                  mtx_unlock(&bqlocks[qindex]);
  
                  /*
                   * Requeue the background write buffer with error and
                   * restart the scan.
                   */
                  if ((bp->b_vflags & BV_BKGRDERR) != 0) {
                          bqrelse(bp);
                          mtx_lock(&bqlocks[qindex]);
                          nbp = TAILQ_FIRST(&bufqueues[qindex]);
                          continue;
                  }
                  bp->b_flags |= B_INVAL;
                  brelse(bp);
                  return (0);
          }
          mtx_unlock(&bqlocks[qindex]);
  
          return (ENOBUFS);
  }
  
  /*
   *      buf_recycle:
   *
   *      Iterate through all clean queues until we find a buf to recycle or
   *      exhaust the search.
   */
  static int
  buf_recycle(bool kva)
  {
          int qindex, first_qindex;
  
          qindex = first_qindex = bqcleanq();
          do {
                  if (buf_qrecycle(qindex, kva) == 0)
                          return (0);
                  if (++qindex == QUEUE_CLEAN + clean_queues)
                          qindex = QUEUE_CLEAN;
          } while (qindex != first_qindex);
  
          return (ENOBUFS);
  }
  
  /*
   *      buf_scan:
   *
   *      Scan the clean queues looking for a buffer to recycle.  needsbuffer
   *      is set on failure so that the caller may optionally bufspace_wait()
   *      in a race-free fashion.
   */
  static int
  buf_scan(bool defrag)
  {
          int error;
  
          /*
           * To avoid heavy synchronization and wakeup races we set
           * needsbuffer and re-poll before failing.  This ensures that
           * no frees can be missed between an unsuccessful poll and
           * going to sleep in a synchronized fashion.
           */
          if ((error = buf_recycle(defrag)) != 0) {
                  atomic_set_int(&needsbuffer, 1);
                  bufspace_daemonwakeup();
                  error = buf_recycle(defrag);
          }
          if (error == 0)
                  atomic_add_int(&getnewbufrestarts, 1);
          return (error);
  }
  
  /*
   *      bremfree:
   *
   *      Mark the buffer for removal from the appropriate free list.
   *      
   */
  void
  bremfree(struct buf *bp)
  {
  
          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_XLOCKED(bp);
  
          bp->b_flags |= B_REMFREE;
  }
  
  /*
   *      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)
  {
          struct mtx *qlock;
  
          qlock = bqlock(bp->b_qindex);
          mtx_lock(qlock);
          bremfreel(bp);
          mtx_unlock(qlock);
  }
  
  /*
   *      bremfreel:
   *
   *      Removes a buffer from the free list, must be called with the
   *      correct qlock held.
   */
  static void
  bremfreel(struct buf *bp)
  {
  
          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));
          if (bp->b_qindex != QUEUE_EMPTY) {
                  BUF_ASSERT_XLOCKED(bp);
          }
          mtx_assert(bqlock(bp->b_qindex), MA_OWNED);
  
          TAILQ_REMOVE(&bufqueues[bp->b_qindex], bp, b_freelist);
  #ifdef INVARIANTS
          KASSERT(bq_len[bp->b_qindex] >= 1, ("queue %d underflow",
              bp->b_qindex));
          bq_len[bp->b_qindex]--;
  #endif
          bp->b_qindex = QUEUE_NONE;
          bp->b_flags &= ~B_REMFREE;
  }
  
  /*
   *      bufkva_free:
   *
   *      Free the kva allocation for a buffer.
   *
   */
  static void
  bufkva_free(struct buf *bp)
  {
  
  #ifdef INVARIANTS
          if (bp->b_kvasize == 0) {
                  KASSERT(bp->b_kvabase == unmapped_buf &&
                      bp->b_data == unmapped_buf,
                      ("Leaked KVA space on %p", bp));
          } else if (buf_mapped(bp))
                  BUF_CHECK_MAPPED(bp);
          else
                  BUF_CHECK_UNMAPPED(bp);
  #endif
          if (bp->b_kvasize == 0)
                  return;
  
          vmem_free(buffer_arena, (vm_offset_t)bp->b_kvabase, bp->b_kvasize);
          atomic_subtract_long(&bufkvaspace, bp->b_kvasize);
          atomic_add_int(&buffreekvacnt, 1);
          bp->b_data = bp->b_kvabase = unmapped_buf;
          bp->b_kvasize = 0;
  }
  
  /*
   *      bufkva_alloc:
   *
   *      Allocate the buffer KVA and set b_kvasize and b_kvabase.
   */
  static int
  bufkva_alloc(struct buf *bp, int maxsize, int gbflags)
  {
          vm_offset_t addr;
          int error;
  
          KASSERT((gbflags & GB_UNMAPPED) == 0 || (gbflags & GB_KVAALLOC) != 0,
              ("Invalid gbflags 0x%x in %s", gbflags, __func__));
  
          bufkva_free(bp);
  
          addr = 0;
          error = vmem_alloc(buffer_arena, maxsize, M_BESTFIT | M_NOWAIT, &addr);
          if (error != 0) {
                  /*
                   * Buffer map is too fragmented.  Request the caller
                   * to defragment the map.
                   */
                  return (error);
          }
          bp->b_kvabase = (caddr_t)addr;
          bp->b_kvasize = maxsize;
          atomic_add_long(&bufkvaspace, bp->b_kvasize);
          if ((gbflags & GB_UNMAPPED) != 0) {
                  bp->b_data = unmapped_buf;
                  BUF_CHECK_UNMAPPED(bp);
          } else {
                  bp->b_data = bp->b_kvabase;
                  BUF_CHECK_MAPPED(bp);
          }
          return (0);
  }
  
  /*
   *      bufkva_reclaim:
   *
   *      Reclaim buffer kva by freeing buffers holding kva.  This is a vmem
   *      callback that fires to avoid returning failure.
   */
  static void
  bufkva_reclaim(vmem_t *vmem, int flags)
  {
          int i;
  
          for (i = 0; i < 5; i++)
                  if (buf_scan(true) != 0)
                          break;
          return;
  }
  
  
  /*
   * 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)) {
  #ifdef RACCT
                                  if (racct_enable) {
                                          PROC_LOCK(curproc);
                                          racct_add_buf(curproc, rabp, 0);
                                          PROC_UNLOCK(curproc);
                                  }
  #endif /* RACCT */
                                  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);
                  }
          }
  }
  
  /*
   * Entry point for bread() and breadn() via #defines in sys/buf.h.
   *
   * 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(). Also starts asynchronous I/O on read-ahead blocks.
   *
   * Always return a NULL buffer pointer (in bpp) when returning an error.
   */
  int
  breadn_flags(struct vnode *vp, daddr_t blkno, int size, daddr_t *rablkno,
      int *rabsize, int cnt, struct ucred *cred, int flags, struct buf **bpp)
  {
          struct buf *bp;
          int rv = 0, readwait = 0;
  
