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

     /*
      * Copyright (c) 1994,1997 John S. Dyson
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
      *    notice immediately at the beginning of the file, without modification,
     *    this list of conditions, and the following disclaimer.
     * 2. Absolutely no warranty of function or purpose is made by the author
     *              John S. Dyson.
     *
     * $FreeBSD: src/sys/kern/vfs_bio.c,v 1.242.2.20 2003/05/28 18:38:10 alc Exp $
     */
    
    /*
     * this file contains a new buffer I/O scheme implementing a coherent
     * VM object and buffer cache scheme.  Pains have been taken to make
     * sure that the performance degradation associated with schemes such
     * as this is not realized.
     *
     * Author:  John S. Dyson
     * Significant help during the development and debugging phases
     * had been provided by David Greenman, also of the FreeBSD core team.
     *
     * see man buf(9) for more info.
     */
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/buf.h>
    #include <sys/conf.h>
    #include <sys/devicestat.h>
    #include <sys/eventhandler.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mount.h>
    #include <sys/kernel.h>
    #include <sys/kthread.h>
    #include <sys/proc.h>
    #include <sys/reboot.h>
    #include <sys/resourcevar.h>
    #include <sys/sysctl.h>
    #include <sys/vmmeter.h>
    #include <sys/vnode.h>
    #include <sys/dsched.h>
    #include <vm/vm.h>
    #include <vm/vm_param.h>
    #include <vm/vm_kern.h>
    #include <vm/vm_pageout.h>
    #include <vm/vm_page.h>
    #include <vm/vm_object.h>
    #include <vm/vm_extern.h>
    #include <vm/vm_map.h>
    #include <vm/vm_pager.h>
    #include <vm/swap_pager.h>
    
    #include <sys/buf2.h>
    #include <sys/thread2.h>
    #include <sys/spinlock2.h>
    #include <sys/mplock2.h>
    #include <vm/vm_page2.h>
    
    #include "opt_ddb.h"
    #ifdef DDB
    #include <ddb/ddb.h>
    #endif
    
    /*
     * Buffer queues.
     */
    enum bufq_type {
            BQUEUE_NONE,            /* not on any queue */
            BQUEUE_LOCKED,          /* locked buffers */
            BQUEUE_CLEAN,           /* non-B_DELWRI buffers */
            BQUEUE_DIRTY,           /* B_DELWRI buffers */
            BQUEUE_DIRTY_HW,        /* B_DELWRI buffers - heavy weight */
            BQUEUE_EMPTYKVA,        /* empty buffer headers with KVA assignment */
            BQUEUE_EMPTY,           /* empty buffer headers */
    
            BUFFER_QUEUES           /* number of buffer queues */
    };
    
    typedef enum bufq_type bufq_type_t;
    
    #define BD_WAKE_SIZE    16384
    #define BD_WAKE_MASK    (BD_WAKE_SIZE - 1)
    
    TAILQ_HEAD(bqueues, buf);
    
    struct bufpcpu {
            struct spinlock spin;
            struct bqueues bufqueues[BUFFER_QUEUES];
    } __cachealign;
    
    struct bufpcpu bufpcpu[MAXCPU];
    
    static MALLOC_DEFINE(M_BIOBUF, "BIO buffer", "BIO buffer");
   
   struct buf *buf;                /* buffer header pool */
   
   static void vfs_clean_pages(struct buf *bp);
   static void vfs_clean_one_page(struct buf *bp, int pageno, vm_page_t m);
   #if 0
   static void vfs_dirty_one_page(struct buf *bp, int pageno, vm_page_t m);
   #endif
   static void vfs_vmio_release(struct buf *bp);
   static int flushbufqueues(struct buf *marker, bufq_type_t q);
   static vm_page_t bio_page_alloc(struct buf *bp, vm_object_t obj,
                                   vm_pindex_t pg, int deficit);
   
   static void bd_signal(long totalspace);
   static void buf_daemon(void);
   static void buf_daemon_hw(void);
   
   /*
    * bogus page -- for I/O to/from partially complete buffers
    * this is a temporary solution to the problem, but it is not
    * really that bad.  it would be better to split the buffer
    * for input in the case of buffers partially already in memory,
    * but the code is intricate enough already.
    */
   vm_page_t bogus_page;
   
   /*
    * These are all static, but make the ones we export globals so we do
    * not need to use compiler magic.
    */
   long bufspace;                  /* locked by buffer_map */
   long maxbufspace;
   static long bufmallocspace;     /* atomic ops */
   long maxbufmallocspace, lobufspace, hibufspace;
   static long bufreusecnt, bufdefragcnt, buffreekvacnt;
   static long lorunningspace;
   static long hirunningspace;
   static long dirtykvaspace;              /* atomic */
   static long dirtybufspace;              /* atomic */
   static long dirtybufcount;              /* atomic */
   static long dirtybufspacehw;            /* atomic */
   static long dirtybufcounthw;            /* atomic */
   static long runningbufspace;            /* atomic */
   static long runningbufcount;            /* atomic */
   long lodirtybufspace;
   long hidirtybufspace;
   static int getnewbufcalls;
   static int getnewbufrestarts;
   static int recoverbufcalls;
   static int needsbuffer;                 /* atomic */
   static int runningbufreq;               /* atomic */
   static int bd_request;                  /* atomic */
   static int bd_request_hw;               /* atomic */
   static u_int bd_wake_ary[BD_WAKE_SIZE];
   static u_int bd_wake_index;
   static u_int vm_cycle_point = 40; /* 23-36 will migrate more act->inact */
   static int debug_commit;
   static int debug_bufbio;
   
   static struct thread *bufdaemon_td;
   static struct thread *bufdaemonhw_td;
   static u_int lowmempgallocs;
   static u_int lowmempgfails;
   
   /*
    * Sysctls for operational control of the buffer cache.
    */
   SYSCTL_LONG(_vfs, OID_AUTO, lodirtybufspace, CTLFLAG_RW, &lodirtybufspace, 0,
           "Number of dirty buffers to flush before bufdaemon becomes inactive");
   SYSCTL_LONG(_vfs, OID_AUTO, hidirtybufspace, CTLFLAG_RW, &hidirtybufspace, 0,
           "High watermark used to trigger explicit flushing of dirty buffers");
   SYSCTL_LONG(_vfs, OID_AUTO, lorunningspace, CTLFLAG_RW, &lorunningspace, 0,
           "Minimum amount of buffer space required for active I/O");
   SYSCTL_LONG(_vfs, OID_AUTO, hirunningspace, CTLFLAG_RW, &hirunningspace, 0,
           "Maximum amount of buffer space to usable for active I/O");
   SYSCTL_UINT(_vfs, OID_AUTO, lowmempgallocs, CTLFLAG_RW, &lowmempgallocs, 0,
           "Page allocations done during periods of very low free memory");
   SYSCTL_UINT(_vfs, OID_AUTO, lowmempgfails, CTLFLAG_RW, &lowmempgfails, 0,
           "Page allocations which failed during periods of very low free memory");
   SYSCTL_UINT(_vfs, OID_AUTO, vm_cycle_point, CTLFLAG_RW, &vm_cycle_point, 0,
           "Recycle pages to active or inactive queue transition pt 0-64");
   /*
    * Sysctls determining current state of the buffer cache.
    */
   SYSCTL_LONG(_vfs, OID_AUTO, nbuf, CTLFLAG_RD, &nbuf, 0,
           "Total number of buffers in buffer cache");
   SYSCTL_LONG(_vfs, OID_AUTO, dirtykvaspace, CTLFLAG_RD, &dirtykvaspace, 0,
           "KVA reserved by dirty buffers (all)");
   SYSCTL_LONG(_vfs, OID_AUTO, dirtybufspace, CTLFLAG_RD, &dirtybufspace, 0,
           "Pending bytes of dirty buffers (all)");
   SYSCTL_LONG(_vfs, OID_AUTO, dirtybufspacehw, CTLFLAG_RD, &dirtybufspacehw, 0,
           "Pending bytes of dirty buffers (heavy weight)");
   SYSCTL_LONG(_vfs, OID_AUTO, dirtybufcount, CTLFLAG_RD, &dirtybufcount, 0,
           "Pending number of dirty buffers");
   SYSCTL_LONG(_vfs, OID_AUTO, dirtybufcounthw, CTLFLAG_RD, &dirtybufcounthw, 0,
           "Pending number of dirty buffers (heavy weight)");
   SYSCTL_LONG(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD, &runningbufspace, 0,
           "I/O bytes currently in progress due to asynchronous writes");
   SYSCTL_LONG(_vfs, OID_AUTO, runningbufcount, CTLFLAG_RD, &runningbufcount, 0,
           "I/O buffers currently in progress due to asynchronous writes");
   SYSCTL_LONG(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RD, &maxbufspace, 0,
           "Hard limit on maximum amount of memory usable for buffer space");
   SYSCTL_LONG(_vfs, OID_AUTO, hibufspace, CTLFLAG_RD, &hibufspace, 0,
           "Soft limit on maximum amount of memory usable for buffer space");
   SYSCTL_LONG(_vfs, OID_AUTO, lobufspace, CTLFLAG_RD, &lobufspace, 0,
           "Minimum amount of memory to reserve for system buffer space");
   SYSCTL_LONG(_vfs, OID_AUTO, bufspace, CTLFLAG_RD, &bufspace, 0,
           "Amount of memory available for buffers");
   SYSCTL_LONG(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RD, &maxbufmallocspace,
           0, "Maximum amount of memory reserved for buffers using malloc");
   SYSCTL_LONG(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD, &bufmallocspace, 0,
           "Amount of memory left for buffers using malloc-scheme");
   SYSCTL_INT(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RD, &getnewbufcalls, 0,
           "New buffer header acquisition requests");
   SYSCTL_INT(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RD, &getnewbufrestarts,
           0, "New buffer header acquisition restarts");
   SYSCTL_INT(_vfs, OID_AUTO, recoverbufcalls, CTLFLAG_RD, &recoverbufcalls, 0,
           "Recover VM space in an emergency");
   SYSCTL_INT(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RD, &bufdefragcnt, 0,
           "Buffer acquisition restarts due to fragmented buffer map");
   SYSCTL_INT(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RD, &buffreekvacnt, 0,
           "Amount of time KVA space was deallocated in an arbitrary buffer");
   SYSCTL_INT(_vfs, OID_AUTO, bufreusecnt, CTLFLAG_RD, &bufreusecnt, 0,
           "Amount of time buffer re-use operations were successful");
   SYSCTL_INT(_vfs, OID_AUTO, debug_commit, CTLFLAG_RW, &debug_commit, 0, "");
   SYSCTL_INT(_vfs, OID_AUTO, debug_bufbio, CTLFLAG_RW, &debug_bufbio, 0, "");
   SYSCTL_INT(_debug_sizeof, OID_AUTO, buf, CTLFLAG_RD, 0, sizeof(struct buf),
           "sizeof(struct buf)");
   
   char *buf_wmesg = BUF_WMESG;
   
   #define VFS_BIO_NEED_ANY        0x01    /* any freeable buffer */
   #define VFS_BIO_NEED_UNUSED02   0x02
   #define VFS_BIO_NEED_UNUSED04   0x04
   #define VFS_BIO_NEED_BUFSPACE   0x08    /* wait for buf space, lo hysteresis */
   
   /*
    * bufspacewakeup:
    *
    *      Called when buffer space is potentially available for recovery.
    *      getnewbuf() will block on this flag when it is unable to free 
    *      sufficient buffer space.  Buffer space becomes recoverable when 
    *      bp's get placed back in the queues.
    */
   static __inline void
   bufspacewakeup(void)
   {
           /*
            * If someone is waiting for BUF space, wake them up.  Even
            * though we haven't freed the kva space yet, the waiting
            * process will be able to now.
            */
           for (;;) {
                   int flags = needsbuffer;
                   cpu_ccfence();
                   if ((flags & VFS_BIO_NEED_BUFSPACE) == 0)
                           break;
                   if (atomic_cmpset_int(&needsbuffer, flags,
                                         flags & ~VFS_BIO_NEED_BUFSPACE)) {
                           wakeup(&needsbuffer);
                           break;
                   }
                   /* retry */
           }
   }
   
   /*
    * runningbufwakeup:
    *
    *      Accounting for I/O in progress.
    *
    */
   static __inline void
   runningbufwakeup(struct buf *bp)
   {
           long totalspace;
           long limit;
           long flags;
   
           if ((totalspace = bp->b_runningbufspace) != 0) {
                   atomic_add_long(&runningbufspace, -totalspace);
                   atomic_add_long(&runningbufcount, -1);
                   bp->b_runningbufspace = 0;
   
                   /*
                    * see waitrunningbufspace() for limit test.
                    */
                   limit = hirunningspace * 3 / 6;
                   for (;;) {
                           flags = runningbufreq;
                           cpu_ccfence();
                           if (flags == 0)
                                   break;
                           if (atomic_cmpset_int(&runningbufreq, flags, 0)) {
                                   wakeup(&runningbufreq);
                                   break;
                           }
                           /* retry */
                   }
                   bd_signal(totalspace);
           }
   }
   
   /*
    * bufcountwakeup:
    *
    *      Called when a buffer has been added to one of the free queues to
    *      account for the buffer and to wakeup anyone waiting for free buffers.
    *      This typically occurs when large amounts of metadata are being handled
    *      by the buffer cache ( else buffer space runs out first, usually ).
    */
   static __inline void
   bufcountwakeup(void) 
   {
           long flags;
   
           for (;;) {
                   flags = needsbuffer;
                   if (flags == 0)
                           break;
                   if (atomic_cmpset_int(&needsbuffer, flags,
                                         (flags & ~VFS_BIO_NEED_ANY))) {
                           wakeup(&needsbuffer);
                           break;
                   }
                   /* retry */
           }
   }
   
   /*
    * waitrunningbufspace()
    *
    * If runningbufspace exceeds 4/6 hirunningspace we block until
    * runningbufspace drops to 3/6 hirunningspace.  We also block if another
    * thread blocked here in order to be fair, even if runningbufspace
    * is now lower than the limit.
    *
    * The caller may be using this function to block in a tight loop, we
    * must block while runningbufspace is greater than at least
    * hirunningspace * 3 / 6.
    */
   void
   waitrunningbufspace(void)
   {
           long limit = hirunningspace * 4 / 6;
           long flags;
   
           while (runningbufspace > limit || runningbufreq) {
                   tsleep_interlock(&runningbufreq, 0);
                   flags = atomic_fetchadd_int(&runningbufreq, 1);
                   if (runningbufspace > limit || flags)
                           tsleep(&runningbufreq, PINTERLOCKED, "wdrn1", hz);
           }
   }
   
   /*
    * buf_dirty_count_severe:
    *
    *      Return true if we have too many dirty buffers.
    */
   int
   buf_dirty_count_severe(void)
   {
           return (runningbufspace + dirtykvaspace >= hidirtybufspace ||
                   dirtybufcount >= nbuf / 2);
   }
   
   /*
    * Return true if the amount of running I/O is severe and BIOQ should
    * start bursting.
    */
   int
   buf_runningbufspace_severe(void)
   {
           return (runningbufspace >= hirunningspace * 4 / 6);
   }
   
   /*
    * vfs_buf_test_cache:
    *
    * Called when a buffer is extended.  This function clears the B_CACHE
    * bit if the newly extended portion of the buffer does not contain
    * valid data.
    *
    * NOTE! Dirty VM pages are not processed into dirty (B_DELWRI) buffer
    * cache buffers.  The VM pages remain dirty, as someone had mmap()'d
    * them while a clean buffer was present.
    */
   static __inline__
   void
   vfs_buf_test_cache(struct buf *bp,
                     vm_ooffset_t foff, vm_offset_t off, vm_offset_t size,
                     vm_page_t m)
   {
           if (bp->b_flags & B_CACHE) {
                   int base = (foff + off) & PAGE_MASK;
                   if (vm_page_is_valid(m, base, size) == 0)
                           bp->b_flags &= ~B_CACHE;
           }
   }
   
   /*
    * bd_speedup()
    *
    * Spank the buf_daemon[_hw] if the total dirty buffer space exceeds the
    * low water mark.
    */
   static __inline__
   void
   bd_speedup(void)
   {
           if (dirtykvaspace < lodirtybufspace && dirtybufcount < nbuf / 2)
                   return;
   
           if (bd_request == 0 &&
               (dirtykvaspace > lodirtybufspace / 2 ||
                dirtybufcount - dirtybufcounthw >= nbuf / 2)) {
                   if (atomic_fetchadd_int(&bd_request, 1) == 0)
                           wakeup(&bd_request);
           }
           if (bd_request_hw == 0 &&
               (dirtykvaspace > lodirtybufspace / 2 ||
                dirtybufcounthw >= nbuf / 2)) {
                   if (atomic_fetchadd_int(&bd_request_hw, 1) == 0)
                           wakeup(&bd_request_hw);
           }
   }
   
   /*
    * bd_heatup()
    *
    *      Get the buf_daemon heated up when the number of running and dirty
    *      buffers exceeds the mid-point.
    *
    *      Return the total number of dirty bytes past the second mid point
    *      as a measure of how much excess dirty data there is in the system.
    */
   long
   bd_heatup(void)
   {
           long mid1;
           long mid2;
           long totalspace;
   
           mid1 = lodirtybufspace + (hidirtybufspace - lodirtybufspace) / 2;
   
           totalspace = runningbufspace + dirtykvaspace;
           if (totalspace >= mid1 || dirtybufcount >= nbuf / 2) {
                   bd_speedup();
                   mid2 = mid1 + (hidirtybufspace - mid1) / 2;
                   if (totalspace >= mid2)
                           return(totalspace - mid2);
           }
           return(0);
   }
   
   /*
    * bd_wait()
    *
    *      Wait for the buffer cache to flush (totalspace) bytes worth of
    *      buffers, then return.
    *
    *      Regardless this function blocks while the number of dirty buffers
    *      exceeds hidirtybufspace.
    */
   void
   bd_wait(long totalspace)
   {
           u_int i;
           u_int j;
           u_int mi;
           int count;
   
           if (curthread == bufdaemonhw_td || curthread == bufdaemon_td)
                   return;
   
           while (totalspace > 0) {
                   bd_heatup();
   
                   /*
                    * Order is important.  Suppliers adjust bd_wake_index after
                    * updating runningbufspace/dirtykvaspace.  We want to fetch
                    * bd_wake_index before accessing.  Any error should thus
                    * be in our favor.
                    */
                   i = atomic_fetchadd_int(&bd_wake_index, 0);
                   if (totalspace > runningbufspace + dirtykvaspace)
                           totalspace = runningbufspace + dirtykvaspace;
                   count = totalspace / BKVASIZE;
                   if (count >= BD_WAKE_SIZE / 2)
                           count = BD_WAKE_SIZE / 2;
                   i = i + count;
                   mi = i & BD_WAKE_MASK;
   
                   /*
                    * This is not a strict interlock, so we play a bit loose
                    * with locking access to dirtybufspace*.  We have to re-check
                    * bd_wake_index to ensure that it hasn't passed us.
                    */
                   tsleep_interlock(&bd_wake_ary[mi], 0);
                   atomic_add_int(&bd_wake_ary[mi], 1);
                   j = atomic_fetchadd_int(&bd_wake_index, 0);
                   if ((int)(i - j) >= 0)
                           tsleep(&bd_wake_ary[mi], PINTERLOCKED, "flstik", hz);
   
                   totalspace = runningbufspace + dirtykvaspace - hidirtybufspace;
           }
   }
   
   /*
    * bd_signal()
    * 
    *      This function is called whenever runningbufspace or dirtykvaspace
    *      is reduced.  Track threads waiting for run+dirty buffer I/O
    *      complete.
    */
   static void
   bd_signal(long totalspace)
   {
           u_int i;
   
           if (totalspace > 0) {
                   if (totalspace > BKVASIZE * BD_WAKE_SIZE)
                           totalspace = BKVASIZE * BD_WAKE_SIZE;
                   while (totalspace > 0) {
                           i = atomic_fetchadd_int(&bd_wake_index, 1);
                           i &= BD_WAKE_MASK;
                           if (atomic_readandclear_int(&bd_wake_ary[i]))
                                   wakeup(&bd_wake_ary[i]);
                           totalspace -= BKVASIZE;
                   }
           }
   }
   
   /*
    * BIO tracking support routines.
    *
    * Release a ref on a bio_track.  Wakeup requests are atomically released
    * along with the last reference so bk_active will never wind up set to
    * only 0x80000000.
    */
   static
   void
   bio_track_rel(struct bio_track *track)
   {
           int     active;
           int     desired;
   