          CTR3(KTR_BUF, "breadn(%p, %jd, %d)", vp, blkno, size);
          /*
           * Can only return NULL if GB_LOCK_NOWAIT flag is specified.
           */
          *bpp = bp = getblk(vp, blkno, size, 0, 0, flags);
          if (bp == NULL)
                  return (EBUSY);
  
          /* if not found in cache, do some I/O */
          if ((bp->b_flags & B_CACHE) == 0) {
                  if (!TD_IS_IDLETHREAD(curthread)) {
  #ifdef RACCT
                          if (racct_enable) {
                                  PROC_LOCK(curproc);
                                  racct_add_buf(curproc, bp, 0);
                                  PROC_UNLOCK(curproc);
                          }
  #endif /* RACCT */
                          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);
                  if (rv != 0) {
                          brelse(bp);
                          *bpp = NULL;
                  }
          }
          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;
          long space;
          int vp_md;
  
          CTR3(KTR_BUF, "bufwrite(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
          if ((bp->b_bufobj->bo_flag & BO_DEAD) != 0) {
                  bp->b_flags |= B_INVAL | B_RELBUF;
                  bp->b_flags &= ~B_CACHE;
                  brelse(bp);
                  return (ENXIO);
          }
          if (bp->b_flags & B_INVAL) {
                  brelse(bp);
                  return (0);
          }
  
          if (bp->b_flags & B_BARRIER)
                  barrierwrites++;
  
          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.  Increment the bufobj write count
           * before bundirty() call, to prevent other thread from seeing
           * empty dirty list and zero counter for writes in progress,
           * falsely indicating that the bufobj is clean.
           */
          bufobj_wref(bp->b_bufobj);
          bundirty(bp);
  
          bp->b_flags &= ~B_DONE;
          bp->b_ioflags &= ~BIO_ERROR;
          bp->b_flags |= B_CACHE;
          bp->b_iocmd = BIO_WRITE;
  
          vfs_busy_pages(bp, 1);
  
          /*
           * Normal bwrites pipeline writes
           */
          bp->b_runningbufspace = bp->b_bufsize;
          space = atomic_fetchadd_long(&runningbufspace, bp->b_runningbufspace);
  
          if (!TD_IS_IDLETHREAD(curthread)) {
  #ifdef RACCT
                  if (racct_enable) {
                          PROC_LOCK(curproc);
                          racct_add_buf(curproc, bp, 1);
                          PROC_UNLOCK(curproc);
                  }
  #endif /* RACCT */
                  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 if (space > hirunningspace) {
                  /*
                   * 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));
          KASSERT((bp->b_flags & B_BARRIER) == 0,
              ("Barrier request in delayed write %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);
  
          /*
           * 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);
                  bdirtyadd();
          }
  }
  
  /*
   *      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);
                  bdirtysub();
          }
          /*
           * 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);
  }
  
  /*
   *      babarrierwrite:
   *
   *      Asynchronous barrier write.  Start output on a buffer, but do not
   *      wait for it to complete.  Place a write barrier after this write so
   *      that this buffer and all buffers written before it are committed to
   *      the disk before any buffers written after this write are committed
   *      to the disk.  The buffer is released when the output completes.
   */
  void
  babarrierwrite(struct buf *bp)
  {
  
          bp->b_flags |= B_ASYNC | B_BARRIER;
          (void) bwrite(bp);
  }
  
  /*
   *      bbarrierwrite:
   *
   *      Synchronous barrier write.  Start output on a buffer and wait for
   *      it to complete.  Place a write barrier after this write so that
   *      this buffer and all buffers written before it are committed to 
   *      the disk before any buffers written after this write are committed
   *      to the disk.  The buffer is released when the output completes.
   */
  int
  bbarrierwrite(struct buf *bp)
  {
  
          bp->b_flags |= B_BARRIER;
          return (bwrite(bp));
  }
  
  /*
   *      bwillwrite:
   *
   *      Called prior to the locking of any vnodes when we are expecting to
   *      write.  We do not want to starve the buffer cache with too many
   *      dirty buffers so we block here.  By blocking prior to the locking
   *      of any vnodes we attempt to avoid the situation where a locked vnode
   *      prevents the various system daemons from flushing related buffers.
   */
  void
  bwillwrite(void)
  {
  
          if (numdirtybuffers >= hidirtybuffers) {
                  mtx_lock(&bdirtylock);
                  while (numdirtybuffers >= hidirtybuffers) {
                          bdirtywait = 1;
                          msleep(&bdirtywait, &bdirtylock, (PRIBIO + 4),
                              "flswai", 0);
                  }
                  mtx_unlock(&bdirtylock);
          }
  }
  
  /*
   * Return true if we have too many dirty buffers.
   */
  int
  buf_dirty_count_severe(void)
  {
  
          return(numdirtybuffers >= hidirtybuffers);
  }
  
  /*
   *      brelse:
   *
   *      Release a busy buffer and, if requested, free its resources.  The
   *      buffer will be stashed in the appropriate bufqueue[] allowing it
   *      to be accessed later as a cache entity or reused for other purposes.
   */
  void
  brelse(struct buf *bp)
  {
          int qindex;
  
          /*
           * Many functions erroneously call brelse with a NULL bp under rare
           * error conditions. Simply return when called with a NULL bp.
           */
          if (bp == NULL)
                  return;
          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));
          KASSERT((bp->b_flags & B_VMIO) != 0 || (bp->b_flags & B_NOREUSE) == 0,
              ("brelse: non-VMIO buffer marked NOREUSE"));
  
          if (BUF_LOCKRECURSED(bp)) {
                  /*
                   * Do not process, in particular, do not handle the
                   * B_INVAL/B_RELBUF and do not release to free list.
                   */
                  BUF_UNLOCK(bp);
                  return;
          }
  
          if (bp->b_flags & B_MANAGED) {
                  bqrelse(bp);
                  return;
          }
  
          if ((bp->b_vflags & (BV_BKGRDINPROG | BV_BKGRDERR)) == BV_BKGRDERR) {
                  BO_LOCK(bp->b_bufobj);
                  bp->b_vflags &= ~BV_BKGRDERR;
                  BO_UNLOCK(bp->b_bufobj);
                  bdirty(bp);
          }
          if (bp->b_iocmd == BIO_WRITE && (bp->b_ioflags & BIO_ERROR) &&
              !(bp->b_flags & B_INVAL)) {
                  /*
                   * Failed write, redirty.  Must clear BIO_ERROR to prevent
                   * pages from being scrapped.
                   */
                  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 read 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)
                          bdirtysub();
                  bp->b_flags &= ~(B_DELWRI | B_CACHE);
                  if ((bp->b_flags & B_VMIO) == 0) {
                          allocbuf(bp, 0);
                          if (bp->b_vp)
                                  brelvp(bp);
                  }
          }
  