           /*
            * Shortcut
            */
           active = track->bk_active;
           if (active == 1 && atomic_cmpset_int(&track->bk_active, 1, 0))
                   return;
   
           /*
            * Full-on.  Note that the wait flag is only atomically released on
            * the 1->0 count transition.
            *
            * We check for a negative count transition using bit 30 since bit 31
            * has a different meaning.
            */
           for (;;) {
                   desired = (active & 0x7FFFFFFF) - 1;
                   if (desired)
                           desired |= active & 0x80000000;
                   if (atomic_cmpset_int(&track->bk_active, active, desired)) {
                           if (desired & 0x40000000)
                                   panic("bio_track_rel: bad count: %p", track);
                           if (active & 0x80000000)
                                   wakeup(track);
                           break;
                   }
                   active = track->bk_active;
           }
   }
   
   /*
    * Wait for the tracking count to reach 0.
    *
    * Use atomic ops such that the wait flag is only set atomically when
    * bk_active is non-zero.
    */
   int
   bio_track_wait(struct bio_track *track, int slp_flags, int slp_timo)
   {
           int     active;
           int     desired;
           int     error;
   
           /*
            * Shortcut
            */
           if (track->bk_active == 0)
                   return(0);
   
           /*
            * Full-on.  Note that the wait flag may only be atomically set if
            * the active count is non-zero.
            *
            * NOTE: We cannot optimize active == desired since a wakeup could
            *       clear active prior to our tsleep_interlock().
            */
           error = 0;
           while ((active = track->bk_active) != 0) {
                   cpu_ccfence();
                   desired = active | 0x80000000;
                   tsleep_interlock(track, slp_flags);
                   if (atomic_cmpset_int(&track->bk_active, active, desired)) {
                           error = tsleep(track, slp_flags | PINTERLOCKED,
                                          "trwait", slp_timo);
                           if (error)
                                   break;
                   }
           }
           return (error);
   }
   
   /*
    * bufinit:
    *
    *      Load time initialisation of the buffer cache, called from machine
    *      dependant initialization code. 
    */
   static
   void
   bufinit(void *dummy __unused)
   {
           struct bufpcpu *pcpu;
           struct buf *bp;
           vm_offset_t bogus_offset;
           int i;
           int j;
           long n;
   
           /* next, make a null set of free lists */
           for (i = 0; i < ncpus; ++i) {
                   pcpu = &bufpcpu[i];
                   spin_init(&pcpu->spin);
                   for (j = 0; j < BUFFER_QUEUES; j++)
                           TAILQ_INIT(&pcpu->bufqueues[j]);
           }
   
           /* finally, initialize each buffer header and stick on empty q */
           i = 0;
           pcpu = &bufpcpu[i];
   
           for (n = 0; n < nbuf; n++) {
                   bp = &buf[n];
                   bzero(bp, sizeof *bp);
                   bp->b_flags = B_INVAL;  /* we're just an empty header */
                   bp->b_cmd = BUF_CMD_DONE;
                   bp->b_qindex = BQUEUE_EMPTY;
                   bp->b_qcpu = i;
                   initbufbio(bp);
                   xio_init(&bp->b_xio);
                   buf_dep_init(bp);
                   TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                     bp, b_freelist);
   
                   i = (i + 1) % ncpus;
                   pcpu = &bufpcpu[i];
           }
   
           /*
            * maxbufspace is the absolute maximum amount of buffer space we are 
            * allowed to reserve in KVM and in real terms.  The absolute maximum
            * is nominally used by buf_daemon.  hibufspace is the nominal maximum
            * used by most other processes.  The differential is required to 
            * ensure that buf_daemon is able to run when other processes might 
            * be blocked waiting for buffer space.
            *
            * maxbufspace is based on BKVASIZE.  Allocating buffers larger then
            * this may result in KVM fragmentation which is not handled optimally
            * by the system.
            */
           maxbufspace = nbuf * BKVASIZE;
           hibufspace = lmax(3 * maxbufspace / 4, maxbufspace - MAXBSIZE * 10);
           lobufspace = hibufspace - MAXBSIZE;
   
           lorunningspace = 512 * 1024;
           /* hirunningspace -- see below */
   
           /*
            * Limit the amount of malloc memory since it is wired permanently
            * into the kernel space.  Even though this is accounted for in
            * the buffer allocation, we don't want the malloced region to grow
            * uncontrolled.  The malloc scheme improves memory utilization
            * significantly on average (small) directories.
            */
           maxbufmallocspace = hibufspace / 20;
   
           /*
            * Reduce the chance of a deadlock occuring by limiting the number
            * of delayed-write dirty buffers we allow to stack up.
            *
            * We don't want too much actually queued to the device at once
            * (XXX this needs to be per-mount!), because the buffers will
            * wind up locked for a very long period of time while the I/O
            * drains.
            */
           hidirtybufspace = hibufspace / 2;       /* dirty + running */
           hirunningspace = hibufspace / 16;       /* locked & queued to device */
           if (hirunningspace < 1024 * 1024)
                   hirunningspace = 1024 * 1024;
   
           dirtykvaspace = 0;
           dirtybufspace = 0;
           dirtybufspacehw = 0;
   
           lodirtybufspace = hidirtybufspace / 2;
   
           /*
            * Maximum number of async ops initiated per buf_daemon loop.  This is
            * somewhat of a hack at the moment, we really need to limit ourselves
            * based on the number of bytes of I/O in-transit that were initiated
            * from buf_daemon.
            */
   
           bogus_offset = kmem_alloc_pageable(&kernel_map, PAGE_SIZE);
           vm_object_hold(&kernel_object);
           bogus_page = vm_page_alloc(&kernel_object,
                                      (bogus_offset >> PAGE_SHIFT),
                                      VM_ALLOC_NORMAL);
           vm_object_drop(&kernel_object);
           vmstats.v_wire_count++;
   
   }
   
   SYSINIT(do_bufinit, SI_BOOT2_MACHDEP, SI_ORDER_FIRST, bufinit, NULL);
   
   /*
    * Initialize the embedded bio structures, typically used by
    * deprecated code which tries to allocate its own struct bufs.
    */
   void
   initbufbio(struct buf *bp)
   {
           bp->b_bio1.bio_buf = bp;
           bp->b_bio1.bio_prev = NULL;
           bp->b_bio1.bio_offset = NOOFFSET;
           bp->b_bio1.bio_next = &bp->b_bio2;
           bp->b_bio1.bio_done = NULL;
           bp->b_bio1.bio_flags = 0;
   
           bp->b_bio2.bio_buf = bp;
           bp->b_bio2.bio_prev = &bp->b_bio1;
           bp->b_bio2.bio_offset = NOOFFSET;
           bp->b_bio2.bio_next = NULL;
           bp->b_bio2.bio_done = NULL;
           bp->b_bio2.bio_flags = 0;
   
           BUF_LOCKINIT(bp);
   }
   
   /*
    * Reinitialize the embedded bio structures as well as any additional
    * translation cache layers.
    */
   void
   reinitbufbio(struct buf *bp)
   {
           struct bio *bio;
   
           for (bio = &bp->b_bio1; bio; bio = bio->bio_next) {
                   bio->bio_done = NULL;
                   bio->bio_offset = NOOFFSET;
           }
   }
   
   /*
    * Undo the effects of an initbufbio().
    */
   void
   uninitbufbio(struct buf *bp)
   {
           dsched_exit_buf(bp);
           BUF_LOCKFREE(bp);
   }
   
   /*
    * Push another BIO layer onto an existing BIO and return it.  The new
    * BIO layer may already exist, holding cached translation data.
    */
   struct bio *
   push_bio(struct bio *bio)
   {
           struct bio *nbio;
   
           if ((nbio = bio->bio_next) == NULL) {
                   int index = bio - &bio->bio_buf->b_bio_array[0];
                   if (index >= NBUF_BIO - 1) {
                           panic("push_bio: too many layers bp %p",
                                   bio->bio_buf);
                   }
                   nbio = &bio->bio_buf->b_bio_array[index + 1];
                   bio->bio_next = nbio;
                   nbio->bio_prev = bio;
                   nbio->bio_buf = bio->bio_buf;
                   nbio->bio_offset = NOOFFSET;
                   nbio->bio_done = NULL;
                   nbio->bio_next = NULL;
           }
           KKASSERT(nbio->bio_done == NULL);
           return(nbio);
   }
   
   /*
    * Pop a BIO translation layer, returning the previous layer.  The
    * must have been previously pushed.
    */
   struct bio *
   pop_bio(struct bio *bio)
   {
           return(bio->bio_prev);
   }
   
   void
   clearbiocache(struct bio *bio)
   {
           while (bio) {
                   bio->bio_offset = NOOFFSET;
                   bio = bio->bio_next;
           }
   }
   
   /*
    * bfreekva:
    *
    *      Free the KVA allocation for buffer 'bp'.
    *
    *      Must be called from a critical section as this is the only locking for
    *      buffer_map.
    *
    *      Since this call frees up buffer space, we call bufspacewakeup().
    */
   static void
   bfreekva(struct buf *bp)
   {
           int count;
   
           if (bp->b_kvasize) {
                   ++buffreekvacnt;
                   count = vm_map_entry_reserve(MAP_RESERVE_COUNT);
                   vm_map_lock(&buffer_map);
                   bufspace -= bp->b_kvasize;
                   vm_map_delete(&buffer_map,
                       (vm_offset_t) bp->b_kvabase,
                       (vm_offset_t) bp->b_kvabase + bp->b_kvasize,
                       &count
                   );
                   vm_map_unlock(&buffer_map);
                   vm_map_entry_release(count);
                   bp->b_kvasize = 0;
                   bp->b_kvabase = NULL;
                   bufspacewakeup();
           }
   }
   
   /*
    * Remove the buffer from the appropriate free list.
    * (caller must be locked)
    */
   static __inline void
   _bremfree(struct buf *bp)
   {
           struct bufpcpu *pcpu = &bufpcpu[bp->b_qcpu];
   
           if (bp->b_qindex != BQUEUE_NONE) {
                   KASSERT(BUF_REFCNTNB(bp) == 1, 
                           ("bremfree: bp %p not locked",bp));
                   TAILQ_REMOVE(&pcpu->bufqueues[bp->b_qindex], bp, b_freelist);
                   bp->b_qindex = BQUEUE_NONE;
           } else {
                   if (BUF_REFCNTNB(bp) <= 1)
                           panic("bremfree: removing a buffer not on a queue");
           }
   }
   
   /*
    * bremfree() - must be called with a locked buffer
    */
   void
   bremfree(struct buf *bp)
   {
           struct bufpcpu *pcpu = &bufpcpu[bp->b_qcpu];
   
           spin_lock(&pcpu->spin);
           _bremfree(bp);
           spin_unlock(&pcpu->spin);
   }
   
   /*
    * bremfree_locked - must be called with pcpu->spin locked
    */
   static void
   bremfree_locked(struct buf *bp)
   {
           _bremfree(bp);
   }
   
   /*
    * This version of bread issues any required I/O asyncnronously and
    * makes a callback on completion.
    *
    * The callback must check whether BIO_DONE is set in the bio and issue
    * the bpdone(bp, 0) if it isn't.  The callback is responsible for clearing
    * BIO_DONE and disposing of the I/O (bqrelse()ing it).
    */
   void
   breadcb(struct vnode *vp, off_t loffset, int size,
           void (*func)(struct bio *), void *arg)
   {
           struct buf *bp;
   
           bp = getblk(vp, loffset, size, 0, 0);
   
           /* if not found in cache, do some I/O */
           if ((bp->b_flags & B_CACHE) == 0) {
                   bp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                   bp->b_cmd = BUF_CMD_READ;
                   bp->b_bio1.bio_done = func;
                   bp->b_bio1.bio_caller_info1.ptr = arg;
                   vfs_busy_pages(vp, bp);
                   BUF_KERNPROC(bp);
                   vn_strategy(vp, &bp->b_bio1);
           } else if (func) {
                   /*
                    * Since we are issuing the callback synchronously it cannot
                    * race the BIO_DONE, so no need for atomic ops here.
                    */
                   /*bp->b_bio1.bio_done = func;*/
                   bp->b_bio1.bio_caller_info1.ptr = arg;
                   bp->b_bio1.bio_flags |= BIO_DONE;
                   func(&bp->b_bio1);
           } else {
                   bqrelse(bp);
           }
   }
   
   /*
    * breadnx() - Terminal function for bread() and breadn().
    *
    * This function will start asynchronous I/O on read-ahead blocks as well
    * as satisfy the primary request.
    *
    * We must clear B_ERROR and B_INVAL prior to initiating I/O.  If B_CACHE is
    * set, the buffer is valid and we do not have to do anything.
    */
   int
   breadnx(struct vnode *vp, off_t loffset, int size, off_t *raoffset,
           int *rabsize, int cnt, struct buf **bpp)
   {
           struct buf *bp, *rabp;
           int i;
           int rv = 0, readwait = 0;
   
           if (*bpp)
                   bp = *bpp;
           else
                   *bpp = bp = getblk(vp, loffset, size, 0, 0);
   
           /* if not found in cache, do some I/O */
           if ((bp->b_flags & B_CACHE) == 0) {
                   bp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                   bp->b_cmd = BUF_CMD_READ;
                   bp->b_bio1.bio_done = biodone_sync;
                   bp->b_bio1.bio_flags |= BIO_SYNC;
                   vfs_busy_pages(vp, bp);
                   vn_strategy(vp, &bp->b_bio1);
                   ++readwait;
           }
   
           for (i = 0; i < cnt; i++, raoffset++, rabsize++) {
                   if (inmem(vp, *raoffset))
                           continue;
                   rabp = getblk(vp, *raoffset, *rabsize, 0, 0);
   
                   if ((rabp->b_flags & B_CACHE) == 0) {
                           rabp->b_flags &= ~(B_ERROR | B_EINTR | B_INVAL);
                           rabp->b_cmd = BUF_CMD_READ;
                           vfs_busy_pages(vp, rabp);
                           BUF_KERNPROC(rabp);
                           vn_strategy(vp, &rabp->b_bio1);
                   } else {
                           brelse(rabp);
                   }
           }
           if (readwait)
                   rv = biowait(&bp->b_bio1, "biord");
           return (rv);
   }
   
   /*
    * bwrite:
    *
    *      Synchronous write, waits for completion.
    *
    *      Write, release buffer on completion.  (Done by iodone
    *      if async).  Do not bother writing anything if the buffer
   *      is invalid.
   *
   *      Note that we set B_CACHE here, indicating that buffer is
   *      fully valid and thus cacheable.  This is true even of NFS
   *      now so we set it generally.  This could be set either here 
   *      or in biodone() since the I/O is synchronous.  We put it
   *      here.
   */
  int
  bwrite(struct buf *bp)
  {
          int error;
  
          if (bp->b_flags & B_INVAL) {
                  brelse(bp);
                  return (0);
          }
          if (BUF_REFCNTNB(bp) == 0)
                  panic("bwrite: buffer is not busy???");
  
          /* Mark the buffer clean */
          bundirty(bp);
  
          bp->b_flags &= ~(B_ERROR | B_EINTR);
          bp->b_flags |= B_CACHE;
          bp->b_cmd = BUF_CMD_WRITE;
          bp->b_bio1.bio_done = biodone_sync;
          bp->b_bio1.bio_flags |= BIO_SYNC;
          vfs_busy_pages(bp->b_vp, bp);
  
          /*
           * Normal bwrites pipeline writes.  NOTE: b_bufsize is only
           * valid for vnode-backed buffers.
           */
          bsetrunningbufspace(bp, bp->b_bufsize);
          vn_strategy(bp->b_vp, &bp->b_bio1);
          error = biowait(&bp->b_bio1, "biows");
          brelse(bp);
  
          return (error);
  }
  
  /*
   * bawrite:
   *
   *      Asynchronous write.  Start output on a buffer, but do not wait for
   *      it to complete.  The buffer is released when the output completes.
   *
   *      bwrite() ( or the VOP routine anyway ) is responsible for handling
   *      B_INVAL buffers.  Not us.
   */
  void
  bawrite(struct buf *bp)
  {
          if (bp->b_flags & B_INVAL) {
                  brelse(bp);
                  return;
          }
          if (BUF_REFCNTNB(bp) == 0)
                  panic("bwrite: buffer is not busy???");
  
          /* Mark the buffer clean */
          bundirty(bp);
  
          bp->b_flags &= ~(B_ERROR | B_EINTR);
          bp->b_flags |= B_CACHE;
          bp->b_cmd = BUF_CMD_WRITE;
          KKASSERT(bp->b_bio1.bio_done == NULL);
          vfs_busy_pages(bp->b_vp, bp);
  
          /*
           * Normal bwrites pipeline writes.  NOTE: b_bufsize is only
           * valid for vnode-backed buffers.
           */
          bsetrunningbufspace(bp, bp->b_bufsize);
          BUF_KERNPROC(bp);
          vn_strategy(bp->b_vp, &bp->b_bio1);
  }
  
  /*
   * bowrite:
   *
   *      Ordered write.  Start output on a buffer, and flag it so that the
   *      device will write it in the order it was queued.  The buffer is
   *      released when the output completes.  bwrite() ( or the VOP routine
   *      anyway ) is responsible for handling B_INVAL buffers.
   */
  int
  bowrite(struct buf *bp)
  {
          bp->b_flags |= B_ORDERED;
          bawrite(bp);
          return (0);
  }
  
  /*
   * bdwrite:
   *
   *      Delayed write. (Buffer is marked dirty).  Do not bother writing
   *      anything if the buffer is marked invalid.
   *
   *      Note that since the buffer must be completely valid, we can safely
   *      set B_CACHE.  In fact, we have to set B_CACHE here rather then in
   *      biodone() in order to prevent getblk from writing the buffer
   *      out synchronously.
   */
  void
  bdwrite(struct buf *bp)
  {
          if (BUF_REFCNTNB(bp) == 0)
                  panic("bdwrite: buffer is not busy");
  
          if (bp->b_flags & B_INVAL) {
                  brelse(bp);
                  return;
          }
          bdirty(bp);
  
          if (dsched_is_clear_buf_priv(bp))
                  dsched_new_buf(bp);
  
          /*
           * Set B_CACHE, indicating that the buffer is fully valid.  This is
           * true even of NFS now.
           */
          bp->b_flags |= B_CACHE;
  
          /*
           * This bmap keeps the system from needing to do the bmap later,
           * perhaps when the system is attempting to do a sync.  Since it
           * is likely that the indirect block -- or whatever other datastructure
           * that the filesystem needs is still in memory now, it is a good
           * thing to do this.  Note also, that if the pageout daemon is
           * requesting a sync -- there might not be enough memory to do
           * the bmap then...  So, this is important to do.
           */
          if (bp->b_bio2.bio_offset == NOOFFSET) {
                  VOP_BMAP(bp->b_vp, bp->b_loffset, &bp->b_bio2.bio_offset,
                           NULL, NULL, BUF_CMD_WRITE);
          }
  
          /*
           * Because the underlying pages may still be mapped and
           * writable trying to set the dirty buffer (b_dirtyoff/end)
           * range here will be inaccurate.
           *
           * However, we must still clean the pages to satisfy the
           * vnode_pager and pageout daemon, so theythink the pages
           * have been "cleaned".  What has really occured is that
           * they've been earmarked for later writing by the buffer
           * cache.
           *
           * So we get the b_dirtyoff/end update but will not actually
           * depend on it (NFS that is) until the pages are busied for
           * writing later on.
           */
          vfs_clean_pages(bp);
          bqrelse(bp);
  
          /*
           * note: we cannot initiate I/O from a bdwrite even if we wanted to,
           * due to the softdep code.
           */
  }
  
  /*
   * Fake write - return pages to VM system as dirty, leave the buffer clean.
   * This is used by tmpfs.
   *
   * It is important for any VFS using this routine to NOT use it for
   * IO_SYNC or IO_ASYNC operations which occur when the system really
   * wants to flush VM pages to backing store.
   */
  void
  buwrite(struct buf *bp)
  {
          vm_page_t m;
          int i;
  
          /*
           * Only works for VMIO buffers.  If the buffer is already
           * marked for delayed-write we can't avoid the bdwrite().
           */
          if ((bp->b_flags & B_VMIO) == 0 || (bp->b_flags & B_DELWRI)) {
                  bdwrite(bp);
                  return;
          }
  
          /*
           * Mark as needing a commit.
           */
          for (i = 0; i < bp->b_xio.xio_npages; i++) {
                  m = bp->b_xio.xio_pages[i];
                  vm_page_need_commit(m);
          }
          bqrelse(bp);
  }
  
  /*
   * bdirty:
   *
   *      Turn buffer into delayed write request by marking it B_DELWRI.
   *      B_RELBUF and B_NOCACHE must be cleared.
   *
   *      We reassign the buffer to itself to properly update it in the
   *      dirty/clean lists. 
   *
   *      Must be called from a critical section.
   *      The buffer must be on BQUEUE_NONE.
   */
  void
  bdirty(struct buf *bp)
  {
          KASSERT(bp->b_qindex == BQUEUE_NONE,
                  ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
          if (bp->b_flags & B_NOCACHE) {
                  kprintf("bdirty: clearing B_NOCACHE on buf %p\n", bp);
                  bp->b_flags &= ~B_NOCACHE;
          }
          if (bp->b_flags & B_INVAL) {
                  kprintf("bdirty: warning, dirtying invalid buffer %p\n", bp);
          }
          bp->b_flags &= ~B_RELBUF;
  
          if ((bp->b_flags & B_DELWRI) == 0) {
                  lwkt_gettoken(&bp->b_vp->v_token);
                  bp->b_flags |= B_DELWRI;
                  reassignbuf(bp);
                  lwkt_reltoken(&bp->b_vp->v_token);
  
                  atomic_add_long(&dirtybufcount, 1);
                  atomic_add_long(&dirtykvaspace, bp->b_kvasize);
                  atomic_add_long(&dirtybufspace, bp->b_bufsize);
                  if (bp->b_flags & B_HEAVY) {
                          atomic_add_long(&dirtybufcounthw, 1);
                          atomic_add_long(&dirtybufspacehw, bp->b_bufsize);
                  }
                  bd_heatup();
          }
  }
  
  /*
   * Set B_HEAVY, indicating that this is a heavy-weight buffer that
   * needs to be flushed with a different buf_daemon thread to avoid
   * deadlocks.  B_HEAVY also imposes restrictions in getnewbuf().
   */
  void
  bheavy(struct buf *bp)
  {
          if ((bp->b_flags & B_HEAVY) == 0) {
                  bp->b_flags |= B_HEAVY;
                  if (bp->b_flags & B_DELWRI) {
                          atomic_add_long(&dirtybufcounthw, 1);
                          atomic_add_long(&dirtybufspacehw, bp->b_bufsize);
                  }
          }
  }
  
  /*
   * bundirty:
   *
   *      Clear B_DELWRI for buffer.
   *
   *      Must be called from a critical section.
   *
   *      The buffer is typically on BQUEUE_NONE but there is one case in 
   *      brelse() that calls this function after placing the buffer on
   *      a different queue.
   */
  void
  bundirty(struct buf *bp)
  {
          if (bp->b_flags & B_DELWRI) {
                  lwkt_gettoken(&bp->b_vp->v_token);
                  bp->b_flags &= ~B_DELWRI;
                  reassignbuf(bp);
                  lwkt_reltoken(&bp->b_vp->v_token);
  
                  atomic_add_long(&dirtybufcount, -1);
                  atomic_add_long(&dirtykvaspace, -bp->b_kvasize);
                  atomic_add_long(&dirtybufspace, -bp->b_bufsize);
                  if (bp->b_flags & B_HEAVY) {
                          atomic_add_long(&dirtybufcounthw, -1);
                          atomic_add_long(&dirtybufspacehw, -bp->b_bufsize);
                  }
                  bd_signal(bp->b_bufsize);
          }
          /*
           * Since it is now being written, we can clear its deferred write flag.
           */
          bp->b_flags &= ~B_DEFERRED;
  }
  
  /*
   * Set the b_runningbufspace field, used to track how much I/O is
   * in progress at any given moment.
   */
  void
  bsetrunningbufspace(struct buf *bp, int bytes)
  {
          bp->b_runningbufspace = bytes;
          if (bytes) {
                  atomic_add_long(&runningbufspace, bytes);
                  atomic_add_long(&runningbufcount, 1);
          }
  }
  
  /*
   * 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)
  {
          struct bufpcpu *pcpu;
  #ifdef INVARIANTS
          int saved_flags = bp->b_flags;
  #endif
  
          KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
                  ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
  
          /*
           * If B_NOCACHE is set we are being asked to destroy the buffer and
           * its backing store.  Clear B_DELWRI.
           *
           * B_NOCACHE is set in two cases: (1) when the caller really wants
           * to destroy the buffer and backing store and (2) when the caller
           * wants to destroy the buffer and backing store after a write 
           * completes.
           */
          if ((bp->b_flags & (B_NOCACHE|B_DELWRI)) == (B_NOCACHE|B_DELWRI)) {
                  bundirty(bp);
          }
  
          if ((bp->b_flags & (B_INVAL | B_DELWRI)) == B_DELWRI) {
                  /*
                   * A re-dirtied buffer is only subject to destruction
                   * by B_INVAL.  B_ERROR and B_NOCACHE are ignored.
                   */
                  /* leave buffer intact */
          } else if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_ERROR)) ||
                     (bp->b_bufsize <= 0)) {
                  /*
                   * Either a failed read or we were asked to free or not
                   * cache the buffer.  This path is reached with B_DELWRI
                   * set only if B_INVAL is already set.  B_NOCACHE governs
                   * backing store destruction.
                   *
                   * NOTE: HAMMER will set B_LOCKED in buf_deallocate if the
                   * buffer cannot be immediately freed.
                   */
                  bp->b_flags |= B_INVAL;
                  if (LIST_FIRST(&bp->b_dep) != NULL)
                          buf_deallocate(bp);
                  if (bp->b_flags & B_DELWRI) {
                          atomic_add_long(&dirtybufcount, -1);
                          atomic_add_long(&dirtykvaspace, -bp->b_kvasize);
                          atomic_add_long(&dirtybufspace, -bp->b_bufsize);
                          if (bp->b_flags & B_HEAVY) {
                                  atomic_add_long(&dirtybufcounthw, -1);
                                  atomic_add_long(&dirtybufspacehw,
                                                  -bp->b_bufsize);
                          }
                          bd_signal(bp->b_bufsize);
                  }
                  bp->b_flags &= ~(B_DELWRI | B_CACHE);
          }
  
          /*
           * We must clear B_RELBUF if B_DELWRI or B_LOCKED is set,
           * or if b_refs is non-zero.
           *
           * If vfs_vmio_release() is called with either bit set, the
           * underlying pages may wind up getting freed causing a previous
           * write (bdwrite()) to get 'lost' because pages associated with
           * a B_DELWRI bp are marked clean.  Pages associated with a
           * B_LOCKED buffer may be mapped by the filesystem.
           *
           * If we want to release the buffer ourselves (rather then the
           * originator asking us to release it), give the originator a
           * chance to countermand the release by setting B_LOCKED.
           * 
           * We still allow the B_INVAL case to call vfs_vmio_release(), even
           * if B_DELWRI is set.
           *
           * If B_DELWRI is not set we may have to set B_RELBUF if we are low
           * on pages to return pages to the VM page queues.
           */
          if ((bp->b_flags & (B_DELWRI | B_LOCKED)) || bp->b_refs) {
                  bp->b_flags &= ~B_RELBUF;
          } else if (vm_page_count_min(0)) {
                  if (LIST_FIRST(&bp->b_dep) != NULL)
                          buf_deallocate(bp);             /* can set B_LOCKED */
                  if (bp->b_flags & (B_DELWRI | B_LOCKED))
                          bp->b_flags &= ~B_RELBUF;
                  else
                          bp->b_flags |= B_RELBUF;
          }
  
          /*
           * Make sure b_cmd is clear.  It may have already been cleared by
           * biodone().
           *
           * At this point destroying the buffer is governed by the B_INVAL 
           * or B_RELBUF flags.
           */
          bp->b_cmd = BUF_CMD_DONE;
          dsched_exit_buf(bp);
  
          /*
           * VMIO buffer rundown.  Make sure the VM page array is restored
           * after an I/O may have replaces some of the pages with bogus pages
           * in order to not destroy dirty pages in a fill-in read.
           *
           * Note that due to the code above, if a buffer is marked B_DELWRI
           * then the B_RELBUF and B_NOCACHE bits will always be clear.
           * B_INVAL may still be set, however.
           *
           * For clean buffers, B_INVAL or B_RELBUF will destroy the buffer
           * but not the backing store.   B_NOCACHE will destroy the backing
           * store.
           *
           * Note that dirty NFS buffers contain byte-granular write ranges
           * and should not be destroyed w/ B_INVAL even if the backing store
           * is left intact.
           */
          if (bp->b_flags & B_VMIO) {
                  /*
                   * Rundown for VMIO buffers which are not dirty NFS buffers.
                   */
                  int i, j, resid;
                  vm_page_t m;
                  off_t foff;
                  vm_pindex_t poff;
                  vm_object_t obj;
                  struct vnode *vp;
  
                  vp = bp->b_vp;
  
                  /*
                   * Get the base offset and length of the buffer.  Note that 
                   * in the VMIO case if the buffer block size is not
                   * page-aligned then b_data pointer may not be page-aligned.
                   * But our b_xio.xio_pages array *IS* page aligned.
                   *
                   * block sizes less then DEV_BSIZE (usually 512) are not 
                   * supported due to the page granularity bits (m->valid,
                   * m->dirty, etc...). 
                   *
                   * See man buf(9) for more information
                   */
  
                  resid = bp->b_bufsize;
                  foff = bp->b_loffset;
  
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          m = bp->b_xio.xio_pages[i];
                          vm_page_flag_clear(m, PG_ZERO);
                          /*
                           * If we hit a bogus page, fixup *all* of them
                           * now.  Note that we left these pages wired
                           * when we removed them so they had better exist,
                           * and they cannot be ripped out from under us so
                           * no critical section protection is necessary.
                           */
                          if (m == bogus_page) {
                                  obj = vp->v_object;
                                  poff = OFF_TO_IDX(bp->b_loffset);
  
                                  vm_object_hold(obj);
                                  for (j = i; j < bp->b_xio.xio_npages; j++) {
                                          vm_page_t mtmp;
  
                                          mtmp = bp->b_xio.xio_pages[j];
                                          if (mtmp == bogus_page) {
                                                  mtmp = vm_page_lookup(obj, poff + j);
                                                  if (!mtmp) {
                                                          panic("brelse: page missing");
                                                  }
                                                  bp->b_xio.xio_pages[j] = mtmp;
                                          }
                                  }
                                  bp->b_flags &= ~B_HASBOGUS;
                                  vm_object_drop(obj);
  
                                  if ((bp->b_flags & B_INVAL) == 0) {
                                          pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                                  bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                                  }
                                  m = bp->b_xio.xio_pages[i];
                          }
  
                          /*
                           * Invalidate the backing store if B_NOCACHE is set
                           * (e.g. used with vinvalbuf()).  If this is NFS
                           * we impose a requirement that the block size be
                           * a multiple of PAGE_SIZE and create a temporary
                           * hack to basically invalidate the whole page.  The
                           * problem is that NFS uses really odd buffer sizes
                           * especially when tracking piecemeal writes and
                           * it also vinvalbuf()'s a lot, which would result
                           * in only partial page validation and invalidation
                           * here.  If the file page is mmap()'d, however,
                           * all the valid bits get set so after we invalidate
                           * here we would end up with weird m->valid values
                           * like 0xfc.  nfs_getpages() can't handle this so
                           * we clear all the valid bits for the NFS case
                           * instead of just some of them.
                           *
                           * The real bug is the VM system having to set m->valid
                           * to VM_PAGE_BITS_ALL for faulted-in pages, which
                           * itself is an artifact of the whole 512-byte
                           * granular mess that exists to support odd block 
                           * sizes and UFS meta-data block sizes (e.g. 6144).
                           * A complete rewrite is required.
                           *
                           * XXX
                           */
                          if (bp->b_flags & (B_NOCACHE|B_ERROR)) {
                                  int poffset = foff & PAGE_MASK;
                                  int presid;
  
                                  presid = PAGE_SIZE - poffset;
                                  if (bp->b_vp->v_tag == VT_NFS &&
                                      bp->b_vp->v_type == VREG) {
                                          ; /* entire page */
                                  } else if (presid > resid) {
                                          presid = resid;
                                  }
                                  KASSERT(presid >= 0, ("brelse: extra page"));
                                  vm_page_set_invalid(m, poffset, presid);
  
                                  /*
                                   * Also make sure any swap cache is removed
                                   * as it is now stale (HAMMER in particular
                                   * uses B_NOCACHE to deal with buffer
                                   * aliasing).
                                   */
                                  swap_pager_unswapped(m);
                          }
                          resid -= PAGE_SIZE - (foff & PAGE_MASK);
                          foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                  }
                  if (bp->b_flags & (B_INVAL | B_RELBUF))
                          vfs_vmio_release(bp);
          } else {
                  /*
                   * Rundown for non-VMIO buffers.
                   */
                  if (bp->b_flags & (B_INVAL | B_RELBUF)) {
                          if (bp->b_bufsize)
                                  allocbuf(bp, 0);
                          KKASSERT (LIST_FIRST(&bp->b_dep) == NULL);
                          if (bp->b_vp)
                                  brelvp(bp);
                  }
          }
                          
          if (bp->b_qindex != BQUEUE_NONE)
                  panic("brelse: free buffer onto another queue???");
          if (BUF_REFCNTNB(bp) > 1) {
                  /* Temporary panic to verify exclusive locking */
                  /* This panic goes away when we allow shared refs */
                  panic("brelse: multiple refs");
                  /* NOT REACHED */
                  return;
          }
  
          /*
           * Figure out the correct queue to place the cleaned up buffer on.
           * Buffers placed in the EMPTY or EMPTYKVA had better already be
           * disassociated from their vnode.
           *
           * Return the buffer to its original pcpu area
           */
          pcpu = &bufpcpu[bp->b_qcpu];
          spin_lock(&pcpu->spin);
  
          if (bp->b_flags & B_LOCKED) {
                  /*
                   * Buffers that are locked are placed in the locked queue
                   * immediately, regardless of their state.
                   */
                  bp->b_qindex = BQUEUE_LOCKED;
                  TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          } else if (bp->b_bufsize == 0) {
                  /*
                   * Buffers with no memory.  Due to conditionals near the top
                   * of brelse() such buffers should probably already be
                   * marked B_INVAL and disassociated from their vnode.
                   */
                  bp->b_flags |= B_INVAL;
                  KASSERT(bp->b_vp == NULL,
                          ("bp1 %p flags %08x/%08x vnode %p "
                           "unexpectededly still associated!",
                          bp, saved_flags, bp->b_flags, bp->b_vp));
                  KKASSERT((bp->b_flags & B_HASHED) == 0);
                  if (bp->b_kvasize) {
                          bp->b_qindex = BQUEUE_EMPTYKVA;
                  } else {
                          bp->b_qindex = BQUEUE_EMPTY;
                  }
                  TAILQ_INSERT_HEAD(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          } else if (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF)) {
                  /*
                   * Buffers with junk contents.   Again these buffers had better
                   * already be disassociated from their vnode.
                   */
                  KASSERT(bp->b_vp == NULL,
                          ("bp2 %p flags %08x/%08x vnode %p unexpectededly "
                           "still associated!",
                          bp, saved_flags, bp->b_flags, bp->b_vp));
                  KKASSERT((bp->b_flags & B_HASHED) == 0);
                  bp->b_flags |= B_INVAL;
                  bp->b_qindex = BQUEUE_CLEAN;
                  TAILQ_INSERT_HEAD(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          } else {
                  /*
                   * Remaining buffers.  These buffers are still associated with
                   * their vnode.
                   */
                  switch(bp->b_flags & (B_DELWRI|B_HEAVY)) {
                  case B_DELWRI:
                          bp->b_qindex = BQUEUE_DIRTY;
                          TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                            bp, b_freelist);
                          break;
                  case B_DELWRI | B_HEAVY:
                          bp->b_qindex = BQUEUE_DIRTY_HW;
                          TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                            bp, b_freelist);
                          break;
                  default:
                          /*
                           * NOTE: Buffers are always placed at the end of the
                           * queue.  If B_AGE is not set the buffer will cycle
                           * through the queue twice.
                           */
                          bp->b_qindex = BQUEUE_CLEAN;
                          TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                            bp, b_freelist);
                          break;
                  }
          }
          spin_unlock(&pcpu->spin);
  
          /*
           * If B_INVAL, clear B_DELWRI.  We've already placed the buffer
           * on the correct queue but we have not yet unlocked it.
           */
          if ((bp->b_flags & (B_INVAL|B_DELWRI)) == (B_INVAL|B_DELWRI))
                  bundirty(bp);
  
          /*
           * The bp is on an appropriate queue unless locked.  If it is not
           * locked or dirty we can wakeup threads waiting for buffer space.
           *
           * We've already handled the B_INVAL case ( B_DELWRI will be clear
           * if B_INVAL is set ).
           */
          if ((bp->b_flags & (B_LOCKED|B_DELWRI)) == 0)
                  bufcountwakeup();
  
          /*
           * Something we can maybe free or reuse
           */
          if (bp->b_bufsize || bp->b_kvasize)
                  bufspacewakeup();
  
          /*
           * Clean up temporary flags and unlock the buffer.
           */
          bp->b_flags &= ~(B_ORDERED | B_NOCACHE | B_RELBUF | B_DIRECT);
          BUF_UNLOCK(bp);
  }
  
  /*
   * bqrelse:
   *
   *      Release a buffer back to the appropriate queue but do not try to free
   *      it.  The buffer is expected to be used again soon.
   *
   *      bqrelse() is used by bdwrite() to requeue a delayed write, and used by
   *      biodone() to requeue an async I/O on completion.  It is also used when
   *      known good buffers need to be requeued but we think we may need the data
   *      again soon.
   *
   *      XXX we should be able to leave the B_RELBUF hint set on completion.
   */
  void
  bqrelse(struct buf *bp)
  {
          struct bufpcpu *pcpu;
  
          KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
                  ("bqrelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
  
          if (bp->b_qindex != BQUEUE_NONE)
                  panic("bqrelse: free buffer onto another queue???");
          if (BUF_REFCNTNB(bp) > 1) {
                  /* do not release to free list */
                  panic("bqrelse: multiple refs");
                  return;
          }
  
          buf_act_advance(bp);
  
          pcpu = &bufpcpu[bp->b_qcpu];
          spin_lock(&pcpu->spin);
  
          if (bp->b_flags & B_LOCKED) {
                  /*
                   * Locked buffers are released to the locked queue.  However,
                   * if the buffer is dirty it will first go into the dirty
                   * queue and later on after the I/O completes successfully it
                   * will be released to the locked queue.
                   */
                  bp->b_qindex = BQUEUE_LOCKED;
                  TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          } else if (bp->b_flags & B_DELWRI) {
                  bp->b_qindex = (bp->b_flags & B_HEAVY) ?
                                 BQUEUE_DIRTY_HW : BQUEUE_DIRTY;
                  TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          } else if (vm_page_count_min(0)) {
                  /*
                   * We are too low on memory, we have to try to free the
                   * buffer (most importantly: the wired pages making up its
                   * backing store) *now*.
                   */
                  spin_unlock(&pcpu->spin);
                  brelse(bp);
                  return;
          } else {
                  bp->b_qindex = BQUEUE_CLEAN;
                  TAILQ_INSERT_TAIL(&pcpu->bufqueues[bp->b_qindex],
                                    bp, b_freelist);
          }
          spin_unlock(&pcpu->spin);
  
          /*
           * We have now placed the buffer on the proper queue, but have yet
           * to unlock it.
           */
          if ((bp->b_flags & B_LOCKED) == 0 &&
              ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0)) {
                  bufcountwakeup();
          }
  
          /*
           * Something we can maybe free or reuse.
           */
          if (bp->b_bufsize && !(bp->b_flags & B_DELWRI))
                  bufspacewakeup();
  
          /*
           * Final cleanup and unlock.  Clear bits that are only used while a
           * buffer is actively locked.
           */
          bp->b_flags &= ~(B_ORDERED | B_NOCACHE | B_RELBUF);
          dsched_exit_buf(bp);
          BUF_UNLOCK(bp);
  }
  