          /*
           * We must clear B_RELBUF if B_DELWRI is set.  If vfs_vmio_truncate() 
           * 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_truncate(), even
           * if B_DELWRI is set.
           */
          if (bp->b_flags & B_DELWRI)
                  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_flags & B_NOCACHE ||
              (bp->b_ioflags & BIO_ERROR && bp->b_iocmd == BIO_READ)) &&
              !(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))) {
                  vfs_vmio_invalidate(bp);
                  allocbuf(bp, 0);
          }
  
          if ((bp->b_flags & (B_INVAL | B_RELBUF)) != 0 ||
              (bp->b_flags & (B_DELWRI | B_NOREUSE)) == B_NOREUSE) {
                  allocbuf(bp, 0);
                  bp->b_flags &= ~B_NOREUSE;
                  if (bp->b_vp != NULL)
                          brelvp(bp);
          }
                          
          /*
           * 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) {
                  buf_free(bp);
                  return;
          }
          /* buffers with junk contents */
          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");
                  qindex = QUEUE_CLEAN;
                  bp->b_flags |= B_AGE;
          /* remaining buffers */
          } else if (bp->b_flags & B_DELWRI)
                  qindex = QUEUE_DIRTY;
          else
                  qindex = QUEUE_CLEAN;
  
          binsfree(bp, qindex);
  
          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);
          if (qindex == QUEUE_CLEAN)
                  bufspace_wakeup();
  }
  
  /*
   * Release a buffer back to the appropriate queue but do not try to free
   * it.  The buffer is expected to be used again soon.
   *
   * bqrelse() is used by bdwrite() to requeue a delayed write, and used by
   * biodone() to requeue an async I/O on completion.  It is also used when
   * known good buffers need to be requeued but we think we may need the data
   * again soon.
   *
   * XXX we should be able to leave the B_RELBUF hint set on completion.
   */
  void
  bqrelse(struct buf *bp)
  {
          int qindex;
  
          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));
  
          qindex = QUEUE_NONE;
          if (BUF_LOCKRECURSED(bp)) {
                  /* do not release to free list */
                  BUF_UNLOCK(bp);
                  return;
          }
          bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
  
          if (bp->b_flags & B_MANAGED) {
                  if (bp->b_flags & B_REMFREE)
                          bremfreef(bp);
                  goto out;
          }
  
          /* buffers with stale but valid contents */
          if ((bp->b_flags & B_DELWRI) != 0 || (bp->b_vflags & (BV_BKGRDINPROG |
              BV_BKGRDERR)) == BV_BKGRDERR) {
                  BO_LOCK(bp->b_bufobj);
                  bp->b_vflags &= ~BV_BKGRDERR;
                  BO_UNLOCK(bp->b_bufobj);
                  qindex = QUEUE_DIRTY;
          } else {
                  if ((bp->b_flags & B_DELWRI) == 0 &&
                      (bp->b_xflags & BX_VNDIRTY))
                          panic("bqrelse: not dirty");
                  if ((bp->b_flags & B_NOREUSE) != 0) {
                          brelse(bp);
                          return;
                  }
                  qindex = QUEUE_CLEAN;
          }
          binsfree(bp, qindex);
  
  out:
          /* unlock */
          BUF_UNLOCK(bp);
          if (qindex == QUEUE_CLEAN)
                  bufspace_wakeup();
  }
  
  /*
   * Complete I/O to a VMIO backed page.  Validate the pages as appropriate,
   * restore bogus pages.
   */
  static void
  vfs_vmio_iodone(struct buf *bp)
  {
          vm_ooffset_t foff;
          vm_page_t m;
          vm_object_t obj;
          struct vnode *vp;
          int bogus, i, iosize;
  
          obj = bp->b_bufobj->bo_object;
          KASSERT(obj->paging_in_progress >= bp->b_npages,
              ("vfs_vmio_iodone: paging in progress(%d) < b_npages(%d)",
              obj->paging_in_progress, bp->b_npages));
  
          vp = bp->b_vp;
          KASSERT(vp->v_holdcnt > 0,
              ("vfs_vmio_iodone: vnode %p has zero hold count", vp));
          KASSERT(vp->v_object != NULL,
              ("vfs_vmio_iodone: vnode %p has no vm_object", vp));
  
          foff = bp->b_offset;
          KASSERT(bp->b_offset != NOOFFSET,
              ("vfs_vmio_iodone: bp %p has no buffer offset", bp));
  
          bogus = 0;
          iosize = bp->b_bcount - bp->b_resid;
          VM_OBJECT_WLOCK(obj);
          for (i = 0; i < bp->b_npages; i++) {
                  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 = 1;
                          m = vm_page_lookup(obj, OFF_TO_IDX(foff));
                          if (m == NULL)
                                  panic("biodone: page disappeared!");
                          bp->b_pages[i] = m;
                  } else if ((bp->b_iocmd == BIO_READ) && resid > 0) {
                          /*
                           * 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.
                           */
                          KASSERT((m->dirty & vm_page_bits(foff & PAGE_MASK,
                              resid)) == 0, ("vfs_vmio_iodone: page %p "
                              "has unexpected dirty bits", m));
                          vfs_page_set_valid(bp, foff, m);
                  }
                  KASSERT(OFF_TO_IDX(foff) == m->pindex,
                      ("vfs_vmio_iodone: foff(%jd)/pindex(%ju) mismatch",
                      (intmax_t)foff, (uintmax_t)m->pindex));
  
                  vm_page_sunbusy(m);
                  foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                  iosize -= resid;
          }
          vm_object_pip_wakeupn(obj, bp->b_npages);
          VM_OBJECT_WUNLOCK(obj);
          if (bogus && buf_mapped(bp)) {
                  BUF_CHECK_MAPPED(bp);
                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                      bp->b_pages, bp->b_npages);
          }
  }
  
  /*
   * Unwire a page held by a buf and place it on the appropriate vm queue.
   */
  static void
  vfs_vmio_unwire(struct buf *bp, vm_page_t m)
  {
          bool freed;
  
          vm_page_lock(m);
          if (vm_page_unwire(m, PQ_NONE)) {
                  /*
                   * Determine if the page should be freed before adding
                   * it to the inactive queue.
                   */
                  if (m->valid == 0) {
                          freed = !vm_page_busied(m);
                          if (freed)
                                  vm_page_free(m);
                  } else if ((bp->b_flags & B_DIRECT) != 0)
                          freed = vm_page_try_to_free(m);
                  else
                          freed = false;
                  if (!freed) {
                          /*
                           * If the page is unlikely to be reused, let the
                           * VM know.  Otherwise, maintain LRU page
                           * ordering and put the page at the tail of the
                           * inactive queue.
                           */
                          if ((bp->b_flags & B_NOREUSE) != 0)
                                  vm_page_deactivate_noreuse(m);
                          else
                                  vm_page_deactivate(m);
                  }
          }
          vm_page_unlock(m);
  }
  
  /*
   * Perform page invalidation when a buffer is released.  The fully invalid
   * pages will be reclaimed later in vfs_vmio_truncate().
   */
  static void
  vfs_vmio_invalidate(struct buf *bp)
  {
          vm_object_t obj;
          vm_page_t m;
          int i, resid, poffset, presid;
  
          if (buf_mapped(bp)) {
                  BUF_CHECK_MAPPED(bp);
                  pmap_qremove(trunc_page((vm_offset_t)bp->b_data), bp->b_npages);
          } else
                  BUF_CHECK_UNMAPPED(bp);
          /*
           * 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
           */
          obj = bp->b_bufobj->bo_object;
          resid = bp->b_bufsize;
          poffset = bp->b_offset & PAGE_MASK;
          VM_OBJECT_WLOCK(obj);
          for (i = 0; i < bp->b_npages; i++) {
                  m = bp->b_pages[i];
                  if (m == bogus_page)
                          panic("vfs_vmio_invalidate: Unexpected bogus page.");
                  bp->b_pages[i] = NULL;
  
                  presid = resid > (PAGE_SIZE - poffset) ?
                      (PAGE_SIZE - poffset) : resid;
                  KASSERT(presid >= 0, ("brelse: extra page"));
                  while (vm_page_xbusied(m)) {
                          vm_page_lock(m);
                          VM_OBJECT_WUNLOCK(obj);
                          vm_page_busy_sleep(m, "mbncsh");
                          VM_OBJECT_WLOCK(obj);
                  }
                  if (pmap_page_wired_mappings(m) == 0)
                          vm_page_set_invalid(m, poffset, presid);
                  vfs_vmio_unwire(bp, m);
                  resid -= presid;
                  poffset = 0;
          }
          VM_OBJECT_WUNLOCK(obj);
          bp->b_npages = 0;
  }
  
  /*
   * Page-granular truncation of an existing VMIO buffer.
   */
  static void
  vfs_vmio_truncate(struct buf *bp, int desiredpages)
  {
          vm_object_t obj;
          vm_page_t m;
          int i;
  
          if (bp->b_npages == desiredpages)
                  return;
  
          if (buf_mapped(bp)) {
                  BUF_CHECK_MAPPED(bp);
                  pmap_qremove((vm_offset_t)trunc_page((vm_offset_t)bp->b_data) +
                      (desiredpages << PAGE_SHIFT), bp->b_npages - desiredpages);
          } else
                  BUF_CHECK_UNMAPPED(bp);
          obj = bp->b_bufobj->bo_object;
          if (obj != NULL)
                  VM_OBJECT_WLOCK(obj);
          for (i = desiredpages; i < bp->b_npages; i++) {
                  m = bp->b_pages[i];
                  KASSERT(m != bogus_page, ("allocbuf: bogus page found"));
                  bp->b_pages[i] = NULL;
                  vfs_vmio_unwire(bp, m);
          }
          if (obj != NULL)
                  VM_OBJECT_WUNLOCK(obj);
          bp->b_npages = desiredpages;
  }
  
  /*
   * Byte granular extension of VMIO buffers.
   */
  static void
  vfs_vmio_extend(struct buf *bp, int desiredpages, int size)
  {
          /*
           * We are growing the buffer, possibly in a 
           * byte-granular fashion.
           */
          vm_object_t obj;
          vm_offset_t toff;
          vm_offset_t tinc;
          vm_page_t m;
  
          /*
           * 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_WLOCK(obj);
          while (bp->b_npages < desiredpages) {
                  /*
                   * We must allocate system pages since blocking
                   * here could interfere with paging I/O, no
                   * matter which process we are.
                   *
                   * Only exclusive busy can be tested here.
                   * Blocking on shared busy might lead to
                   * deadlocks once allocbuf() is called after
                   * pages are vfs_busy_pages().
                   */
                  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_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 ), not the
           * aligned range ( newbsize ).
           *
           * The VM test is against m->valid, which is DEV_BSIZE
           * aligned.  Needless to say, the validity of the data
           * needs to also be DEV_BSIZE aligned.  Note that this
           * fails with NFS if the server or some other client
           * extends the file's EOF.  If our buffer is resized, 
           * B_CACHE may remain set! XXX
           */
          toff = bp->b_bcount;
          tinc = PAGE_SIZE - ((bp->b_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;
                  m = bp->b_pages[pi];
                  vfs_buf_test_cache(bp, bp->b_offset, toff, tinc, m);
                  toff += tinc;
                  tinc = PAGE_SIZE;
          }
          VM_OBJECT_WUNLOCK(obj);
  
          /*
           * Step 3, fixup the KVA pmap.
           */
          if (buf_mapped(bp))
                  bpmap_qenter(bp);
          else
                  BUF_CHECK_UNMAPPED(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;
          int gbflags;
  
          bo = &vp->v_bufobj;
          gbflags = (bp->b_data == unmapped_buf) ? GB_UNMAPPED : 0;
          /*
           * 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_RLOCK(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_RUNLOCK(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,
                              gbflags);
                          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_kva:
   *
   *      Allocate KVA for an empty buf header according to gbflags.
   */
  static int
  getnewbuf_kva(struct buf *bp, int gbflags, int maxsize)
  {
  
          if ((gbflags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_UNMAPPED) {
                  /*
                   * In order to keep fragmentation sane we only allocate kva
                   * in BKVASIZE chunks.  XXX with vmem we can do page size.
                   */
                  maxsize = (maxsize + BKVAMASK) & ~BKVAMASK;
  
                  if (maxsize != bp->b_kvasize &&
                      bufkva_alloc(bp, maxsize, gbflags))
                          return (ENOSPC);
          }
          return (0);
  }
  