  /*
   * Hold a buffer, preventing it from being reused.  This will prevent
   * normal B_RELBUF operations on the buffer but will not prevent B_INVAL
   * operations.  If a B_INVAL operation occurs the buffer will remain held
   * but the underlying pages may get ripped out.
   *
   * These functions are typically used in VOP_READ/VOP_WRITE functions
   * to hold a buffer during a copyin or copyout, preventing deadlocks
   * or recursive lock panics when read()/write() is used over mmap()'d
   * space.
   *
   * NOTE: bqhold() requires that the buffer be locked at the time of the
   *       hold.  bqdrop() has no requirements other than the buffer having
   *       previously been held.
   */
  void
  bqhold(struct buf *bp)
  {
          atomic_add_int(&bp->b_refs, 1);
  }
  
  void
  bqdrop(struct buf *bp)
  {
          KKASSERT(bp->b_refs > 0);
          atomic_add_int(&bp->b_refs, -1);
  }
  
  /*
   * Return backing pages held by the buffer 'bp' back to the VM system.
   * This routine is called when the bp is invalidated, released, or
   * reused.
   *
   * The KVA mapping (b_data) for the underlying pages is removed by
   * this function.
   *
   * WARNING! This routine is integral to the low memory critical path
   *          when a buffer is B_RELBUF'd.  If the system has a severe page
   *          deficit we need to get the page(s) onto the PQ_FREE or PQ_CACHE
   *          queues so they can be reused in the current pageout daemon
   *          pass.
   */
  static void
  vfs_vmio_release(struct buf *bp)
  {
          int i;
          vm_page_t m;
  
          for (i = 0; i < bp->b_xio.xio_npages; i++) {
                  m = bp->b_xio.xio_pages[i];
                  bp->b_xio.xio_pages[i] = NULL;
  
                  /*
                   * We need to own the page in order to safely unwire it.
                   */
                  vm_page_busy_wait(m, FALSE, "vmiopg");
  
                  /*
                   * The VFS is telling us this is not a meta-data buffer
                   * even if it is backed by a block device.
                   */
                  if (bp->b_flags & B_NOTMETA)
                          vm_page_flag_set(m, PG_NOTMETA);
  
                  /*
                   * This is a very important bit of code.  We try to track
                   * VM page use whether the pages are wired into the buffer
                   * cache or not.  While wired into the buffer cache the
                   * bp tracks the act_count.
                   *
                   * We can choose to place unwired pages on the inactive
                   * queue (0) or active queue (1).  If we place too many
                   * on the active queue the queue will cycle the act_count
                   * on pages we'd like to keep, just from single-use pages
                   * (such as when doing a tar-up or file scan).
                   */
                  if (bp->b_act_count < vm_cycle_point)
                          vm_page_unwire(m, 0);
                  else
                          vm_page_unwire(m, 1);
  
                  /*
                   * If the wire_count has dropped to 0 we may need to take
                   * further action before unbusying the page.
                   *
                   * WARNING: vm_page_try_*() also checks PG_NEED_COMMIT for us.
                   */
                  if (m->wire_count == 0) {
                          vm_page_flag_clear(m, PG_ZERO);
  
                          if (bp->b_flags & B_DIRECT) {
                                  /*
                                   * Attempt to free the page if B_DIRECT is
                                   * set, the caller does not desire the page
                                   * to be cached.
                                   */
                                  vm_page_wakeup(m);
                                  vm_page_try_to_free(m);
                          } else if ((bp->b_flags & B_NOTMETA) ||
                                     vm_page_count_min(0)) {
                                  /*
                                   * Attempt to move the page to PQ_CACHE
                                   * if B_NOTMETA is set.  This flag is set
                                   * by HAMMER to remove one of the two pages
                                   * present when double buffering is enabled.
                                   *
                                   * Attempt to move the page to PQ_CACHE
                                   * If we have a severe page deficit.  This
                                   * will cause buffer cache operations related
                                   * to pageouts to recycle the related pages
                                   * in order to avoid a low memory deadlock.
                                   */
                                  m->act_count = bp->b_act_count;
                                  vm_page_wakeup(m);
                                  vm_page_try_to_cache(m);
                          } else {
                                  /*
                                   * Nominal case, leave the page on the
                                   * queue the original unwiring placed it on
                                   * (active or inactive).
                                   */
                                  m->act_count = bp->b_act_count;
                                  vm_page_wakeup(m);
                          }
                  } else {
                          vm_page_wakeup(m);
                  }
          }
  
          pmap_qremove(trunc_page((vm_offset_t) bp->b_data),
                       bp->b_xio.xio_npages);
          if (bp->b_bufsize) {
                  bufspacewakeup();
                  bp->b_bufsize = 0;
          }
          bp->b_xio.xio_npages = 0;
          bp->b_flags &= ~B_VMIO;
          KKASSERT (LIST_FIRST(&bp->b_dep) == NULL);
          if (bp->b_vp)
                  brelvp(bp);
  }
  
  /*
   * Find and initialize a new buffer header, freeing up existing buffers
   * in the bufqueues as necessary.  The new buffer is returned locked.
   *
   * Important:  B_INVAL is not set.  If the caller wishes to throw the
   * buffer away, the caller must set B_INVAL prior to calling brelse().
   *
   * We block if:
   *      We have insufficient buffer headers
   *      We have insufficient buffer space
   *      buffer_map is too fragmented ( space reservation fails )
   *      If we have to flush dirty buffers ( but we try to avoid this )
   *
   * To avoid VFS layer recursion we do not flush dirty buffers ourselves.
   * Instead we ask the buf daemon to do it for us.  We attempt to
   * avoid piecemeal wakeups of the pageout daemon.
   */
  struct buf *
  getnewbuf(int blkflags, int slptimeo, int size, int maxsize)
  {
          struct bufpcpu *pcpu;
          struct buf *bp;
          struct buf *nbp;
          int defrag = 0;
          int nqindex;
          int nqcpu;
          int slpflags = (blkflags & GETBLK_PCATCH) ? PCATCH : 0;
          int maxloops = 200000;
          int restart_reason = 0;
          struct buf *restart_bp = NULL;
          static int flushingbufs;
  
          /*
           * We can't afford to block since we might be holding a vnode lock,
           * which may prevent system daemons from running.  We deal with
           * low-memory situations by proactively returning memory and running
           * async I/O rather then sync I/O.
           */
          
          ++getnewbufcalls;
          --getnewbufrestarts;
          nqcpu = mycpu->gd_cpuid;
  restart:
          ++getnewbufrestarts;
  
          if (debug_bufbio && --maxloops == 0)
                  panic("getnewbuf, excessive loops on cpu %d restart %d (%p)",
                          mycpu->gd_cpuid, restart_reason, restart_bp);
  
          /*
           * Setup for scan.  If we do not have enough free buffers,
           * we setup a degenerate case that immediately fails.  Note
           * that if we are specially marked process, we are allowed to
           * dip into our reserves.
           *
           * The scanning sequence is nominally:  EMPTY->EMPTYKVA->CLEAN
           *
           * We start with EMPTYKVA.  If the list is empty we backup to EMPTY.
           * However, there are a number of cases (defragging, reusing, ...)
           * where we cannot backup.
           */
          pcpu = &bufpcpu[nqcpu];
          nqindex = BQUEUE_EMPTYKVA;
          spin_lock(&pcpu->spin);
  
          nbp = TAILQ_FIRST(&pcpu->bufqueues[BQUEUE_EMPTYKVA]);
  
          if (nbp == NULL) {
                  /*
                   * If no EMPTYKVA buffers and we are either
                   * defragging or reusing, locate a CLEAN buffer
                   * to free or reuse.  If bufspace useage is low
                   * skip this step so we can allocate a new buffer.
                   */
                  if (defrag || bufspace >= lobufspace) {
                          nqindex = BQUEUE_CLEAN;
                          nbp = TAILQ_FIRST(&pcpu->bufqueues[BQUEUE_CLEAN]);
                  }
  
                  /*
                   * If we could not find or were not allowed to reuse a
                   * CLEAN buffer, check to see if it is ok to use an EMPTY
                   * buffer.  We can only use an EMPTY buffer if allocating
                   * its KVA would not otherwise run us out of buffer space.
                   */
                  if (nbp == NULL && defrag == 0 &&
                      bufspace + maxsize < hibufspace) {
                          nqindex = BQUEUE_EMPTY;
                          nbp = TAILQ_FIRST(&pcpu->bufqueues[BQUEUE_EMPTY]);
                  }
          }
  
          /*
           * Run scan, possibly freeing data and/or kva mappings on the fly
           * depending.
           *
           * WARNING! spin is held!
           */
          while ((bp = nbp) != NULL) {
                  int qindex = nqindex;
  
                  nbp = TAILQ_NEXT(bp, b_freelist);
  
                  /*
                   * BQUEUE_CLEAN - B_AGE special case.  If not set the bp
                   * cycles through the queue twice before being selected.
                   */
                  if (qindex == BQUEUE_CLEAN && 
                      (bp->b_flags & B_AGE) == 0 && nbp) {
                          bp->b_flags |= B_AGE;
                          TAILQ_REMOVE(&pcpu->bufqueues[qindex],
                                       bp, b_freelist);
                          TAILQ_INSERT_TAIL(&pcpu->bufqueues[qindex],
                                            bp, b_freelist);
                          continue;
                  }
  
                  /*
                   * Calculate next bp ( we can only use it if we do not block
                   * or do other fancy things ).
                   */
                  if (nbp == NULL) {
                          switch(qindex) {
                          case BQUEUE_EMPTY:
                                  nqindex = BQUEUE_EMPTYKVA;
                                  if ((nbp = TAILQ_FIRST(&pcpu->bufqueues[BQUEUE_EMPTYKVA])))
                                          break;
                                  /* fall through */
                          case BQUEUE_EMPTYKVA:
                                  nqindex = BQUEUE_CLEAN;
                                  if ((nbp = TAILQ_FIRST(&pcpu->bufqueues[BQUEUE_CLEAN])))
                                          break;
                                  /* fall through */
                          case BQUEUE_CLEAN:
                                  /*
                                   * nbp is NULL. 
                                   */
                                  break;
                          }
                  }
  
                  /*
                   * Sanity Checks
                   */
                  KASSERT(bp->b_qindex == qindex,
                          ("getnewbuf: inconsistent queue %d bp %p", qindex, bp));
  
                  /*
                   * Note: we no longer distinguish between VMIO and non-VMIO
                   * buffers.
                   */
                  KASSERT((bp->b_flags & B_DELWRI) == 0,
                          ("delwri buffer %p found in queue %d", bp, qindex));
  
                  /*
                   * Do not try to reuse a buffer with a non-zero b_refs.
                   * This is an unsynchronized test.  A synchronized test
                   * is also performed after we lock the buffer.
                   */
                  if (bp->b_refs)
                          continue;
  
                  /*
                   * If we are defragging then we need a buffer with 
                   * b_kvasize != 0.  XXX this situation should no longer
                   * occur, if defrag is non-zero the buffer's b_kvasize
                   * should also be non-zero at this point.  XXX
                   */
                  if (defrag && bp->b_kvasize == 0) {
                          kprintf("Warning: defrag empty buffer %p\n", bp);
                          continue;
                  }
  
                  /*
                   * Start freeing the bp.  This is somewhat involved.  nbp
                   * remains valid only for BQUEUE_EMPTY[KVA] bp's.  Buffers
                   * on the clean list must be disassociated from their 
                   * current vnode.  Buffers on the empty[kva] lists have
                   * already been disassociated.
                   *
                   * b_refs is checked after locking along with queue changes.
                   * We must check here to deal with zero->nonzero transitions
                   * made by the owner of the buffer lock, which is used by
                   * VFS's to hold the buffer while issuing an unlocked
                   * uiomove()s.  We cannot invalidate the buffer's pages
                   * for this case.  Once we successfully lock a buffer the
                   * only 0->1 transitions of b_refs will occur via findblk().
                   *
                   * We must also check for queue changes after successful
                   * locking as the current lock holder may dispose of the
                   * buffer and change its queue.
                   */
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
                          spin_unlock(&pcpu->spin);
                          tsleep(&bd_request, 0, "gnbxxx", (hz + 99) / 100);
                          restart_reason = 1;
                          restart_bp = bp;
                          goto restart;
                  }
                  if (bp->b_qindex != qindex || bp->b_refs) {
                          spin_unlock(&pcpu->spin);
                          BUF_UNLOCK(bp);
                          restart_reason = 2;
                          restart_bp = bp;
                          goto restart;
                  }
                  bremfree_locked(bp);
                  spin_unlock(&pcpu->spin);
  
                  /*
                   * Dependancies must be handled before we disassociate the
                   * vnode.
                   *
                   * NOTE: HAMMER will set B_LOCKED if the buffer cannot
                   * be immediately disassociated.  HAMMER then becomes
                   * responsible for releasing the buffer.
                   *
                   * NOTE: spin is UNLOCKED now.
                   */
                  if (LIST_FIRST(&bp->b_dep) != NULL) {
                          buf_deallocate(bp);
                          if (bp->b_flags & B_LOCKED) {
                                  bqrelse(bp);
                                  restart_reason = 3;
                                  restart_bp = bp;
                                  goto restart;
                          }
                          KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                  }
  
                  if (qindex == BQUEUE_CLEAN) {
                          if (bp->b_flags & B_VMIO)
                                  vfs_vmio_release(bp);
                          if (bp->b_vp)
                                  brelvp(bp);
                  }
  
                  /*
                   * NOTE:  nbp is now entirely invalid.  We can only restart
                   * the scan from this point on.
                   *
                   * Get the rest of the buffer freed up.  b_kva* is still
                   * valid after this operation.
                   */
                  KASSERT(bp->b_vp == NULL,
                          ("bp3 %p flags %08x vnode %p qindex %d "
                           "unexpectededly still associated!",
                           bp, bp->b_flags, bp->b_vp, qindex));
                  KKASSERT((bp->b_flags & B_HASHED) == 0);
  
                  /*
                   * critical section protection is not required when
                   * scrapping a buffer's contents because it is already 
                   * wired.
                   */
                  if (bp->b_bufsize)
                          allocbuf(bp, 0);
  
                  bp->b_flags = B_BNOCLIP;
                  bp->b_cmd = BUF_CMD_DONE;
                  bp->b_vp = NULL;
                  bp->b_error = 0;
                  bp->b_resid = 0;
                  bp->b_bcount = 0;
                  bp->b_xio.xio_npages = 0;
                  bp->b_dirtyoff = bp->b_dirtyend = 0;
                  bp->b_act_count = ACT_INIT;
                  reinitbufbio(bp);
                  KKASSERT(LIST_FIRST(&bp->b_dep) == NULL);
                  buf_dep_init(bp);
                  if (blkflags & GETBLK_BHEAVY)
                          bp->b_flags |= B_HEAVY;
  
                  /*
                   * If we are defragging then free the buffer.
                   */
                  if (defrag) {
                          bp->b_flags |= B_INVAL;
                          bfreekva(bp);
                          brelse(bp);
                          defrag = 0;
                          restart_reason = 4;
                          restart_bp = bp;
                          goto restart;
                  }
  
                  /*
                   * If we are overcomitted then recover the buffer and its
                   * KVM space.  This occurs in rare situations when multiple
                   * processes are blocked in getnewbuf() or allocbuf().
                   *
                   * On 64-bit systems BKVASIZE == MAXBSIZE and overcommit
                   * should not be possible.
                   */
                  if (bufspace >= hibufspace)
                          flushingbufs = 1;
                  if (BKVASIZE != MAXBSIZE) {
                          if (flushingbufs && bp->b_kvasize != 0) {
                                  bp->b_flags |= B_INVAL;
                                  bfreekva(bp);
                                  brelse(bp);
                                  restart_reason = 5;
                                  restart_bp = bp;
                                  goto restart;
                          }
                  }
                  if (bufspace < lobufspace)
                          flushingbufs = 0;
  
                  /*
                   * b_refs can transition to a non-zero value while we hold
                   * the buffer locked due to a findblk().  Our brelvp() above
                   * interlocked any future possible transitions due to
                   * findblk()s.
                   *
                   * If we find b_refs to be non-zero we can destroy the
                   * buffer's contents but we cannot yet reuse the buffer.
                   */
                  if (bp->b_refs) {
                          bp->b_flags |= B_INVAL;
                          if (BKVASIZE != MAXBSIZE)
                                  bfreekva(bp);
                          brelse(bp);
                          restart_reason = 6;
                          restart_bp = bp;
                          goto restart;
                  }
                  break;
                  /* NOT REACHED, spin not held */
          }
  
          /*
           * If we exhausted our list, iterate other cpus.  If that fails,
           * sleep as appropriate.  We may have to wakeup various daemons
           * and write out some dirty buffers.
           *
           * Generally we are sleeping due to insufficient buffer space.
           *
           * NOTE: spin is held if bp is NULL, else it is not held.
           */
          if (bp == NULL) {
                  int flags;
                  char *waitmsg;
  
                  spin_unlock(&pcpu->spin);
  
                  nqcpu = (nqcpu + 1) % ncpus;
                  if (nqcpu != mycpu->gd_cpuid) {
                          restart_reason = 7;
                          restart_bp = bp;
                          goto restart;
                  }
  
                  if (defrag) {
                          flags = VFS_BIO_NEED_BUFSPACE;
                          waitmsg = "nbufkv";
                  } else if (bufspace >= hibufspace) {
                          waitmsg = "nbufbs";
                          flags = VFS_BIO_NEED_BUFSPACE;
                  } else {
                          waitmsg = "newbuf";
                          flags = VFS_BIO_NEED_ANY;
                  }
  
                  bd_speedup();   /* heeeelp */
                  atomic_set_int(&needsbuffer, flags);
                  while (needsbuffer & flags) {
                          int value;
  
                          tsleep_interlock(&needsbuffer, 0);
                          value = atomic_fetchadd_int(&needsbuffer, 0);
                          if (value & flags) {
                                  if (tsleep(&needsbuffer, PINTERLOCKED|slpflags,
                                             waitmsg, slptimeo)) {
                                          return (NULL);
                                  }
                          }
                  }
          } else {
                  /*
                   * We finally have a valid bp.  We aren't quite out of the
                   * woods, we still have to reserve kva space.  In order
                   * to keep fragmentation sane we only allocate kva in
                   * BKVASIZE chunks.
                   *
                   * (spin is not held)
                   */
                  maxsize = (maxsize + BKVAMASK) & ~BKVAMASK;
  
                  if (maxsize != bp->b_kvasize) {
                          vm_offset_t addr = 0;
                          int count;
  
                          bfreekva(bp);
  
                          count = vm_map_entry_reserve(MAP_RESERVE_COUNT);
                          vm_map_lock(&buffer_map);
  
                          if (vm_map_findspace(&buffer_map,
                                      vm_map_min(&buffer_map), maxsize,
                                      maxsize, 0, &addr)) {
                                  /*
                                   * Uh oh.  Buffer map is too fragmented.  We
                                   * must defragment the map.
                                   */
                                  vm_map_unlock(&buffer_map);
                                  vm_map_entry_release(count);
                                  ++bufdefragcnt;
                                  defrag = 1;
                                  bp->b_flags |= B_INVAL;
                                  brelse(bp);
                                  restart_reason = 8;
                                  restart_bp = bp;
                                  goto restart;
                          }
                          if (addr) {
                                  vm_map_insert(&buffer_map, &count,
                                          NULL, 0,
                                          addr, addr + maxsize,
                                          VM_MAPTYPE_NORMAL,
                                          VM_PROT_ALL, VM_PROT_ALL,
                                          MAP_NOFAULT);
  
                                  bp->b_kvabase = (caddr_t) addr;
                                  bp->b_kvasize = maxsize;
                                  bufspace += bp->b_kvasize;
                                  ++bufreusecnt;
                          }
                          vm_map_unlock(&buffer_map);
                          vm_map_entry_release(count);
                  }
                  bp->b_data = bp->b_kvabase;
          }
          return(bp);
  }
  
  /*
   * buf_daemon:
   *
   *      Buffer flushing daemon.  Buffers are normally flushed by the
   *      update daemon but if it cannot keep up this process starts to
   *      take the load in an attempt to prevent getnewbuf() from blocking.
   *
   *      Once a flush is initiated it does not stop until the number
   *      of buffers falls below lodirtybuffers, but we will wake up anyone
   *      waiting at the mid-point.
   */
  static struct kproc_desc buf_kp = {
          "bufdaemon",
          buf_daemon,
          &bufdaemon_td
  };
  SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST,
          kproc_start, &buf_kp)
  
  static struct kproc_desc bufhw_kp = {
          "bufdaemon_hw",
          buf_daemon_hw,
          &bufdaemonhw_td
  };
  SYSINIT(bufdaemon_hw, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST,
          kproc_start, &bufhw_kp)
  
  static void
  buf_daemon1(struct thread *td, int queue, int (*buf_limit_fn)(long), 
              int *bd_req)
  {
          long limit;
          struct buf *marker;
  
          marker = kmalloc(sizeof(*marker), M_BIOBUF, M_WAITOK | M_ZERO);
          marker->b_flags |= B_MARKER;
          marker->b_qindex = BQUEUE_NONE;
          marker->b_qcpu = 0;
  