  /*
   *      getnewbuf:
   *
   *      Find and initialize a new buffer header, freeing up existing buffers
   *      in the bufqueues as necessary.  The new buffer is returned locked.
   *
   *      We block if:
   *              We have insufficient buffer headers
   *              We have insufficient buffer space
   *              buffer_arena is too fragmented ( space reservation fails )
   *              If we have to flush dirty buffers ( but we try to avoid this )
   *
   *      The caller is responsible for releasing the reserved bufspace after
   *      allocbuf() is called.
   */
  static struct buf *
  getnewbuf(struct vnode *vp, int slpflag, int slptimeo, int maxsize, int gbflags)
  {
          struct buf *bp;
          bool metadata, reserved;
  
          bp = NULL;
          KASSERT((gbflags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_KVAALLOC,
              ("GB_KVAALLOC only makes sense with GB_UNMAPPED"));
          if (!unmapped_buf_allowed)
                  gbflags &= ~(GB_UNMAPPED | GB_KVAALLOC);
  
          if (vp == NULL || (vp->v_vflag & (VV_MD | VV_SYSTEM)) != 0 ||
              vp->v_type == VCHR)
                  metadata = true;
          else
                  metadata = false;
          atomic_add_int(&getnewbufcalls, 1);
          reserved = false;
          do {
                  if (reserved == false &&
                      bufspace_reserve(maxsize, metadata) != 0)
                          continue;
                  reserved = true;
                  if ((bp = buf_alloc()) == NULL)
                          continue;
                  if (getnewbuf_kva(bp, gbflags, maxsize) == 0)
                          return (bp);
                  break;
          } while(buf_scan(false) == 0);
  
          if (reserved)
                  atomic_subtract_long(&bufspace, maxsize);
          if (bp != NULL) {
                  bp->b_flags |= B_INVAL;
                  brelse(bp);
          }
          bufspace_wait(vp, gbflags, slpflag, slptimeo);
  
          return (NULL);
  }
  
  /*
   *      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_flush(struct vnode *vp, int target)
  {
          int flushed;
  
          flushed = flushbufqueues(vp, target, 0);
          if (flushed == 0) {
                  /*
                   * Could not find any buffers without rollback
                   * dependencies, so just write the first one
                   * in the hopes of eventually making progress.
                   */
                  if (vp != NULL && target > 2)
                          target /= 2;
                  flushbufqueues(vp, target, 1);
          }
          return (flushed);
  }
  
  static void
  buf_daemon()
  {
          int lodirty;
  
          /*
           * 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);
                  lodirty = lodirtybuffers;
                  if (bd_speedupreq) {
                          lodirty = 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.
                   */
                  while (numdirtybuffers > lodirty) {
                          if (buf_flush(NULL, numdirtybuffers - lodirty) == 0)
                                  break;
                          kern_yield(PRI_USER);
                  }
  
                  /*
                   * 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 for a short period
                   * to avoid endless loops on unlockable buffers.
                   */
                  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;
                          /*
                           * Do an extra wakeup in case dirty threshold
                           * changed via sysctl and the explicit transition
                           * out of shortfall was missed.
                           */
                          bdirtywakeup();
                          if (runningbufspace <= lorunningspace)
                                  runningwakeup();
                          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 target, int flushdeps)
  {
          struct buf *sentinel;
          struct vnode *vp;
          struct mount *mp;
          struct buf *bp;
          int hasdeps;
          int flushed;
          int queue;
          int error;
          bool unlock;
  
          flushed = 0;
          queue = QUEUE_DIRTY;
          bp = NULL;
          sentinel = malloc(sizeof(struct buf), M_TEMP, M_WAITOK | M_ZERO);
          sentinel->b_qindex = QUEUE_SENTINEL;
          mtx_lock(&bqlocks[queue]);
          TAILQ_INSERT_HEAD(&bufqueues[queue], sentinel, b_freelist);
          mtx_unlock(&bqlocks[queue]);
          while (flushed != target) {
                  maybe_yield();
                  mtx_lock(&bqlocks[queue]);
                  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 {
                          mtx_unlock(&bqlocks[queue]);
                          break;
                  }
                  /*
                   * Skip sentinels inserted by other invocations of the
                   * flushbufqueues(), taking care to not reorder them.
                   *
                   * Only flush the buffers that belong to the
                   * vnode locked by the curthread.
                   */
                  if (bp->b_qindex == QUEUE_SENTINEL || (lvp != NULL &&
                      bp->b_vp != lvp)) {
                          mtx_unlock(&bqlocks[queue]);
                          continue;
                  }
                  error = BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL);
                  mtx_unlock(&bqlocks[queue]);
                  if (error != 0)
                          continue;
                  if (bp->b_pin_count > 0) {
                          BUF_UNLOCK(bp);
                          continue;
                  }
                  /*
                   * BKGRDINPROG can only be set with the buf and bufobj
                   * locks both held.  We tolerate a race to clear it here.
                   */
                  if ((bp->b_vflags & BV_BKGRDINPROG) != 0 ||
                      (bp->b_flags & B_DELWRI) == 0) {
                          BUF_UNLOCK(bp);
                          continue;
                  }
                  if (bp->b_flags & B_INVAL) {
                          bremfreef(bp);
                          brelse(bp);
                          flushed++;
                          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 (lvp == NULL) {
                          unlock = true;
                          error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT);
                  } else {
                          ASSERT_VOP_LOCKED(vp, "getbuf");
                          unlock = false;
                          error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 :
                              vn_lock(vp, LK_TRYUPGRADE);
                  }
                  if (error == 0) {
                          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);
                                  notbufdflushes++;
                          }
                          vn_finished_write(mp);
                          if (unlock)
                                  VOP_UNLOCK(vp, 0);
                          flushwithdeps += hasdeps;
                          flushed++;
  
                          /*
                           * Sleeping on runningbufspace while holding
                           * vnode lock leads to deadlock.
                           */
                          if (curproc == bufdaemonproc &&
                              runningbufspace > hirunningspace)
                                  waitrunningbufspace();
                          continue;
                  }
                  vn_finished_write(mp);
                  BUF_UNLOCK(bp);
          }
          mtx_lock(&bqlocks[queue]);
          TAILQ_REMOVE(&bufqueues[queue], sentinel, b_freelist);
          mtx_unlock(&bqlocks[queue]);
          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_RLOCK(bo);
          bp = gbincore(bo, blkno);
          BO_RUNLOCK(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_RLOCK(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_RUNLOCK(obj);
          return 1;
  
  notinmem:
          VM_OBJECT_RUNLOCK(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_WLOCK(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_WUNLOCK(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_ASSERT_WLOCKED(object);
  
          /*
           * We qualify the scan for modified pages on whether the
           * object has been flushed yet.
           */
          if ((object->flags & OBJ_MIGHTBEDIRTY) != 0) {
                  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;
                  }
          }
  }
  