          /*
           * This process needs to be suspended prior to shutdown sync.
           */
          EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_kproc,
                                td, SHUTDOWN_PRI_LAST);
          curthread->td_flags |= TDF_SYSTHREAD;
  
          /*
           * This process is allowed to take the buffer cache to the limit
           */
          for (;;) {
                  kproc_suspend_loop();
  
                  /*
                   * Do the flush as long as the number of dirty buffers
                   * (including those running) exceeds lodirtybufspace.
                   *
                   * When flushing limit running I/O to hirunningspace
                   * Do the flush.  Limit the amount of in-transit I/O we
                   * allow to build up, otherwise we would completely saturate
                   * the I/O system.  Wakeup any waiting processes before we
                   * normally would so they can run in parallel with our drain.
                   *
                   * Our aggregate normal+HW lo water mark is lodirtybufspace,
                   * but because we split the operation into two threads we
                   * have to cut it in half for each thread.
                   */
                  waitrunningbufspace();
                  limit = lodirtybufspace / 2;
                  while (buf_limit_fn(limit)) {
                          if (flushbufqueues(marker, queue) == 0)
                                  break;
                          if (runningbufspace < hirunningspace)
                                  continue;
                          waitrunningbufspace();
                  }
  
                  /*
                   * We reached our low water mark, reset the
                   * request and sleep until we are needed again.
                   * The sleep is just so the suspend code works.
                   */
                  tsleep_interlock(bd_req, 0);
                  if (atomic_swap_int(bd_req, 0) == 0)
                          tsleep(bd_req, PINTERLOCKED, "psleep", hz);
          }
          /* NOT REACHED */
          /*kfree(marker, M_BIOBUF);*/
  }
  
  static int
  buf_daemon_limit(long limit)
  {
          return (runningbufspace + dirtykvaspace > limit ||
                  dirtybufcount - dirtybufcounthw >= nbuf / 2);
  }
  
  static int
  buf_daemon_hw_limit(long limit)
  {
          return (runningbufspace + dirtykvaspace > limit ||
                  dirtybufcounthw >= nbuf / 2);
  }
  
  static void
  buf_daemon(void)
  {
          buf_daemon1(bufdaemon_td, BQUEUE_DIRTY, buf_daemon_limit, 
                      &bd_request);
  }
  
  static void
  buf_daemon_hw(void)
  {
          buf_daemon1(bufdaemonhw_td, BQUEUE_DIRTY_HW, buf_daemon_hw_limit,
                      &bd_request_hw);
  }
  
  /*
   * flushbufqueues:
   *
   *      Try to flush a buffer in the dirty queue.  We must be careful to
   *      free up B_INVAL buffers instead of write them, which NFS is 
   *      particularly sensitive to.
   *
   *      B_RELBUF may only be set by VFSs.  We do set B_AGE to indicate
   *      that we really want to try to get the buffer out and reuse it
   *      due to the write load on the machine.
   *
   *      We must lock the buffer in order to check its validity before we
   *      can mess with its contents.  spin isn't enough.
   */
  static int
  flushbufqueues(struct buf *marker, bufq_type_t q)
  {
          struct bufpcpu *pcpu;
          struct buf *bp;
          int r = 0;
          int lcpu = marker->b_qcpu;
  
          KKASSERT(marker->b_qindex == BQUEUE_NONE);
          KKASSERT(marker->b_flags & B_MARKER);
  
  again:
          /*
           * Spinlock needed to perform operations on the queue and may be
           * held through a non-blocking BUF_LOCK(), but cannot be held when
           * BUF_UNLOCK()ing or through any other major operation.
           */
          pcpu = &bufpcpu[marker->b_qcpu];
          spin_lock(&pcpu->spin);
          marker->b_qindex = q;
          TAILQ_INSERT_HEAD(&pcpu->bufqueues[q], marker, b_freelist);
          bp = marker;
  
          while ((bp = TAILQ_NEXT(bp, b_freelist)) != NULL) {
                  /*
                   * NOTE: spinlock is always held at the top of the loop
                   */
                  if (bp->b_flags & B_MARKER)
                          continue;
                  if ((bp->b_flags & B_DELWRI) == 0) {
                          kprintf("Unexpected clean buffer %p\n", bp);
                          continue;
                  }
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT))
                          continue;
                  KKASSERT(bp->b_qcpu == marker->b_qcpu && bp->b_qindex == q);
  
                  /*
                   * Once the buffer is locked we will have no choice but to
                   * unlock the spinlock around a later BUF_UNLOCK and re-set
                   * bp = marker when looping.  Move the marker now to make
                   * things easier.
                   */
                  TAILQ_REMOVE(&pcpu->bufqueues[q], marker, b_freelist);
                  TAILQ_INSERT_AFTER(&pcpu->bufqueues[q], bp, marker, b_freelist);
  
                  /*
                   * Must recheck B_DELWRI after successfully locking
                   * the buffer.
                   */
                  if ((bp->b_flags & B_DELWRI) == 0) {
                          spin_unlock(&pcpu->spin);
                          BUF_UNLOCK(bp);
                          spin_lock(&pcpu->spin);
                          bp = marker;
                          continue;
                  }
  
                  /*
                   * Remove the buffer from its queue.  We still own the
                   * spinlock here.
                   */
                  _bremfree(bp);
  
                  /*
                   * Disposing of an invalid buffer counts as a flush op
                   */
                  if (bp->b_flags & B_INVAL) {
                          spin_unlock(&pcpu->spin);
                          brelse(bp);
                          spin_lock(&pcpu->spin);
                          ++r;
                          break;
                  }
  
                  /*
                   * Release the spinlock for the more complex ops we
                   * are now going to do.
                   */
                  spin_unlock(&pcpu->spin);
                  lwkt_yield();
  
                  /*
                   * This is a bit messy
                   */
                  if (LIST_FIRST(&bp->b_dep) != NULL &&
                      (bp->b_flags & B_DEFERRED) == 0 &&
                      buf_countdeps(bp, 0)) {
                          spin_lock(&pcpu->spin);
                          TAILQ_INSERT_TAIL(&pcpu->bufqueues[q], bp, b_freelist);
                          bp->b_qindex = q;
                          bp->b_flags |= B_DEFERRED;
                          spin_unlock(&pcpu->spin);
                          BUF_UNLOCK(bp);
                          spin_lock(&pcpu->spin);
                          bp = marker;
                          continue;
                  }
  
                  /*
                   * spinlock not held here.
                   *
                   * If the buffer has a dependancy, buf_checkwrite() must
                   * also return 0 for us to be able to initate the write.
                   *
                   * If the buffer is flagged B_ERROR it may be requeued
                   * over and over again, we try to avoid a live lock.
                   */
                  if (LIST_FIRST(&bp->b_dep) != NULL && buf_checkwrite(bp)) {
                          brelse(bp);
                  } else if (bp->b_flags & B_ERROR) {
                          tsleep(bp, 0, "bioer", 1);
                          bp->b_flags &= ~B_AGE;
                          cluster_awrite(bp);
                  } else {
                          bp->b_flags |= B_AGE;
                          cluster_awrite(bp);
                  }
                  spin_lock(&pcpu->spin);
                  ++r;
                  break;
          }
  
          TAILQ_REMOVE(&pcpu->bufqueues[q], marker, b_freelist);
          marker->b_qindex = BQUEUE_NONE;
          spin_unlock(&pcpu->spin);
  
          /*
           * Advance the marker to be fair.
           */
          marker->b_qcpu = (marker->b_qcpu + 1) % ncpus;
          if (bp == NULL) {
                  if (marker->b_qcpu != lcpu)
                          goto again;
          }
  
          return (r);
  }
  
  /*
   * inmem:
   *
   *      Returns true if no I/O is needed to access the associated VM object.
   *      This is like findblk except it also hunts around in the VM system for
   *      the data.
   *
   *      Note that we ignore vm_page_free() races from interrupts against our
   *      lookup, since if the caller is not protected our return value will not
   *      be any more valid then otherwise once we exit the critical section.
   */
  int
  inmem(struct vnode *vp, off_t loffset)
  {
          vm_object_t obj;
          vm_offset_t toff, tinc, size;
          vm_page_t m;
          int res = 1;
  
          if (findblk(vp, loffset, FINDBLK_TEST))
                  return 1;
          if (vp->v_mount == NULL)
                  return 0;
          if ((obj = vp->v_object) == NULL)
                  return 0;
  
          size = PAGE_SIZE;
          if (size > vp->v_mount->mnt_stat.f_iosize)
                  size = vp->v_mount->mnt_stat.f_iosize;
  
          vm_object_hold(obj);
          for (toff = 0; toff < vp->v_mount->mnt_stat.f_iosize; toff += tinc) {
                  m = vm_page_lookup(obj, OFF_TO_IDX(loffset + toff));
                  if (m == NULL) {
                          res = 0;
                          break;
                  }
                  tinc = size;
                  if (tinc > PAGE_SIZE - ((toff + loffset) & PAGE_MASK))
                          tinc = PAGE_SIZE - ((toff + loffset) & PAGE_MASK);
                  if (vm_page_is_valid(m,
                      (vm_offset_t) ((toff + loffset) & PAGE_MASK), tinc) == 0) {
                          res = 0;
                          break;
                  }
          }
          vm_object_drop(obj);
          return (res);
  }
  
  /*
   * findblk:
   *
   *      Locate and return the specified buffer.  Unless flagged otherwise,
   *      a locked buffer will be returned if it exists or NULL if it does not.
   *
   *      findblk()'d buffers are still on the bufqueues and if you intend
   *      to use your (locked NON-TEST) buffer you need to bremfree(bp)
   *      and possibly do other stuff to it.
   *
   *      FINDBLK_TEST    - Do not lock the buffer.  The caller is responsible
   *                        for locking the buffer and ensuring that it remains
   *                        the desired buffer after locking.
   *
   *      FINDBLK_NBLOCK  - Lock the buffer non-blocking.  If we are unable
   *                        to acquire the lock we return NULL, even if the
   *                        buffer exists.
   *
   *      FINDBLK_REF     - Returns the buffer ref'd, which prevents normal
   *                        reuse by getnewbuf() but does not prevent
   *                        disassociation (B_INVAL).  Used to avoid deadlocks
   *                        against random (vp,loffset)s due to reassignment.
   *
   *      (0)             - Lock the buffer blocking.
   */
  struct buf *
  findblk(struct vnode *vp, off_t loffset, int flags)
  {
          struct buf *bp;
          int lkflags;
  
          lkflags = LK_EXCLUSIVE;
          if (flags & FINDBLK_NBLOCK)
                  lkflags |= LK_NOWAIT;
  
          for (;;) {
                  /*
                   * Lookup.  Ref the buf while holding v_token to prevent
                   * reuse (but does not prevent diassociation).
                   */
                  lwkt_gettoken_shared(&vp->v_token);
                  bp = buf_rb_hash_RB_LOOKUP(&vp->v_rbhash_tree, loffset);
                  if (bp == NULL) {
                          lwkt_reltoken(&vp->v_token);
                          return(NULL);
                  }
                  bqhold(bp);
                  lwkt_reltoken(&vp->v_token);
  
                  /*
                   * If testing only break and return bp, do not lock.
                   */
                  if (flags & FINDBLK_TEST)
                          break;
  
                  /*
                   * Lock the buffer, return an error if the lock fails.
                   * (only FINDBLK_NBLOCK can cause the lock to fail).
                   */
                  if (BUF_LOCK(bp, lkflags)) {
                          atomic_subtract_int(&bp->b_refs, 1);
                          /* bp = NULL; not needed */
                          return(NULL);
                  }
  
                  /*
                   * Revalidate the locked buf before allowing it to be
                   * returned.
                   */
                  if (bp->b_vp == vp && bp->b_loffset == loffset)
                          break;
                  atomic_subtract_int(&bp->b_refs, 1);
                  BUF_UNLOCK(bp);
          }
  
          /*
           * Success
           */
          if ((flags & FINDBLK_REF) == 0)
                  atomic_subtract_int(&bp->b_refs, 1);
          return(bp);
  }
  
  /*
   * getcacheblk:
   *
   *      Similar to getblk() except only returns the buffer if it is
   *      B_CACHE and requires no other manipulation.  Otherwise NULL
   *      is returned.
   *
   *      If B_RAM is set the buffer might be just fine, but we return
   *      NULL anyway because we want the code to fall through to the
   *      cluster read.  Otherwise read-ahead breaks.
   *
   *      If blksize is 0 the buffer cache buffer must already be fully
   *      cached.
   *
   *      If blksize is non-zero getblk() will be used, allowing a buffer
   *      to be reinstantiated from its VM backing store.  The buffer must
   *      still be fully cached after reinstantiation to be returned.
   */
  struct buf *
  getcacheblk(struct vnode *vp, off_t loffset, int blksize, int blkflags)
  {
          struct buf *bp;
          int fndflags = (blkflags & GETBLK_NOWAIT) ? FINDBLK_NBLOCK : 0;
  
          if (blksize) {
                  bp = getblk(vp, loffset, blksize, blkflags, 0);
                  if (bp) {
                          if ((bp->b_flags & (B_INVAL | B_CACHE | B_RAM)) ==
                              B_CACHE) {
                                  bp->b_flags &= ~B_AGE;
                          } else {
                                  brelse(bp);
                                  bp = NULL;
                          }
                  }
          } else {
                  bp = findblk(vp, loffset, fndflags);
                  if (bp) {
                          if ((bp->b_flags & (B_INVAL | B_CACHE | B_RAM)) ==
                              B_CACHE) {
                                  bp->b_flags &= ~B_AGE;
                                  bremfree(bp);
                          } else {
                                  BUF_UNLOCK(bp);
                                  bp = NULL;
                          }
                  }
          }
          return (bp);
  }
  
  /*
   * getblk:
   *
   *      Get a block given a specified block and offset into a file/device.
   *      B_INVAL may or may not be set on return.  The caller should clear
   *      B_INVAL prior to initiating a READ.
   *
   *      IT IS IMPORTANT TO UNDERSTAND THAT IF YOU CALL GETBLK() AND B_CACHE
   *      IS NOT SET, YOU MUST INITIALIZE THE RETURNED BUFFER, ISSUE A READ,
   *      OR SET B_INVAL BEFORE RETIRING IT.  If you retire a getblk'd buffer
   *      without doing any of those things the system will likely believe
   *      the buffer to be valid (especially if it is not B_VMIO), and the
   *      next getblk() will return the buffer with B_CACHE set.
   *
   *      For a non-VMIO buffer, B_CACHE is set to the opposite of B_INVAL for
   *      an existing buffer.
   *
   *      For a VMIO buffer, B_CACHE is modified according to the backing VM.
   *      If getblk()ing a previously 0-sized invalid buffer, B_CACHE is set
   *      and then cleared based on the backing VM.  If the previous buffer is
   *      non-0-sized but invalid, B_CACHE will be cleared.
   *
   *      If getblk() must create a new buffer, the new buffer is returned with
   *      both B_INVAL and B_CACHE clear unless it is a VMIO buffer, in which
   *      case it is returned with B_INVAL clear and B_CACHE set based on the
   *      backing VM.
   *
   *      getblk() also forces a bwrite() for any B_DELWRI buffer whos
   *      B_CACHE bit is clear.
   *      
   *      What this means, basically, is that the caller should use B_CACHE to
   *      determine whether the buffer is fully valid or not and should clear
   *      B_INVAL prior to issuing a read.  If the caller intends to validate
   *      the buffer by loading its data area with something, the caller needs
   *      to clear B_INVAL.  If the caller does this without issuing an I/O, 
   *      the caller should set B_CACHE ( as an optimization ), else the caller
   *      should issue the I/O and biodone() will set B_CACHE if the I/O was
   *      a write attempt or if it was a successfull read.  If the caller 
   *      intends to issue a READ, the caller must clear B_INVAL and B_ERROR
   *      prior to issuing the READ.  biodone() will *not* clear B_INVAL.
   *
   *      getblk flags:
   *
   *      GETBLK_PCATCH - catch signal if blocked, can cause NULL return
   *      GETBLK_BHEAVY - heavy-weight buffer cache buffer
   */
  struct buf *
  getblk(struct vnode *vp, off_t loffset, int size, int blkflags, int slptimeo)
  {
          struct buf *bp;
          int slpflags = (blkflags & GETBLK_PCATCH) ? PCATCH : 0;
          int error;
          int lkflags;
  
          if (size > MAXBSIZE)
                  panic("getblk: size(%d) > MAXBSIZE(%d)", size, MAXBSIZE);
          if (vp->v_object == NULL)
                  panic("getblk: vnode %p has no object!", vp);
  
  loop:
          if ((bp = findblk(vp, loffset, FINDBLK_REF | FINDBLK_TEST)) != NULL) {
                  /*
                   * The buffer was found in the cache, but we need to lock it.
                   * We must acquire a ref on the bp to prevent reuse, but
                   * this will not prevent disassociation (brelvp()) so we
                   * must recheck (vp,loffset) after acquiring the lock.
                   *
                   * Without the ref the buffer could potentially be reused
                   * before we acquire the lock and create a deadlock
                   * situation between the thread trying to reuse the buffer
                   * and us due to the fact that we would wind up blocking
                   * on a random (vp,loffset).
                   */
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                          if (blkflags & GETBLK_NOWAIT) {
                                  bqdrop(bp);
                                  return(NULL);
                          }
                          lkflags = LK_EXCLUSIVE | LK_SLEEPFAIL;
                          if (blkflags & GETBLK_PCATCH)
                                  lkflags |= LK_PCATCH;
                          error = BUF_TIMELOCK(bp, lkflags, "getblk", slptimeo);
                          if (error) {
                                  bqdrop(bp);
                                  if (error == ENOLCK)
                                          goto loop;
                                  return (NULL);
                          }
                          /* buffer may have changed on us */
                  }
                  bqdrop(bp);
  
                  /*
                   * Once the buffer has been locked, make sure we didn't race
                   * a buffer recyclement.  Buffers that are no longer hashed
                   * will have b_vp == NULL, so this takes care of that check
                   * as well.
                   */
                  if (bp->b_vp != vp || bp->b_loffset != loffset) {
                          kprintf("Warning buffer %p (vp %p loffset %lld) "
                                  "was recycled\n",
                                  bp, vp, (long long)loffset);
                          BUF_UNLOCK(bp);
                          goto loop;
                  }
  
                  /*
                   * If SZMATCH any pre-existing buffer must be of the requested
                   * size or NULL is returned.  The caller absolutely does not
                   * want getblk() to bwrite() the buffer on a size mismatch.
                   */
                  if ((blkflags & GETBLK_SZMATCH) && size != bp->b_bcount) {
                          BUF_UNLOCK(bp);
                          return(NULL);
                  }
  
                  /*
                   * All vnode-based buffers must be backed by a VM object.
                   */
                  KKASSERT(bp->b_flags & B_VMIO);
                  KKASSERT(bp->b_cmd == BUF_CMD_DONE);
                  bp->b_flags &= ~B_AGE;
  
                  /*
                   * Make sure that B_INVAL buffers do not have a cached
                   * block number translation.
                   */
                  if ((bp->b_flags & B_INVAL) && (bp->b_bio2.bio_offset != NOOFFSET)) {
                          kprintf("Warning invalid buffer %p (vp %p loffset %lld)"
                                  " did not have cleared bio_offset cache\n",
                                  bp, vp, (long long)loffset);
                          clearbiocache(&bp->b_bio2);
                  }
  
                  /*
                   * The buffer is locked.  B_CACHE is cleared if the buffer is 
                   * invalid.
                   */
                  if (bp->b_flags & B_INVAL)
                          bp->b_flags &= ~B_CACHE;
                  bremfree(bp);
  