  /*
   * Allocate the KVA mapping for an existing buffer.
   * If an unmapped buffer is provided but a mapped buffer is requested, take
   * also care to properly setup mappings between pages and KVA.
   */
  static void
  bp_unmapped_get_kva(struct buf *bp, daddr_t blkno, int size, int gbflags)
  {
          int bsize, maxsize, need_mapping, need_kva;
          off_t offset;
  
          need_mapping = bp->b_data == unmapped_buf &&
              (gbflags & GB_UNMAPPED) == 0;
          need_kva = bp->b_kvabase == unmapped_buf &&
              bp->b_data == unmapped_buf &&
              (gbflags & GB_KVAALLOC) != 0;
          if (!need_mapping && !need_kva)
                  return;
  
          BUF_CHECK_UNMAPPED(bp);
  
          if (need_mapping && bp->b_kvabase != unmapped_buf) {
                  /*
                   * Buffer is not mapped, but the KVA was already
                   * reserved at the time of the instantiation.  Use the
                   * allocated space.
                   */
                  goto has_addr;
          }
  
          /*
           * Calculate the amount of the address space we would reserve
           * if the buffer was mapped.
           */
          bsize = vn_isdisk(bp->b_vp, NULL) ? DEV_BSIZE : bp->b_bufobj->bo_bsize;
          KASSERT(bsize != 0, ("bsize == 0, check bo->bo_bsize"));
          offset = blkno * bsize;
          maxsize = size + (offset & PAGE_MASK);
          maxsize = imax(maxsize, bsize);
  
          while (bufkva_alloc(bp, maxsize, gbflags) != 0) {
                  if ((gbflags & GB_NOWAIT_BD) != 0) {
                          /*
                           * XXXKIB: defragmentation cannot
                           * succeed, not sure what else to do.
                           */
                          panic("GB_NOWAIT_BD and GB_UNMAPPED %p", bp);
                  }
                  atomic_add_int(&mappingrestarts, 1);
                  bufspace_wait(bp->b_vp, gbflags, 0, 0);
          }
  has_addr:
          if (need_mapping) {
                  /* b_offset is handled by bpmap_qenter. */
                  bp->b_data = bp->b_kvabase;
                  BUF_CHECK_MAPPED(bp);
                  bpmap_qenter(bp);
          }
  }
  
  /*
   *      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 successful 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 bsize, error, maxsize, vmio;
          off_t offset;
  
          CTR3(KTR_BUF, "getblk(%p, %ld, %d)", vp, (long)blkno, size);
          KASSERT((flags & (GB_UNMAPPED | GB_KVAALLOC)) != GB_KVAALLOC,
              ("GB_KVAALLOC only makes sense with GB_UNMAPPED"));
          ASSERT_VOP_LOCKED(vp, "getblk");
          if (size > MAXBCACHEBUF)
                  panic("getblk: size(%d) > MAXBCACHEBUF(%d)\n", size,
                      MAXBCACHEBUF);
          if (!unmapped_buf_allowed)
                  flags &= ~(GB_UNMAPPED | GB_KVAALLOC);
  
          bo = &vp->v_bufobj;
  loop:
          BO_RLOCK(bo);
          bp = gbincore(bo, blkno);
          if (bp != NULL) {
                  int lockflags;
                  /*
                   * Buffer is in-core.  If the buffer is not busy nor managed,
                   * 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_LOCKPTR(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);
                  /* If recursed, assume caller knows the rules. */
                  else if (BUF_LOCKRECURSED(bp))
                          goto end;
  
                  /*
                   * 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;
                  if (bp->b_flags & B_MANAGED)
                          MPASS(bp->b_qindex == QUEUE_NONE);
                  else
                          bremfree(bp);
  
                  /*
                   * check for size inconsistencies 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;
                          }
                  }
  
                  /*
                   * Handle the case of unmapped buffer which should
                   * become mapped, or the buffer for which KVA
                   * reservation is requested.
                   */
                  bp_unmapped_get_kva(bp, blkno, size, flags);
  
                  /*
                   * If the size is inconsistent 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.
                   */
                  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 {
                  /*
                   * 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_RUNLOCK(bo);
                  /*
                   * If the user does not want us to create the buffer, bail out
                   * here.
                   */
                  if (flags & GB_NOCREAT)
                          return NULL;
                  if (numfreebuffers == 0 && TD_IS_IDLETHREAD(curthread))
                          return NULL;
  
                  bsize = vn_isdisk(vp, NULL) ? DEV_BSIZE : bo->bo_bsize;
                  KASSERT(bsize != 0, ("bsize == 0, check bo->bo_bsize"));
                  offset = blkno * bsize;
                  vmio = vp->v_object != NULL;
                  if (vmio) {
                          maxsize = size + (offset & PAGE_MASK);
                  } else {
                          maxsize = size;
                          /* Do not allow non-VMIO notmapped buffers. */
                          flags &= ~(GB_UNMAPPED | GB_KVAALLOC);
                  }
                  maxsize = imax(maxsize, bsize);
  
                  bp = getnewbuf(vp, slpflag, slptimeo, maxsize, flags);
                  if (bp == NULL) {
                          if (slpflag || slptimeo)
                                  return NULL;
                          /*
                           * XXX This is here until the sleep path is diagnosed
                           * enough to work under very low memory conditions.
                           *
                           * There's an issue on low memory, 4BSD+non-preempt
                           * systems (eg MIPS routers with 32MB RAM) where buffer
                           * exhaustion occurs without sleeping for buffer
                           * reclaimation.  This just sticks in a loop and
                           * constantly attempts to allocate a buffer, which
                           * hits exhaustion and tries to wakeup bufdaemon.
                           * This never happens because we never yield.
                           *
                           * The real solution is to identify and fix these cases
                           * so we aren't effectively busy-waiting in a loop
                           * until the reclaimation path has cycles to run.
                           */
                          kern_yield(PRI_USER);
                          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);
                          bufspace_release(maxsize);
                          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;
                          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));
                          BUF_CHECK_MAPPED(bp);
                  }
  
                  allocbuf(bp, size);
                  bufspace_release(maxsize);
                  bp->b_flags &= ~B_DONE;
          }
          CTR4(KTR_BUF, "getblk(%p, %ld, %d) = %p", vp, (long)blkno, size, bp);
          BUF_ASSERT_HELD(bp);
  end:
          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, maxsize, flags)) == NULL) {
                  if ((flags & GB_NOWAIT_BD) &&
                      (curthread->td_pflags & TDP_BUFNEED) != 0)
                          return (NULL);
          }
          allocbuf(bp, size);
          bufspace_release(maxsize);
          bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
          BUF_ASSERT_HELD(bp);
          return (bp);
  }
  