                  /*
                   * Any size inconsistancy with a dirty buffer or a buffer
                   * with a softupdates dependancy must be resolved.  Resizing
                   * the buffer in such circumstances can lead to problems.
                   *
                   * Dirty or dependant buffers are written synchronously.
                   * Other types of buffers are simply released and
                   * reconstituted as they may be backed by valid, dirty VM
                   * pages (but not marked B_DELWRI).
                   *
                   * NFS NOTE: NFS buffers which straddle EOF are oddly-sized
                   * and may be left over from a prior truncation (and thus
                   * no longer represent the actual EOF point), so we
                   * definitely do not want to B_NOCACHE the backing store.
                   */
                  if (size != bp->b_bcount) {
                          if (bp->b_flags & B_DELWRI) {
                                  bp->b_flags |= B_RELBUF;
                                  bwrite(bp);
                          } else if (LIST_FIRST(&bp->b_dep)) {
                                  bp->b_flags |= B_RELBUF;
                                  bwrite(bp);
                          } else {
                                  bp->b_flags |= B_RELBUF;
                                  brelse(bp);
                          }
                          goto loop;
                  }
                  KKASSERT(size <= bp->b_kvasize);
                  KASSERT(bp->b_loffset != NOOFFSET, 
                          ("getblk: no buffer offset"));
  
                  /*
                   * A buffer with B_DELWRI set and B_CACHE clear must
                   * be committed before we can return the buffer in
                   * order to prevent the caller from issuing a read
                   * ( due to B_CACHE not being set ) and overwriting
                   * it.
                   *
                   * Most callers, including NFS and FFS, need this to
                   * operate properly either because they assume they
                   * can issue a read if B_CACHE is not set, or because
                   * ( for example ) an uncached B_DELWRI might loop due 
                   * to softupdates re-dirtying the buffer.  In the latter
                   * case, B_CACHE is set after the first write completes,
                   * preventing further loops.
                   *
                   * NOTE!  b*write() sets B_CACHE.  If we cleared B_CACHE
                   * above while extending the buffer, we cannot allow the
                   * buffer to remain with B_CACHE set after the write
                   * completes or it will represent a corrupt state.  To
                   * deal with this we set B_NOCACHE to scrap the buffer
                   * after the write.
                   *
                   * XXX Should this be B_RELBUF instead of B_NOCACHE?
                   *     I'm not even sure this state is still possible
                   *     now that getblk() writes out any dirty buffers
                   *     on size changes.
                   *
                   * We might be able to do something fancy, like setting
                   * B_CACHE in bwrite() except if B_DELWRI is already set,
                   * so the below call doesn't set B_CACHE, but that gets real
                   * confusing.  This is much easier.
                   */
  
                  if ((bp->b_flags & (B_CACHE|B_DELWRI)) == B_DELWRI) {
                          kprintf("getblk: Warning, bp %p loff=%jx DELWRI set "
                                  "and CACHE clear, b_flags %08x\n",
                                  bp, (uintmax_t)bp->b_loffset, bp->b_flags);
                          bp->b_flags |= B_NOCACHE;
                          bwrite(bp);
                          goto loop;
                  }
          } else {
                  /*
                   * Buffer is not in-core, create new buffer.  The buffer
                   * returned by getnewbuf() is locked.  Note that the returned
                   * buffer is also considered valid (not marked B_INVAL).
                   *
                   * Calculating the offset for the I/O requires figuring out
                   * the block size.  We use DEV_BSIZE for VBLK or VCHR and
                   * the mount's f_iosize otherwise.  If the vnode does not
                   * have an associated mount we assume that the passed size is 
                   * the block size.  
                   *
                   * Note that vn_isdisk() cannot be used here since it may
                   * return a failure for numerous reasons.   Note that the
                   * buffer size may be larger then the block size (the caller
                   * will use block numbers with the proper multiple).  Beware
                   * of using any v_* fields which are part of unions.  In
                   * particular, in DragonFly the mount point overloading 
                   * mechanism uses the namecache only and the underlying
                   * directory vnode is not a special case.
                   */
                  int bsize, maxsize;
  
                  if (vp->v_type == VBLK || vp->v_type == VCHR)
                          bsize = DEV_BSIZE;
                  else if (vp->v_mount)
                          bsize = vp->v_mount->mnt_stat.f_iosize;
                  else
                          bsize = size;
  
                  maxsize = size + (loffset & PAGE_MASK);
                  maxsize = imax(maxsize, bsize);
  
                  bp = getnewbuf(blkflags, slptimeo, size, maxsize);
                  if (bp == NULL) {
                          if (slpflags || slptimeo)
                                  return NULL;
                          goto loop;
                  }
  
                  /*
                   * Atomically insert the buffer into the hash, so that it can
                   * be found by findblk().
                   *
                   * If bgetvp() returns non-zero a collision occured, and the
                   * bp will not be associated with the vnode.
                   *
                   * Make sure the translation layer has been cleared.
                   */
                  bp->b_loffset = loffset;
                  bp->b_bio2.bio_offset = NOOFFSET;
                  /* bp->b_bio2.bio_next = NULL; */
  
                  if (bgetvp(vp, bp, size)) {
                          bp->b_flags |= B_INVAL;
                          brelse(bp);
                          goto loop;
                  }
  
                  /*
                   * All vnode-based buffers must be backed by a VM object.
                   */
                  KKASSERT(vp->v_object != NULL);
                  bp->b_flags |= B_VMIO;
                  KKASSERT(bp->b_cmd == BUF_CMD_DONE);
  
                  allocbuf(bp, size);
          }
          KKASSERT(dsched_is_clear_buf_priv(bp));
          return (bp);
  }
  
  /*
   * regetblk(bp)
   *
   * Reacquire a buffer that was previously released to the locked queue,
   * or reacquire a buffer which is interlocked by having bioops->io_deallocate
   * set B_LOCKED (which handles the acquisition race).
   *
   * To this end, either B_LOCKED must be set or the dependancy list must be
   * non-empty.
   */
  void
  regetblk(struct buf *bp)
  {
          KKASSERT((bp->b_flags & B_LOCKED) || LIST_FIRST(&bp->b_dep) != NULL);
          BUF_LOCK(bp, LK_EXCLUSIVE | LK_RETRY);
          bremfree(bp);
  }
  
  /*
   * geteblk:
   *
   *      Get an empty, disassociated buffer of given size.  The buffer is
   *      initially set to B_INVAL.
   *
   *      critical section protection is not required for the allocbuf()
   *      call because races are impossible here.
   */
  struct buf *
  geteblk(int size)
  {
          struct buf *bp;
          int maxsize;
  
          maxsize = (size + BKVAMASK) & ~BKVAMASK;
  
          while ((bp = getnewbuf(0, 0, size, maxsize)) == NULL)
                  ;
          allocbuf(bp, size);
          bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
          KKASSERT(dsched_is_clear_buf_priv(bp));
          return (bp);
  }
  
  
  /*
   * allocbuf:
   *
   *      This code constitutes the buffer memory from either anonymous system
   *      memory (in the case of non-VMIO operations) or from an associated
   *      VM object (in the case of VMIO operations).  This code is able to
   *      resize a buffer up or down.
   *
   *      Note that this code is tricky, and has many complications to resolve
   *      deadlock or inconsistant data situations.  Tread lightly!!! 
   *      There are B_CACHE and B_DELWRI interactions that must be dealt with by 
   *      the caller.  Calling this code willy nilly can result in the loss of
   *      data.
   *
   *      allocbuf() only adjusts B_CACHE for VMIO buffers.  getblk() deals with
   *      B_CACHE for the non-VMIO case.
   *
   *      This routine does not need to be called from a critical section but you
   *      must own the buffer.
   */
  int
  allocbuf(struct buf *bp, int size)
  {
          int newbsize, mbsize;
          int i;
  
          if (BUF_REFCNT(bp) == 0)
                  panic("allocbuf: buffer not busy");
  
          if (bp->b_kvasize < size)
                  panic("allocbuf: buffer too small");
  
          if ((bp->b_flags & B_VMIO) == 0) {
                  caddr_t origbuf;
                  int origbufsize;
                  /*
                   * Just get anonymous memory from the kernel.  Don't
                   * mess with B_CACHE.
                   */
                  mbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                  if (bp->b_flags & B_MALLOC)
                          newbsize = mbsize;
                  else
                          newbsize = round_page(size);
  
                  if (newbsize < bp->b_bufsize) {
                          /*
                           * Malloced buffers are not shrunk
                           */
                          if (bp->b_flags & B_MALLOC) {
                                  if (newbsize) {
                                          bp->b_bcount = size;
                                  } else {
                                          kfree(bp->b_data, M_BIOBUF);
                                          if (bp->b_bufsize) {
                                                  atomic_subtract_long(&bufmallocspace, bp->b_bufsize);
                                                  bufspacewakeup();
                                                  bp->b_bufsize = 0;
                                          }
                                          bp->b_data = bp->b_kvabase;
                                          bp->b_bcount = 0;
                                          bp->b_flags &= ~B_MALLOC;
                                  }
                                  return 1;
                          }               
                          vm_hold_free_pages(
                              bp,
                              (vm_offset_t) bp->b_data + newbsize,
                              (vm_offset_t) bp->b_data + bp->b_bufsize);
                  } else if (newbsize > bp->b_bufsize) {
                          /*
                           * We only use malloced memory on the first allocation.
                           * and revert to page-allocated memory when the buffer
                           * grows.
                           */
                          if ((bufmallocspace < maxbufmallocspace) &&
                                  (bp->b_bufsize == 0) &&
                                  (mbsize <= PAGE_SIZE/2)) {
  
                                  bp->b_data = kmalloc(mbsize, M_BIOBUF, M_WAITOK);
                                  bp->b_bufsize = mbsize;
                                  bp->b_bcount = size;
                                  bp->b_flags |= B_MALLOC;
                                  atomic_add_long(&bufmallocspace, mbsize);
                                  return 1;
                          }
                          origbuf = NULL;
                          origbufsize = 0;
                          /*
                           * If the buffer is growing on its other-than-first
                           * allocation, then we revert to the page-allocation
                           * scheme.
                           */
                          if (bp->b_flags & B_MALLOC) {
                                  origbuf = bp->b_data;
                                  origbufsize = bp->b_bufsize;
                                  bp->b_data = bp->b_kvabase;
                                  if (bp->b_bufsize) {
                                          atomic_subtract_long(&bufmallocspace,
                                                               bp->b_bufsize);
                                          bufspacewakeup();
                                          bp->b_bufsize = 0;
                                  }
                                  bp->b_flags &= ~B_MALLOC;
                                  newbsize = round_page(newbsize);
                          }
                          vm_hold_load_pages(
                              bp,
                              (vm_offset_t) bp->b_data + bp->b_bufsize,
                              (vm_offset_t) bp->b_data + newbsize);
                          if (origbuf) {
                                  bcopy(origbuf, bp->b_data, origbufsize);
                                  kfree(origbuf, M_BIOBUF);
                          }
                  }
          } else {
                  vm_page_t m;
                  int desiredpages;
  
                  newbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                  desiredpages = ((int)(bp->b_loffset & PAGE_MASK) +
                                  newbsize + PAGE_MASK) >> PAGE_SHIFT;
                  KKASSERT(desiredpages <= XIO_INTERNAL_PAGES);
  
                  if (bp->b_flags & B_MALLOC)
                          panic("allocbuf: VMIO buffer can't be malloced");
                  /*
                   * Set B_CACHE initially if buffer is 0 length or will become
                   * 0-length.
                   */
                  if (size == 0 || bp->b_bufsize == 0)
                          bp->b_flags |= B_CACHE;
  
                  if (newbsize < bp->b_bufsize) {
                          /*
                           * DEV_BSIZE aligned new buffer size is less then the
                           * DEV_BSIZE aligned existing buffer size.  Figure out
                           * if we have to remove any pages.
                           */
                          if (desiredpages < bp->b_xio.xio_npages) {
                                  for (i = desiredpages; i < bp->b_xio.xio_npages; i++) {
                                          /*
                                           * the page is not freed here -- it
                                           * is the responsibility of 
                                           * vnode_pager_setsize
                                           */
                                          m = bp->b_xio.xio_pages[i];
                                          KASSERT(m != bogus_page,
                                              ("allocbuf: bogus page found"));
                                          vm_page_busy_wait(m, TRUE, "biodep");
                                          bp->b_xio.xio_pages[i] = NULL;
                                          vm_page_unwire(m, 0);
                                          vm_page_wakeup(m);
                                  }
                                  pmap_qremove((vm_offset_t) trunc_page((vm_offset_t)bp->b_data) +
                                      (desiredpages << PAGE_SHIFT), (bp->b_xio.xio_npages - desiredpages));
                                  bp->b_xio.xio_npages = desiredpages;
                          }
                  } else if (size > bp->b_bcount) {
                          /*
                           * We are growing the buffer, possibly in a 
                           * byte-granular fashion.
                           */
                          struct vnode *vp;
                          vm_object_t obj;
                          vm_offset_t toff;
                          vm_offset_t tinc;
  
                          /*
                           * Step 1, bring in the VM pages from the object, 
                           * allocating them if necessary.  We must clear
                           * B_CACHE if these pages are not valid for the 
                           * range covered by the buffer.
                           *
                           * critical section protection is required to protect
                           * against interrupts unbusying and freeing pages
                           * between our vm_page_lookup() and our
                           * busycheck/wiring call.
                           */
                          vp = bp->b_vp;
                          obj = vp->v_object;
  
                          vm_object_hold(obj);
                          while (bp->b_xio.xio_npages < desiredpages) {
                                  vm_page_t m;
                                  vm_pindex_t pi;
                                  int error;
  
                                  pi = OFF_TO_IDX(bp->b_loffset) +
                                       bp->b_xio.xio_npages;
  
                                  /*
                                   * Blocking on m->busy might lead to a
                                   * deadlock:
                                   *
                                   *  vm_fault->getpages->cluster_read->allocbuf
                                   */
                                  m = vm_page_lookup_busy_try(obj, pi, FALSE,
                                                              &error);
                                  if (error) {
                                          vm_page_sleep_busy(m, FALSE, "pgtblk");
                                          continue;
                                  }
                                  if (m == NULL) {
                                          /*
                                           * note: must allocate system pages
                                           * since blocking here could intefere
                                           * with paging I/O, no matter which
                                           * process we are.
                                           */
                                          m = bio_page_alloc(bp, obj, pi, desiredpages - bp->b_xio.xio_npages);
                                          if (m) {
                                                  vm_page_wire(m);
                                                  vm_page_flag_clear(m, PG_ZERO);
                                                  vm_page_wakeup(m);
                                                  bp->b_flags &= ~B_CACHE;
                                                  bp->b_xio.xio_pages[bp->b_xio.xio_npages] = m;
                                                  ++bp->b_xio.xio_npages;
                                          }
                                          continue;
                                  }
  
                                  /*
                                   * We found a page and were able to busy it.
                                   */
                                  vm_page_flag_clear(m, PG_ZERO);
                                  vm_page_wire(m);
                                  vm_page_wakeup(m);
                                  bp->b_xio.xio_pages[bp->b_xio.xio_npages] = m;
                                  ++bp->b_xio.xio_npages;
                                  if (bp->b_act_count < m->act_count)
                                          bp->b_act_count = m->act_count;
                          }
                          vm_object_drop(obj);
  
                          /*
                           * Step 2.  We've loaded the pages into the buffer,
                           * we have to figure out if we can still have B_CACHE
                           * set.  Note that B_CACHE is set according to the
                           * byte-granular range ( bcount and size ), not the
                           * aligned range ( newbsize ).
                           *
                           * The VM test is against m->valid, which is DEV_BSIZE
                           * aligned.  Needless to say, the validity of the data
                           * needs to also be DEV_BSIZE aligned.  Note that this
                           * fails with NFS if the server or some other client
                           * extends the file's EOF.  If our buffer is resized, 
                           * B_CACHE may remain set! XXX
                           */
  
                          toff = bp->b_bcount;
                          tinc = PAGE_SIZE - ((bp->b_loffset + toff) & PAGE_MASK);
  
                          while ((bp->b_flags & B_CACHE) && toff < size) {
                                  vm_pindex_t pi;
  
                                  if (tinc > (size - toff))
                                          tinc = size - toff;
  
                                  pi = ((bp->b_loffset & PAGE_MASK) + toff) >> 
                                      PAGE_SHIFT;
  
                                  vfs_buf_test_cache(
                                      bp, 
                                      bp->b_loffset,
                                      toff, 
                                      tinc, 
                                      bp->b_xio.xio_pages[pi]
                                  );
                                  toff += tinc;
                                  tinc = PAGE_SIZE;
                          }
  
                          /*
                           * Step 3, fixup the KVM pmap.  Remember that
                           * bp->b_data is relative to bp->b_loffset, but 
                           * bp->b_loffset may be offset into the first page.
                           */
  
                          bp->b_data = (caddr_t)
                              trunc_page((vm_offset_t)bp->b_data);
                          pmap_qenter(
                              (vm_offset_t)bp->b_data,
                              bp->b_xio.xio_pages, 
                              bp->b_xio.xio_npages
                          );
                          bp->b_data = (caddr_t)((vm_offset_t)bp->b_data | 
                              (vm_offset_t)(bp->b_loffset & PAGE_MASK));
                  }
          }
  
          /* adjust space use on already-dirty buffer */
          if (bp->b_flags & B_DELWRI) {
                  /* dirtykvaspace unchanged */
                  atomic_add_long(&dirtybufspace, newbsize - bp->b_bufsize);
                  if (bp->b_flags & B_HEAVY) {
                          atomic_add_long(&dirtybufspacehw,
                                          newbsize - bp->b_bufsize);
                  }
          }
          if (newbsize < bp->b_bufsize)
                  bufspacewakeup();
          bp->b_bufsize = newbsize;       /* actual buffer allocation     */
          bp->b_bcount = size;            /* requested buffer size        */
          return 1;
  }
  
  /*
   * biowait:
   *
   *      Wait for buffer I/O completion, returning error status. B_EINTR
   *      is converted into an EINTR error but not cleared (since a chain
   *      of biowait() calls may occur).
   *
   *      On return bpdone() will have been called but the buffer will remain
   *      locked and will not have been brelse()'d.
   *
   *      NOTE!  If a timeout is specified and ETIMEDOUT occurs the I/O is
   *      likely still in progress on return.
   *
   *      NOTE!  This operation is on a BIO, not a BUF.
   *
   *      NOTE!  BIO_DONE is cleared by vn_strategy()
   */
  static __inline int
  _biowait(struct bio *bio, const char *wmesg, int to)
  {
          struct buf *bp = bio->bio_buf;
          u_int32_t flags;
          u_int32_t nflags;
          int error;
  
          KKASSERT(bio == &bp->b_bio1);
          for (;;) {
                  flags = bio->bio_flags;
                  if (flags & BIO_DONE)
                          break;
                  nflags = flags | BIO_WANT;
                  tsleep_interlock(bio, 0);
                  if (atomic_cmpset_int(&bio->bio_flags, flags, nflags)) {
                          if (wmesg)
                                  error = tsleep(bio, PINTERLOCKED, wmesg, to);
                          else if (bp->b_cmd == BUF_CMD_READ)
                                  error = tsleep(bio, PINTERLOCKED, "biord", to);
                          else
                                  error = tsleep(bio, PINTERLOCKED, "biowr", to);
                          if (error) {
                                  kprintf("tsleep error biowait %d\n", error);
                                  return (error);
                          }
                  }
          }
  
          /*
           * Finish up.
           */
          KKASSERT(bp->b_cmd == BUF_CMD_DONE);
          bio->bio_flags &= ~(BIO_DONE | BIO_SYNC);
          if (bp->b_flags & B_EINTR)
                  return (EINTR);
          if (bp->b_flags & B_ERROR)
                  return (bp->b_error ? bp->b_error : EIO);
          return (0);
  }
  
  int
  biowait(struct bio *bio, const char *wmesg)
  {
          return(_biowait(bio, wmesg, 0));
  }
  
  int
  biowait_timeout(struct bio *bio, const char *wmesg, int to)
  {
          return(_biowait(bio, wmesg, to));
  }
  
  /*
   * This associates a tracking count with an I/O.  vn_strategy() and
   * dev_dstrategy() do this automatically but there are a few cases
   * where a vnode or device layer is bypassed when a block translation
   * is cached.  In such cases bio_start_transaction() may be called on
   * the bypassed layers so the system gets an I/O in progress indication 
   * for those higher layers.
   */
  void
  bio_start_transaction(struct bio *bio, struct bio_track *track)
  {
          bio->bio_track = track;
          if (dsched_is_clear_buf_priv(bio->bio_buf))
                  dsched_new_buf(bio->bio_buf);
          bio_track_ref(track);
  }
  
  /*
   * Initiate I/O on a vnode.
   *
   * SWAPCACHE OPERATION:
   *
   *      Real buffer cache buffers have a non-NULL bp->b_vp.  Unfortunately
   *      devfs also uses b_vp for fake buffers so we also have to check
   *      that B_PAGING is 0.  In this case the passed 'vp' is probably the
   *      underlying block device.  The swap assignments are related to the
   *      buffer cache buffer's b_vp, not the passed vp.
   *
   *      The passed vp == bp->b_vp only in the case where the strategy call
   *      is made on the vp itself for its own buffers (a regular file or
   *      block device vp).  The filesystem usually then re-calls vn_strategy()
   *      after translating the request to an underlying device.
   *
   *      Cluster buffers set B_CLUSTER and the passed vp is the vp of the
   *      underlying buffer cache buffers.
   *
   *      We can only deal with page-aligned buffers at the moment, because
   *      we can't tell what the real dirty state for pages straddling a buffer
   *      are.
   *
   *      In order to call swap_pager_strategy() we must provide the VM object
   *      and base offset for the underlying buffer cache pages so it can find
   *      the swap blocks.
   */
  void
  vn_strategy(struct vnode *vp, struct bio *bio)
  {
          struct bio_track *track;
          struct buf *bp = bio->bio_buf;
  
          KKASSERT(bp->b_cmd != BUF_CMD_DONE);
  
          /*
           * Set when an I/O is issued on the bp.  Cleared by consumers
           * (aka HAMMER), allowing the consumer to determine if I/O had
           * actually occurred.
           */
          bp->b_flags |= B_IODEBUG;
  
          /*
           * Handle the swap cache intercept.
           */
          if (vn_cache_strategy(vp, bio))
                  return;
  
          /*
           * Otherwise do the operation through the filesystem
           */
          if (bp->b_cmd == BUF_CMD_READ)
                  track = &vp->v_track_read;
          else
                  track = &vp->v_track_write;
          KKASSERT((bio->bio_flags & BIO_DONE) == 0);
          bio->bio_track = track;
          if (dsched_is_clear_buf_priv(bio->bio_buf))
                  dsched_new_buf(bio->bio_buf);
          bio_track_ref(track);
          vop_strategy(*vp->v_ops, vp, bio);
  }
  
  static void vn_cache_strategy_callback(struct bio *bio);
  
  int
  vn_cache_strategy(struct vnode *vp, struct bio *bio)
  {
          struct buf *bp = bio->bio_buf;
          struct bio *nbio;
          vm_object_t object;
          vm_page_t m;
          int i;
  
          /*
           * Is this buffer cache buffer suitable for reading from
           * the swap cache?
           */
          if (vm_swapcache_read_enable == 0 ||
              bp->b_cmd != BUF_CMD_READ ||
              ((bp->b_flags & B_CLUSTER) == 0 &&
               (bp->b_vp == NULL || (bp->b_flags & B_PAGING))) ||
              ((int)bp->b_loffset & PAGE_MASK) != 0 ||
              (bp->b_bcount & PAGE_MASK) != 0) {
                  return(0);
          }
  
          /*
           * Figure out the original VM object (it will match the underlying
           * VM pages).  Note that swap cached data uses page indices relative
           * to that object, not relative to bio->bio_offset.
           */
          if (bp->b_flags & B_CLUSTER)
                  object = vp->v_object;
          else
                  object = bp->b_vp->v_object;
  
          /*
           * In order to be able to use the swap cache all underlying VM
           * pages must be marked as such, and we can't have any bogus pages.
           */
          for (i = 0; i < bp->b_xio.xio_npages; ++i) {
                  m = bp->b_xio.xio_pages[i];
                  if ((m->flags & PG_SWAPPED) == 0)
                          break;
                  if (m == bogus_page)
                          break;
          }
  
          /*
           * If we are good then issue the I/O using swap_pager_strategy().
           *
           * We can only do this if the buffer actually supports object-backed
           * I/O.  If it doesn't npages will be 0.
           */
          if (i && i == bp->b_xio.xio_npages) {
                  m = bp->b_xio.xio_pages[0];
                  nbio = push_bio(bio);
                  nbio->bio_done = vn_cache_strategy_callback;
                  nbio->bio_offset = ptoa(m->pindex);
                  KKASSERT(m->object == object);
                  swap_pager_strategy(object, nbio);
                  return(1);
          }
          return(0);
  }
  
  /*
   * This is a bit of a hack but since the vn_cache_strategy() function can
   * override a VFS's strategy function we must make sure that the bio, which
   * is probably bio2, doesn't leak an unexpected offset value back to the
   * filesystem.  The filesystem (e.g. UFS) might otherwise assume that the
   * bio went through its own file strategy function and the the bio2 offset
   * is a cached disk offset when, in fact, it isn't.
   */
  static void
  vn_cache_strategy_callback(struct bio *bio)
  {
          bio->bio_offset = NOOFFSET;
          biodone(pop_bio(bio));
  }
  
  /*
   * bpdone:
   *
   *      Finish I/O on a buffer after all BIOs have been processed.
   *      Called when the bio chain is exhausted or by biowait.  If called
   *      by biowait, elseit is typically 0.
   *
   *      bpdone is also responsible for setting B_CACHE in a B_VMIO bp.
   *      In a non-VMIO bp, B_CACHE will be set on the next getblk() 
   *      assuming B_INVAL is clear.
   *
   *      For the VMIO case, we set B_CACHE if the op was a read and no
   *      read error occured, or if the op was a write.  B_CACHE is never
   *      set if the buffer is invalid or otherwise uncacheable.
   *
   *      bpdone does not mess with B_INVAL, allowing the I/O routine or the
   *      initiator to leave B_INVAL set to brelse the buffer out of existance
   *      in the biodone routine.
   */
  void
  bpdone(struct buf *bp, int elseit)
  {
          buf_cmd_t cmd;
  
          KASSERT(BUF_REFCNTNB(bp) > 0, 
                  ("biodone: bp %p not busy %d", bp, BUF_REFCNTNB(bp)));
          KASSERT(bp->b_cmd != BUF_CMD_DONE, 
                  ("biodone: bp %p already done!", bp));
  
          /*
           * No more BIOs are left.  All completion functions have been dealt
           * with, now we clean up the buffer.
           */
          cmd = bp->b_cmd;
          bp->b_cmd = BUF_CMD_DONE;
  
          /*
           * Only reads and writes are processed past this point.
           */
          if (cmd != BUF_CMD_READ && cmd != BUF_CMD_WRITE) {
                  if (cmd == BUF_CMD_FREEBLKS)
                          bp->b_flags |= B_NOCACHE;
                  if (elseit)
                          brelse(bp);
                  return;
          }
  
          /*
           * Warning: softupdates may re-dirty the buffer, and HAMMER can do
           * a lot worse.  XXX - move this above the clearing of b_cmd
           */
          if (LIST_FIRST(&bp->b_dep) != NULL)
                  buf_complete(bp);
  
          /*
           * A failed write must re-dirty the buffer unless B_INVAL
           * was set.  Only applicable to normal buffers (with VPs).
           * vinum buffers may not have a vp.
           */
          if (cmd == BUF_CMD_WRITE &&
              (bp->b_flags & (B_ERROR | B_INVAL)) == B_ERROR) {
                  bp->b_flags &= ~B_NOCACHE;
                  if (bp->b_vp)
                          bdirty(bp);
          }
  
          if (bp->b_flags & B_VMIO) {
                  int i;
                  vm_ooffset_t foff;
                  vm_page_t m;
                  vm_object_t obj;
                  int iosize;
                  struct vnode *vp = bp->b_vp;
  
                  obj = vp->v_object;
  
  #if defined(VFS_BIO_DEBUG)
                  if (vp->v_auxrefs == 0)
                          panic("biodone: zero vnode hold count");
                  if ((vp->v_flag & VOBJBUF) == 0)
                          panic("biodone: vnode is not setup for merged cache");
  #endif
  
                  foff = bp->b_loffset;
                  KASSERT(foff != NOOFFSET, ("biodone: no buffer offset"));
                  KASSERT(obj != NULL, ("biodone: missing VM object"));
  
  #if defined(VFS_BIO_DEBUG)
                  if (obj->paging_in_progress < bp->b_xio.xio_npages) {
                          kprintf("biodone: paging in progress(%d) < "
                                  "bp->b_xio.xio_npages(%d)\n",
                                  obj->paging_in_progress,
                                  bp->b_xio.xio_npages);
                  }
  #endif
  
                  /*
                   * Set B_CACHE if the op was a normal read and no error
                   * occured.  B_CACHE is set for writes in the b*write()
                   * routines.
                   */
                  iosize = bp->b_bcount - bp->b_resid;
                  if (cmd == BUF_CMD_READ &&
                      (bp->b_flags & (B_INVAL|B_NOCACHE|B_ERROR)) == 0) {
                          bp->b_flags |= B_CACHE;
                  }
  
                  vm_object_hold(obj);
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          int bogusflag = 0;
                          int resid;
  
                          resid = ((foff + PAGE_SIZE) & ~(off_t)PAGE_MASK) - foff;
                          if (resid > iosize)
                                  resid = iosize;
  
                          /*
                           * cleanup bogus pages, restoring the originals.  Since
                           * the originals should still be wired, we don't have
                           * to worry about interrupt/freeing races destroying
                           * the VM object association.
                           */
                          m = bp->b_xio.xio_pages[i];
                          if (m == bogus_page) {
                                  bogusflag = 1;
                                  m = vm_page_lookup(obj, OFF_TO_IDX(foff));
                                  if (m == NULL)
                                          panic("biodone: page disappeared");
                                  bp->b_xio.xio_pages[i] = m;
                                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                          bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                          }
  #if defined(VFS_BIO_DEBUG)
                          if (OFF_TO_IDX(foff) != m->pindex) {
                                  kprintf("biodone: foff(%lu)/m->pindex(%ld) "
                                          "mismatch\n",
                                          (unsigned long)foff, (long)m->pindex);
                          }
  #endif
  
                          /*
                           * In the write case, the valid and clean bits are
                           * already changed correctly (see bdwrite()), so we
                           * only need to do this here in the read case.
                           */
                          vm_page_busy_wait(m, FALSE, "bpdpgw");
                          if (cmd == BUF_CMD_READ && !bogusflag && resid > 0) {
                                  vfs_clean_one_page(bp, i, m);
                          }
                          vm_page_flag_clear(m, PG_ZERO);
  
                          /*
                           * when debugging new filesystems or buffer I/O
                           * methods, this is the most common error that pops
                           * up.  if you see this, you have not set the page
                           * busy flag correctly!!!
                           */
                          if (m->busy == 0) {
                                  kprintf("biodone: page busy < 0, "
                                      "pindex: %d, foff: 0x(%x,%x), "
                                      "resid: %d, index: %d\n",
                                      (int) m->pindex, (int)(foff >> 32),
                                                  (int) foff & 0xffffffff, resid, i);
                                  if (!vn_isdisk(vp, NULL))
                                          kprintf(" iosize: %ld, loffset: %lld, "
                                                  "flags: 0x%08x, npages: %d\n",
                                              bp->b_vp->v_mount->mnt_stat.f_iosize,
                                              (long long)bp->b_loffset,
                                              bp->b_flags, bp->b_xio.xio_npages);
                                  else
                                          kprintf(" VDEV, loffset: %lld, flags: 0x%08x, npages: %d\n",
                                              (long long)bp->b_loffset,
                                              bp->b_flags, bp->b_xio.xio_npages);
                                  kprintf(" valid: 0x%x, dirty: 0x%x, "
                                          "wired: %d\n",
                                          m->valid, m->dirty,
                                          m->wire_count);
                                  panic("biodone: page busy < 0");
                          }
                          vm_page_io_finish(m);
                          vm_page_wakeup(m);
                          vm_object_pip_wakeup(obj);
                          foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                          iosize -= resid;
                  }
                  bp->b_flags &= ~B_HASBOGUS;
                  vm_object_drop(obj);
          }
  
          /*
           * Finish up by releasing the buffer.  There are no more synchronous
           * or asynchronous completions, those were handled by bio_done
           * callbacks.
           */
          if (elseit) {
                  if (bp->b_flags & (B_NOCACHE|B_INVAL|B_ERROR|B_RELBUF))
                          brelse(bp);
                  else
                          bqrelse(bp);
          }
  }
  
  /*
   * Normal biodone.
   */
  void
  biodone(struct bio *bio)
  {
          struct buf *bp = bio->bio_buf;
  
          runningbufwakeup(bp);
  
          /*
           * Run up the chain of BIO's.   Leave b_cmd intact for the duration.
           */
          while (bio) {
                  biodone_t *done_func;
                  struct bio_track *track;
  
                  /*
                   * BIO tracking.  Most but not all BIOs are tracked.
                   */
                  if ((track = bio->bio_track) != NULL) {
                          bio_track_rel(track);
                          bio->bio_track = NULL;
                  }
  
                  /*
                   * A bio_done function terminates the loop.  The function
                   * will be responsible for any further chaining and/or
                   * buffer management.
                   *
                   * WARNING!  The done function can deallocate the buffer!
                   */
                  if ((done_func = bio->bio_done) != NULL) {
                          bio->bio_done = NULL;
                          done_func(bio);
                          return;
                  }
                  bio = bio->bio_prev;
          }
  
          /*
           * If we've run out of bio's do normal [a]synchronous completion.
           */
          bpdone(bp, 1);
  }
  
  /*
   * Synchronous biodone - this terminates a synchronous BIO.
   *
   * bpdone() is called with elseit=FALSE, leaving the buffer completed
   * but still locked.  The caller must brelse() the buffer after waiting
   * for completion.
   */
  void
  biodone_sync(struct bio *bio)
  {
          struct buf *bp = bio->bio_buf;
          int flags;
          int nflags;
  
          KKASSERT(bio == &bp->b_bio1);
          bpdone(bp, 0);
  
          for (;;) {
                  flags = bio->bio_flags;
                  nflags = (flags | BIO_DONE) & ~BIO_WANT;
  
                  if (atomic_cmpset_int(&bio->bio_flags, flags, nflags)) {
                          if (flags & BIO_WANT)
                                  wakeup(bio);
                          break;
                  }
          }
  }
  
  /*
   * vfs_unbusy_pages:
   *
   *      This routine is called in lieu of iodone in the case of
   *      incomplete I/O.  This keeps the busy status for pages
   *      consistant.
   */
  void
  vfs_unbusy_pages(struct buf *bp)
  {
          int i;
  
          runningbufwakeup(bp);
  
          if (bp->b_flags & B_VMIO) {
                  struct vnode *vp = bp->b_vp;
                  vm_object_t obj;
  
                  obj = vp->v_object;
                  vm_object_hold(obj);
  
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          vm_page_t m = bp->b_xio.xio_pages[i];
  
                          /*
                           * When restoring bogus changes the original pages
                           * should still be wired, so we are in no danger of
                           * losing the object association and do not need
                           * critical section protection particularly.
                           */
                          if (m == bogus_page) {
                                  m = vm_page_lookup(obj, OFF_TO_IDX(bp->b_loffset) + i);
                                  if (!m) {
                                          panic("vfs_unbusy_pages: page missing");
                                  }
                                  bp->b_xio.xio_pages[i] = m;
                                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                          bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                          }
                          vm_page_busy_wait(m, FALSE, "bpdpgw");
                          vm_page_flag_clear(m, PG_ZERO);
                          vm_page_io_finish(m);
                          vm_page_wakeup(m);
                          vm_object_pip_wakeup(obj);
                  }
                  bp->b_flags &= ~B_HASBOGUS;
                  vm_object_drop(obj);
          }
  }
  
  /*
   * vfs_busy_pages:
   *
   *      This routine is called before a device strategy routine.
   *      It is used to tell the VM system that paging I/O is in
   *      progress, and treat the pages associated with the buffer
   *      almost as being PG_BUSY.  Also the object 'paging_in_progress'
   *      flag is handled to make sure that the object doesn't become
   *      inconsistant.
   *
   *      Since I/O has not been initiated yet, certain buffer flags
   *      such as B_ERROR or B_INVAL may be in an inconsistant state
   *      and should be ignored.
   */
  void
  vfs_busy_pages(struct vnode *vp, struct buf *bp)
  {
          int i, bogus;
          struct lwp *lp = curthread->td_lwp;
  
          /*
           * The buffer's I/O command must already be set.  If reading,
           * B_CACHE must be 0 (double check against callers only doing
           * I/O when B_CACHE is 0).
           */
          KKASSERT(bp->b_cmd != BUF_CMD_DONE);
          KKASSERT(bp->b_cmd == BUF_CMD_WRITE || (bp->b_flags & B_CACHE) == 0);
  
          if (bp->b_flags & B_VMIO) {
                  vm_object_t obj;
  
                  obj = vp->v_object;
                  KASSERT(bp->b_loffset != NOOFFSET,
                          ("vfs_busy_pages: no buffer offset"));
  
                  /*
                   * Busy all the pages.  We have to busy them all at once
                   * to avoid deadlocks.
                   */
  retry:
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          vm_page_t m = bp->b_xio.xio_pages[i];
  
                          if (vm_page_busy_try(m, FALSE)) {
                                  vm_page_sleep_busy(m, FALSE, "vbpage");
                                  while (--i >= 0)
                                          vm_page_wakeup(bp->b_xio.xio_pages[i]);
                                  goto retry;
                          }
                  }
  
                  /*
                   * Setup for I/O, soft-busy the page right now because
                   * the next loop may block.
                   */
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          vm_page_t m = bp->b_xio.xio_pages[i];
  
                          vm_page_flag_clear(m, PG_ZERO);
                          if ((bp->b_flags & B_CLUSTER) == 0) {
                                  vm_object_pip_add(obj, 1);
                                  vm_page_io_start(m);
                          }
                  }
  
                  /*
                   * Adjust protections for I/O and do bogus-page mapping.
                   * Assume that vm_page_protect() can block (it can block
                   * if VM_PROT_NONE, don't take any chances regardless).
                   *
                   * In particular note that for writes we must incorporate
                   * page dirtyness from the VM system into the buffer's
                   * dirty range.
                   *
                   * For reads we theoretically must incorporate page dirtyness
                   * from the VM system to determine if the page needs bogus
                   * replacement, but we shortcut the test by simply checking
                   * that all m->valid bits are set, indicating that the page
                   * is fully valid and does not need to be re-read.  For any
                   * VM system dirtyness the page will also be fully valid
                   * since it was mapped at one point.
                   */
                  bogus = 0;
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          vm_page_t m = bp->b_xio.xio_pages[i];
  
                          vm_page_flag_clear(m, PG_ZERO); /* XXX */
                          if (bp->b_cmd == BUF_CMD_WRITE) {
                                  /*
                                   * When readying a vnode-backed buffer for
                                   * a write we must zero-fill any invalid
                                   * portions of the backing VM pages, mark
                                   * it valid and clear related dirty bits.
                                   *
                                   * vfs_clean_one_page() incorporates any
                                   * VM dirtyness and updates the b_dirtyoff
                                   * range (after we've made the page RO).
                                   *
                                   * It is also expected that the pmap modified
                                   * bit has already been cleared by the
                                   * vm_page_protect().  We may not be able
                                   * to clear all dirty bits for a page if it
                                   * was also memory mapped (NFS).
                                   *
                                   * Finally be sure to unassign any swap-cache
                                   * backing store as it is now stale.
                                   */
                                  vm_page_protect(m, VM_PROT_READ);
                                  vfs_clean_one_page(bp, i, m);
                                  swap_pager_unswapped(m);
                          } else if (m->valid == VM_PAGE_BITS_ALL) {
                                  /*
                                   * When readying a vnode-backed buffer for
                                   * read we must replace any dirty pages with
                                   * a bogus page so dirty data is not destroyed
                                   * when filling gaps.
                                   *
                                   * To avoid testing whether the page is
                                   * dirty we instead test that the page was
                                   * at some point mapped (m->valid fully
                                   * valid) with the understanding that
                                   * this also covers the dirty case.
                                   */
                                  bp->b_xio.xio_pages[i] = bogus_page;
                                  bp->b_flags |= B_HASBOGUS;
                                  bogus++;
                          } else if (m->valid & m->dirty) {
                                  /*
                                   * This case should not occur as partial
                                   * dirtyment can only happen if the buffer
                                   * is B_CACHE, and this code is not entered
                                   * if the buffer is B_CACHE.
                                   */
                                  kprintf("Warning: vfs_busy_pages - page not "
                                          "fully valid! loff=%jx bpf=%08x "
                                          "idx=%d val=%02x dir=%02x\n",
                                          (uintmax_t)bp->b_loffset, bp->b_flags,
                                          i, m->valid, m->dirty);
                                  vm_page_protect(m, VM_PROT_NONE);
                          } else {
                                  /*
                                   * The page is not valid and can be made
                                   * part of the read.
                                   */
                                  vm_page_protect(m, VM_PROT_NONE);
                          }
                          vm_page_wakeup(m);
                  }
                  if (bogus) {
                          pmap_qenter(trunc_page((vm_offset_t)bp->b_data),
                                  bp->b_xio.xio_pages, bp->b_xio.xio_npages);
                  }
          }
  