  /*
   * Truncate the backing store for a non-vmio buffer.
   */
  static void
  vfs_nonvmio_truncate(struct buf *bp, int newbsize)
  {
  
          if (bp->b_flags & B_MALLOC) {
                  /*
                   * malloced buffers are not shrunk
                   */
                  if (newbsize == 0) {
                          bufmallocadjust(bp, 0);
                          free(bp->b_data, M_BIOBUF);
                          bp->b_data = bp->b_kvabase;
                          bp->b_flags &= ~B_MALLOC;
                  }
                  return;
          }
          vm_hold_free_pages(bp, newbsize);
          bufspace_adjust(bp, newbsize);
  }
  
  /*
   * Extend the backing for a non-VMIO buffer.
   */
  static void
  vfs_nonvmio_extend(struct buf *bp, int newbsize)
  {
          caddr_t origbuf;
          int origbufsize;
  
          /*
           * 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 (bp->b_bufsize == 0 && newbsize <= PAGE_SIZE/2 &&
              bufmallocspace < maxbufmallocspace) {
                  bp->b_data = malloc(newbsize, M_BIOBUF, M_WAITOK);
                  bp->b_flags |= B_MALLOC;
                  bufmallocadjust(bp, newbsize);
                  return;
          }
  
          /*
           * If the buffer is growing on its other-than-first
           * allocation then we revert to the page-allocation
           * scheme.
           */
          origbuf = NULL;
          origbufsize = 0;
          if (bp->b_flags & B_MALLOC) {
                  origbuf = bp->b_data;
                  origbufsize = bp->b_bufsize;
                  bp->b_data = bp->b_kvabase;
                  bufmallocadjust(bp, 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 != NULL) {
                  bcopy(origbuf, bp->b_data, origbufsize);
                  free(origbuf, M_BIOBUF);
          }
          bufspace_adjust(bp, newbsize);
  }
  
  /*
   * 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 inconsistent 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;
  
          BUF_ASSERT_HELD(bp);
  
          if (bp->b_bcount == size)
                  return (1);
  
          if (bp->b_kvasize != 0 && bp->b_kvasize < size)
                  panic("allocbuf: buffer too small");
  
          newbsize = roundup2(size, DEV_BSIZE);
          if ((bp->b_flags & B_VMIO) == 0) {
                  if ((bp->b_flags & B_MALLOC) == 0)
                          newbsize = round_page(newbsize);
                  /*
                   * Just get anonymous memory from the kernel.  Don't
                   * mess with B_CACHE.
                   */
                  if (newbsize < bp->b_bufsize)
                          vfs_nonvmio_truncate(bp, newbsize);
                  else if (newbsize > bp->b_bufsize)
                          vfs_nonvmio_extend(bp, newbsize);
          } else {
                  int desiredpages;
  
                  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)
                          vfs_vmio_truncate(bp, desiredpages);
                  /* XXX This looks as if it should be newbsize > b_bufsize */
                  else if (size > bp->b_bcount)
                          vfs_vmio_extend(bp, desiredpages, size);
                  bufspace_adjust(bp, newbsize);
          }
          bp->b_bcount = size;            /* requested buffer size. */
          return (1);
  }
  
  extern int inflight_transient_maps;
  
  void
  biodone(struct bio *bp)
  {
          struct mtx *mtxp;
          void (*done)(struct bio *);
          vm_offset_t start, end;
  
          if ((bp->bio_flags & BIO_TRANSIENT_MAPPING) != 0) {
                  bp->bio_flags &= ~BIO_TRANSIENT_MAPPING;
                  bp->bio_flags |= BIO_UNMAPPED;
                  start = trunc_page((vm_offset_t)bp->bio_data);
                  end = round_page((vm_offset_t)bp->bio_data + bp->bio_length);
                  bp->bio_data = unmapped_buf;
                  pmap_qremove(start, OFF_TO_IDX(end - start));
                  vmem_free(transient_arena, start, end - start);
                  atomic_add_int(&inflight_transient_maps, -1);
          }
          done = bp->bio_done;
          if (done == NULL) {
                  mtxp = mtx_pool_find(mtxpool_sleep, bp);
                  mtx_lock(mtxp);
                  bp->bio_flags |= BIO_DONE;
                  wakeup(bp);
                  mtx_unlock(mtxp);
          } else {
                  bp->bio_flags |= BIO_DONE;
                  done(bp);
          }
  }
  
  /*
   * Wait for a BIO to finish.
   */
  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, 0);
          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);
          }
  }
  
  /*
   *      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 occurred, 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 existence
   *      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) {
                  /*
                   * Set B_CACHE if the op was a normal read and no error
                   * occurred.  B_CACHE is set for writes in the b*write()
                   * routines.
                   */
                  if (bp->b_iocmd == BIO_READ &&
                      !(bp->b_flags & (B_INVAL|B_NOCACHE)) &&
                      !(bp->b_ioflags & BIO_ERROR))
                          bp->b_flags |= B_CACHE;
                  vfs_vmio_iodone(bp);
          }
  
          /*
           * 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
   * consistent.
   */
  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_WLOCK(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;
                          if (buf_mapped(bp)) {
                                  BUF_CHECK_MAPPED(bp);
                                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                      bp->b_pages, bp->b_npages);
                          } else
                                  BUF_CHECK_UNMAPPED(bp);
                  }
                  vm_page_sunbusy(m);
          }
          vm_object_pip_wakeupn(obj, bp->b_npages);
          VM_OBJECT_WUNLOCK(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_range(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 boundary 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 exclusive busied.  If any page is
   * exclusive busy, drain it.
   */
  void
  vfs_drain_busy_pages(struct buf *bp)
  {
          vm_page_t m;
          int i, last_busied;
  
          VM_OBJECT_ASSERT_WLOCKED(bp->b_bufobj->bo_object);
          last_busied = 0;
          for (i = 0; i < bp->b_npages; i++) {
                  m = bp->b_pages[i];
                  if (vm_page_xbusied(m)) {
                          for (; last_busied < i; last_busied++)
                                  vm_page_sbusy(bp->b_pages[last_busied]);
                          while (vm_page_xbusied(m)) {
                                  vm_page_lock(m);
                                  VM_OBJECT_WUNLOCK(bp->b_bufobj->bo_object);
                                  vm_page_busy_sleep(m, "vbpage");
                                  VM_OBJECT_WLOCK(bp->b_bufobj->bo_object);
                          }
                  }
          }
          for (i = 0; i < last_busied; i++)
                  vm_page_sunbusy(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 exclusive busy.  Also the object paging_in_progress
   * flag is handled to make sure that the object doesn't become
   * inconsistent.
   *
   * Since I/O has not been initiated yet, certain buffer flags
   * such as BIO_ERROR or B_INVAL may be in an inconsistent 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_WLOCK(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_sbusy(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_WUNLOCK(obj);
          if (bogus && buf_mapped(bp)) {
                  BUF_CHECK_MAPPED(bp);
                  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_WLOCK(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_range(m, base & PAGE_MASK, n);
                  base += n;
                  size -= n;
                  n = PAGE_SIZE;
          }
          VM_OBJECT_WUNLOCK(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, sa, ea, slide;
  
          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_WLOCK(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_ASSERT_WLOCKED(bp->b_pages[0]->object);
                  if ((bp->b_pages[0]->valid & mask) == mask)
                          goto unlock;
                  if ((bp->b_pages[0]->valid & mask) == 0) {
                          pmap_zero_page_area(bp->b_pages[0], 0, bp->b_bufsize);
                          bp->b_pages[0]->valid |= mask;
                          goto unlock;
                  }
          }
          sa = bp->b_offset & PAGE_MASK;
          slide = 0;
          for (i = 0; i < bp->b_npages; i++, sa = 0) {
                  slide = imin(slide + PAGE_SIZE, bp->b_offset + bp->b_bufsize);
                  ea = slide & PAGE_MASK;
                  if (ea == 0)
                          ea = PAGE_SIZE;
                  if (bp->b_pages[i] == bogus_page)
                          continue;
                  j = sa / DEV_BSIZE;
                  mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                  VM_OBJECT_ASSERT_WLOCKED(bp->b_pages[i]->object);
                  if ((bp->b_pages[i]->valid & mask) == mask)
                          continue;
                  if ((bp->b_pages[i]->valid & mask) == 0)
                          pmap_zero_page_area(bp->b_pages[i], sa, ea - sa);
                  else {
                          for (; sa < ea; sa += DEV_BSIZE, j++) {
                                  if ((bp->b_pages[i]->valid & (1 << j)) == 0) {
                                          pmap_zero_page_area(bp->b_pages[i],
                                              sa, DEV_BSIZE);
                                  }
                          }
                  }
                  bp->b_pages[i]->valid |= mask;
          }
  unlock:
          VM_OBJECT_WUNLOCK(bp->b_bufobj->bo_object);
          bp->b_resid = 0;
  }
  
  void
  vfs_bio_bzero_buf(struct buf *bp, int base, int size)
  {
          vm_page_t m;
          int i, n;
  
          if (buf_mapped(bp)) {
                  BUF_CHECK_MAPPED(bp);
                  bzero(bp->b_data + base, size);
          } else {
                  BUF_CHECK_UNMAPPED(bp);
                  n = PAGE_SIZE - (base & PAGE_MASK);
                  for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                          m = bp->b_pages[i];
                          if (n > size)
                                  n = size;
                          pmap_zero_page_area(m, base & PAGE_MASK, n);
                          base += n;
                          size -= n;
                          n = PAGE_SIZE;
                  }
          }
  }
  
  /*
   * 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;
  
          BUF_CHECK_MAPPED(bp);
  
          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, 0, VM_ALLOC_SYSTEM | VM_ALLOC_NOOBJ |
                      VM_ALLOC_WIRED | VM_ALLOC_COUNT((to - pg) >> PAGE_SHIFT));
                  if (p == NULL) {
                          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;
  
          BUF_CHECK_MAPPED(bp);
  
          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 (vm_page_sbusied(p))
                          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(&vm_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.
   *
   * This function only works with pager buffers.
   */
  int
  vmapbuf(struct buf *bp, int mapbuf)
  {
          vm_prot_t prot;
          int pidx;
  
          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 */
          if ((pidx = vm_fault_quick_hold_pages(&curproc->p_vmspace->vm_map,
              (vm_offset_t)bp->b_data, bp->b_bufsize, prot, bp->b_pages,
              btoc(MAXPHYS))) < 0)
                  return (-1);
          bp->b_npages = pidx;
          bp->b_offset = ((vm_offset_t)bp->b_data) & PAGE_MASK;
          if (mapbuf || !unmapped_buf_allowed) {
                  pmap_qenter((vm_offset_t)bp->b_kvabase, bp->b_pages, pidx);
                  bp->b_data = bp->b_kvabase + bp->b_offset;
          } else
                  bp->b_data = unmapped_buf;
          return(0);
  }
  
  /*
   * Free the io map PTEs associated with this IO operation.
   * We also invalidate the TLB entries and restore the original b_addr.
   *
   * This function only works with pager buffers.
   */
  void
  vunmapbuf(struct buf *bp)
  {
          int npages;
  
          npages = bp->b_npages;
          if (buf_mapped(bp))
                  pmap_qremove(trunc_page((vm_offset_t)bp->b_data), npages);
          vm_page_unhold_pages(bp->b_pages, npages);
  
          bp->b_data = unmapped_buf;
  }
  
  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_WLOCKED(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_WLOCKED(bo);
          error = 0;
          while (bo->bo_numoutput) {
                  bo->bo_flag |= BO_WWAIT;
                  error = msleep(&bo->bo_numoutput, BO_LOCKPTR(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);
  }
  
  /*
   * Set bio_data or bio_ma for struct bio from the struct buf.
   */
  void
  bdata2bio(struct buf *bp, struct bio *bip)
  {
  
          if (!buf_mapped(bp)) {
                  KASSERT(unmapped_buf_allowed, ("unmapped"));
                  bip->bio_ma = bp->b_pages;
                  bip->bio_ma_n = bp->b_npages;
                  bip->bio_data = unmapped_buf;
                  bip->bio_ma_offset = (vm_offset_t)bp->b_offset & PAGE_MASK;
                  bip->bio_flags |= BIO_UNMAPPED;
                  KASSERT(round_page(bip->bio_ma_offset + bip->bio_length) /
                      PAGE_SIZE == bp->b_npages,
                      ("Buffer %p too short: %d %lld %d", bp, bip->bio_ma_offset,
                      (long long)bip->bio_length, bip->bio_ma_n));
          } else {
                  bip->bio_data = bp->b_data;
                  bip->bio_ma = NULL;
          }
  }
  
  #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, b_xflags=0x%b, b_vflags=0x%b\n",
              (u_int)bp->b_flags, PRINT_BUF_FLAGS, (u_int)bp->b_xflags,
              PRINT_BUF_XFLAGS, (u_int)bp->b_vflags, PRINT_BUF_VFLAGS);
          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_lblkno = %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,
              (intmax_t)bp->b_lblkno, bp->b_dep.lh_first);
          db_printf("b_kvabase = %p, b_kvasize = %d\n",
              bp->b_kvabase, bp->b_kvasize);
          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(" ");
          BUF_LOCKPRINTINFO(bp);
  }
  
  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_qindex == QUEUE_EMPTY)
                          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 */