          /*
           * This is the easiest place to put the process accounting for the I/O
           * for now.
           */
          if (lp != NULL) {
                  if (bp->b_cmd == BUF_CMD_READ)
                          lp->lwp_ru.ru_inblock++;
                  else
                          lp->lwp_ru.ru_oublock++;
          }
  }
  
  /*
   * Tell the VM system that the pages associated with this buffer
   * are clean.  This is used for delayed writes where the data is
   * going to go to disk eventually without additional VM intevention.
   *
   * NOTE: While we only really need to clean through to b_bcount, we
   *       just go ahead and clean through to b_bufsize.
   */
  static void
  vfs_clean_pages(struct buf *bp)
  {
          vm_page_t m;
          int i;
  
          if ((bp->b_flags & B_VMIO) == 0)
                  return;
  
          KASSERT(bp->b_loffset != NOOFFSET,
                  ("vfs_clean_pages: no buffer offset"));
  
          for (i = 0; i < bp->b_xio.xio_npages; i++) {
                  m = bp->b_xio.xio_pages[i];
                  vfs_clean_one_page(bp, i, m);
          }
  }
  
  /*
   * vfs_clean_one_page:
   *
   *      Set the valid bits and clear the dirty bits in a page within a
   *      buffer.  The range is restricted to the buffer's size and the
   *      buffer's logical offset might index into the first page.
   *
   *      The caller has busied or soft-busied the page and it is not mapped,
   *      test and incorporate the dirty bits into b_dirtyoff/end before
   *      clearing them.  Note that we need to clear the pmap modified bits
   *      after determining the the page was dirty, vm_page_set_validclean()
   *      does not do it for us.
   *
   *      This routine is typically called after a read completes (dirty should
   *      be zero in that case as we are not called on bogus-replace pages),
   *      or before a write is initiated.
   */
  static void
  vfs_clean_one_page(struct buf *bp, int pageno, vm_page_t m)
  {
          int bcount;
          int xoff;
          int soff;
          int eoff;
  
          /*
           * Calculate offset range within the page but relative to buffer's
           * loffset.  loffset might be offset into the first page.
           */
          xoff = (int)bp->b_loffset & PAGE_MASK;  /* loffset offset into pg 0 */
          bcount = bp->b_bcount + xoff;           /* offset adjusted */
  
          if (pageno == 0) {
                  soff = xoff;
                  eoff = PAGE_SIZE;
          } else {
                  soff = (pageno << PAGE_SHIFT);
                  eoff = soff + PAGE_SIZE;
          }
          if (eoff > bcount)
                  eoff = bcount;
          if (soff >= eoff)
                  return;
  
          /*
           * Test dirty bits and adjust b_dirtyoff/end.
           *
           * If dirty pages are incorporated into the bp any prior
           * B_NEEDCOMMIT state (NFS) must be cleared because the
           * caller has not taken into account the new dirty data.
           *
           * If the page was memory mapped the dirty bits might go beyond the
           * end of the buffer, but we can't really make the assumption that
           * a file EOF straddles the buffer (even though this is the case for
           * NFS if B_NEEDCOMMIT is also set).  So for the purposes of clearing
           * B_NEEDCOMMIT we only test the dirty bits covered by the buffer.
           * This also saves some console spam.
           *
           * When clearing B_NEEDCOMMIT we must also clear B_CLUSTEROK,
           * NFS can handle huge commits but not huge writes.
           */
          vm_page_test_dirty(m);
          if (m->dirty) {
                  if ((bp->b_flags & B_NEEDCOMMIT) &&
                      (m->dirty & vm_page_bits(soff & PAGE_MASK, eoff - soff))) {
                          if (debug_commit)
                                  kprintf("Warning: vfs_clean_one_page: bp %p "
                                      "loff=%jx,%d flgs=%08x clr B_NEEDCOMMIT"
                                      " cmd %d vd %02x/%02x x/s/e %d %d %d "
                                      "doff/end %d %d\n",
                                      bp, (uintmax_t)bp->b_loffset, bp->b_bcount,
                                      bp->b_flags, bp->b_cmd,
                                      m->valid, m->dirty, xoff, soff, eoff,
                                      bp->b_dirtyoff, bp->b_dirtyend);
                          bp->b_flags &= ~(B_NEEDCOMMIT | B_CLUSTEROK);
                          if (debug_commit)
                                  print_backtrace(-1);
                  }
                  /*
                   * Only clear the pmap modified bits if ALL the dirty bits
                   * are set, otherwise the system might mis-clear portions
                   * of a page.
                   */
                  if (m->dirty == VM_PAGE_BITS_ALL &&
                      (bp->b_flags & B_NEEDCOMMIT) == 0) {
                          pmap_clear_modify(m);
                  }
                  if (bp->b_dirtyoff > soff - xoff)
                          bp->b_dirtyoff = soff - xoff;
                  if (bp->b_dirtyend < eoff - xoff)
                          bp->b_dirtyend = eoff - xoff;
          }
  
          /*
           * Set related valid bits, clear related dirty bits.
           * Does not mess with the pmap modified bit.
           *
           * WARNING!  We cannot just clear all of m->dirty here as the
           *           buffer cache buffers may use a DEV_BSIZE'd aligned
           *           block size, or have an odd size (e.g. NFS at file EOF).
           *           The putpages code can clear m->dirty to 0.
           *
           *           If a VOP_WRITE generates a buffer cache buffer which
           *           covers the same space as mapped writable pages the
           *           buffer flush might not be able to clear all the dirty
           *           bits and still require a putpages from the VM system
           *           to finish it off.
           *
           * WARNING!  vm_page_set_validclean() currently assumes vm_token
           *           is held.  The page might not be busied (bdwrite() case).
           *           XXX remove this comment once we've validated that this
           *           is no longer an issue.
           */
          vm_page_set_validclean(m, soff & PAGE_MASK, eoff - soff);
  }
  
  #if 0
  /*
   * Similar to vfs_clean_one_page() but sets the bits to valid and dirty.
   * The page data is assumed to be valid (there is no zeroing here).
   */
  static void
  vfs_dirty_one_page(struct buf *bp, int pageno, vm_page_t m)
  {
          int bcount;
          int xoff;
          int soff;
          int eoff;
  
          /*
           * Calculate offset range within the page but relative to buffer's
           * loffset.  loffset might be offset into the first page.
           */
          xoff = (int)bp->b_loffset & PAGE_MASK;  /* loffset offset into pg 0 */
          bcount = bp->b_bcount + xoff;           /* offset adjusted */
  
          if (pageno == 0) {
                  soff = xoff;
                  eoff = PAGE_SIZE;
          } else {
                  soff = (pageno << PAGE_SHIFT);
                  eoff = soff + PAGE_SIZE;
          }
          if (eoff > bcount)
                  eoff = bcount;
          if (soff >= eoff)
                  return;
          vm_page_set_validdirty(m, soff & PAGE_MASK, eoff - soff);
  }
  #endif
  
  /*
   * vfs_bio_clrbuf:
   *
   *      Clear a buffer.  This routine essentially fakes an I/O, so we need
   *      to clear B_ERROR and B_INVAL.
   *
   *      Note that while we only theoretically need to clear through b_bcount,
   *      we go ahead and clear through b_bufsize.
   */
  
  void
  vfs_bio_clrbuf(struct buf *bp)
  {
          int i, mask = 0;
          caddr_t sa, ea;
          if ((bp->b_flags & (B_VMIO | B_MALLOC)) == B_VMIO) {
                  bp->b_flags &= ~(B_INVAL | B_EINTR | B_ERROR);
                  if ((bp->b_xio.xio_npages == 1) && (bp->b_bufsize < PAGE_SIZE) &&
                      (bp->b_loffset & PAGE_MASK) == 0) {
                          mask = (1 << (bp->b_bufsize / DEV_BSIZE)) - 1;
                          if ((bp->b_xio.xio_pages[0]->valid & mask) == mask) {
                                  bp->b_resid = 0;
                                  return;
                          }
                          if (((bp->b_xio.xio_pages[0]->flags & PG_ZERO) == 0) &&
                              ((bp->b_xio.xio_pages[0]->valid & mask) == 0)) {
                                  bzero(bp->b_data, bp->b_bufsize);
                                  bp->b_xio.xio_pages[0]->valid |= mask;
                                  bp->b_resid = 0;
                                  return;
                          }
                  }
                  sa = bp->b_data;
                  for(i=0;i<bp->b_xio.xio_npages;i++,sa=ea) {
                          int j = ((vm_offset_t)sa & PAGE_MASK) / DEV_BSIZE;
                          ea = (caddr_t)trunc_page((vm_offset_t)sa + PAGE_SIZE);
                          ea = (caddr_t)(vm_offset_t)ulmin(
                              (u_long)(vm_offset_t)ea,
                              (u_long)(vm_offset_t)bp->b_data + bp->b_bufsize);
                          mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                          if ((bp->b_xio.xio_pages[i]->valid & mask) == mask)
                                  continue;
                          if ((bp->b_xio.xio_pages[i]->valid & mask) == 0) {
                                  if ((bp->b_xio.xio_pages[i]->flags & PG_ZERO) == 0) {
                                          bzero(sa, ea - sa);
                                  }
                          } else {
                                  for (; sa < ea; sa += DEV_BSIZE, j++) {
                                          if (((bp->b_xio.xio_pages[i]->flags & PG_ZERO) == 0) &&
                                                  (bp->b_xio.xio_pages[i]->valid & (1<<j)) == 0)
                                                  bzero(sa, DEV_BSIZE);
                                  }
                          }
                          bp->b_xio.xio_pages[i]->valid |= mask;
                          vm_page_flag_clear(bp->b_xio.xio_pages[i], PG_ZERO);
                  }
                  bp->b_resid = 0;
          } else {
                  clrbuf(bp);
          }
  }
  
  /*
   * vm_hold_load_pages:
   *
   *      Load pages into the buffer's address space.  The pages are
   *      allocated from the kernel object in order to reduce interference
   *      with the any VM paging I/O activity.  The range of loaded
   *      pages will be wired.
   *
   *      If a page cannot be allocated, the 'pagedaemon' is woken up to
   *      retrieve the full range (to - from) of pages.
   */
  void
  vm_hold_load_pages(struct buf *bp, vm_offset_t from, vm_offset_t to)
  {
          vm_offset_t pg;
          vm_page_t p;
          int index;
  
          to = round_page(to);
          from = round_page(from);
          index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
  
          pg = from;
          while (pg < to) {
                  /*
                   * Note: must allocate system pages since blocking here
                   * could intefere with paging I/O, no matter which
                   * process we are.
                   */
                  vm_object_hold(&kernel_object);
                  p = bio_page_alloc(bp, &kernel_object, pg >> PAGE_SHIFT,
                                     (vm_pindex_t)((to - pg) >> PAGE_SHIFT));
                  vm_object_drop(&kernel_object);
                  if (p) {
                          vm_page_wire(p);
                          p->valid = VM_PAGE_BITS_ALL;
                          vm_page_flag_clear(p, PG_ZERO);
                          pmap_kenter(pg, VM_PAGE_TO_PHYS(p));
                          bp->b_xio.xio_pages[index] = p;
                          vm_page_wakeup(p);
  
                          pg += PAGE_SIZE;
                          ++index;
                  }
          }
          bp->b_xio.xio_npages = index;
  }
  
  /*
   * Allocate a page for a buffer cache buffer.
   *
   * If NULL is returned the caller is expected to retry (typically check if
   * the page already exists on retry before trying to allocate one).
   *
   * NOTE! Low-memory handling is dealt with in b[q]relse(), not here.  This
   *       function will use the system reserve with the hope that the page
   *       allocations can be returned to PQ_CACHE/PQ_FREE when the caller
   *       is done with the buffer.
   *
   * NOTE! However, TMPFS is a special case because flushing a dirty buffer
   *       to TMPFS doesn't clean the page.  For TMPFS, only the pagedaemon
   *       is capable of retiring pages (to swap).  For TMPFS we don't dig
   *       into the system reserve because doing so could stall out pretty
   *       much every process running on the system.
   */
  static
  vm_page_t
  bio_page_alloc(struct buf *bp, vm_object_t obj, vm_pindex_t pg, int deficit)
  {
          int vmflags = VM_ALLOC_NORMAL | VM_ALLOC_NULL_OK;
          vm_page_t p;
  
          ASSERT_LWKT_TOKEN_HELD(vm_object_token(obj));
  
          /*
           * Try a normal allocation first.
           */
          p = vm_page_alloc(obj, pg, vmflags);
          if (p)
                  return(p);
          if (vm_page_lookup(obj, pg))
                  return(NULL);
          vm_pageout_deficit += deficit;
  
          /*
           * Try again, digging into the system reserve.
           *
           * Trying to recover pages from the buffer cache here can deadlock
           * against other threads trying to busy underlying pages so we
           * depend on the code in brelse() and bqrelse() to free/cache the
           * underlying buffer cache pages when memory is low.
           */
          if (curthread->td_flags & TDF_SYSTHREAD)
                  vmflags |= VM_ALLOC_SYSTEM | VM_ALLOC_INTERRUPT;
          else if (bp->b_vp && bp->b_vp->v_tag == VT_TMPFS)
                  vmflags |= 0;
          else
                  vmflags |= VM_ALLOC_SYSTEM;
  
          /*recoverbufpages();*/
          p = vm_page_alloc(obj, pg, vmflags);
          if (p)
                  return(p);
          if (vm_page_lookup(obj, pg))
                  return(NULL);
  
          /*
           * Wait for memory to free up and try again
           */
          if (vm_page_count_severe())
                  ++lowmempgallocs;
          vm_wait(hz / 20 + 1);
  
          p = vm_page_alloc(obj, pg, vmflags);
          if (p)
                  return(p);
          if (vm_page_lookup(obj, pg))
                  return(NULL);
  
          /*
           * Ok, now we are really in trouble.
           */
          {
                  static struct krate biokrate = { .freq = 1 };
                  krateprintf(&biokrate,
                              "Warning: bio_page_alloc: memory exhausted "
                              "during bufcache page allocation from %s\n",
                              curthread->td_comm);
          }
          if (curthread->td_flags & TDF_SYSTHREAD)
                  vm_wait(hz / 20 + 1);
          else
                  vm_wait(hz / 2 + 1);
          return (NULL);
  }
  
  /*
   * vm_hold_free_pages:
   *
   *      Return pages associated with the buffer back to the VM system.
   *
   *      The range of pages underlying the buffer's address space will
   *      be unmapped and un-wired.
   */
  void
  vm_hold_free_pages(struct buf *bp, vm_offset_t from, vm_offset_t to)
  {
          vm_offset_t pg;
          vm_page_t p;
          int index, newnpages;
  
          from = round_page(from);
          to = round_page(to);
          index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
          newnpages = index;
  
          for (pg = from; pg < to; pg += PAGE_SIZE, index++) {
                  p = bp->b_xio.xio_pages[index];
                  if (p && (index < bp->b_xio.xio_npages)) {
                          if (p->busy) {
                                  kprintf("vm_hold_free_pages: doffset: %lld, "
                                          "loffset: %lld\n",
                                          (long long)bp->b_bio2.bio_offset,
                                          (long long)bp->b_loffset);
                          }
                          bp->b_xio.xio_pages[index] = NULL;
                          pmap_kremove(pg);
                          vm_page_busy_wait(p, FALSE, "vmhldpg");
                          vm_page_unwire(p, 0);
                          vm_page_free(p);
                  }
          }
          bp->b_xio.xio_npages = newnpages;
  }
  
  /*
   * vmapbuf:
   *
   *      Map a user buffer into KVM via a pbuf.  On return the buffer's
   *      b_data, b_bufsize, and b_bcount will be set, and its XIO page array
   *      initialized.
   */
  int
  vmapbuf(struct buf *bp, caddr_t udata, int bytes)
  {
          caddr_t addr;
          vm_offset_t va;
          vm_page_t m;
          int vmprot;
          int error;
          int pidx;
          int i;
  
          /* 
           * bp had better have a command and it better be a pbuf.
           */
          KKASSERT(bp->b_cmd != BUF_CMD_DONE);
          KKASSERT(bp->b_flags & B_PAGING);
          KKASSERT(bp->b_kvabase);
  
          if (bytes < 0)
                  return (-1);
  
          /*
           * Map the user data into KVM.  Mappings have to be page-aligned.
           */
          addr = (caddr_t)trunc_page((vm_offset_t)udata);
          pidx = 0;
  
          vmprot = VM_PROT_READ;
          if (bp->b_cmd == BUF_CMD_READ)
                  vmprot |= VM_PROT_WRITE;
  
          while (addr < udata + bytes) {
                  /*
                   * Do the vm_fault if needed; do the copy-on-write thing
                   * when reading stuff off device into memory.
                   *
                   * vm_fault_page*() returns a held VM page.
                   */
                  va = (addr >= udata) ? (vm_offset_t)addr : (vm_offset_t)udata;
                  va = trunc_page(va);
  
                  m = vm_fault_page_quick(va, vmprot, &error);
                  if (m == NULL) {
                          for (i = 0; i < pidx; ++i) {
                              vm_page_unhold(bp->b_xio.xio_pages[i]);
                              bp->b_xio.xio_pages[i] = NULL;
                          }
                          return(-1);
                  }
                  bp->b_xio.xio_pages[pidx] = m;
                  addr += PAGE_SIZE;
                  ++pidx;
          }
  
          /*
           * Map the page array and set the buffer fields to point to
           * the mapped data buffer.
           */
          if (pidx > btoc(MAXPHYS))
                  panic("vmapbuf: mapped more than MAXPHYS");
          pmap_qenter((vm_offset_t)bp->b_kvabase, bp->b_xio.xio_pages, pidx);
  
          bp->b_xio.xio_npages = pidx;
          bp->b_data = bp->b_kvabase + ((int)(intptr_t)udata & PAGE_MASK);
          bp->b_bcount = bytes;
          bp->b_bufsize = bytes;
          return(0);
  }
  
  /*
   * vunmapbuf:
   *
   *      Free the io map PTEs associated with this IO operation.
   *      We also invalidate the TLB entries and restore the original b_addr.
   */
  void
  vunmapbuf(struct buf *bp)
  {
          int pidx;
          int npages;
  
          KKASSERT(bp->b_flags & B_PAGING);
  
          npages = bp->b_xio.xio_npages;
          pmap_qremove(trunc_page((vm_offset_t)bp->b_data), npages);
          for (pidx = 0; pidx < npages; ++pidx) {
                  vm_page_unhold(bp->b_xio.xio_pages[pidx]);
                  bp->b_xio.xio_pages[pidx] = NULL;
          }
          bp->b_xio.xio_npages = 0;
          bp->b_data = bp->b_kvabase;
  }
  
  /*
   * Scan all buffers in the system and issue the callback.
   */
  int
  scan_all_buffers(int (*callback)(struct buf *, void *), void *info)
  {
          int count = 0;
          int error;
          long n;
  
          for (n = 0; n < nbuf; ++n) {
                  if ((error = callback(&buf[n], info)) < 0) {
                          count = error;
                          break;
                  }
                  count += error;
          }
          return (count);
  }
  
  /*
   * nestiobuf_iodone: biodone callback for nested buffers and propagate
   * completion to the master buffer.
   */
  static void
  nestiobuf_iodone(struct bio *bio)
  {
          struct bio *mbio;
          struct buf *mbp, *bp;
          struct devstat *stats;
          int error;
          int donebytes;
  
          bp = bio->bio_buf;
          mbio = bio->bio_caller_info1.ptr;
          stats = bio->bio_caller_info2.ptr;
          mbp = mbio->bio_buf;
  
          KKASSERT(bp->b_bcount <= bp->b_bufsize);
          KKASSERT(mbp != bp);
  
          error = bp->b_error;
          if (bp->b_error == 0 &&
              (bp->b_bcount < bp->b_bufsize || bp->b_resid > 0)) {
                  /*
                   * Not all got transfered, raise an error. We have no way to
                   * propagate these conditions to mbp.
                   */
                  error = EIO;
          }
  
          donebytes = bp->b_bufsize;
  
          relpbuf(bp, NULL);
  
          nestiobuf_done(mbio, donebytes, error, stats);
  }
  
  void
  nestiobuf_done(struct bio *mbio, int donebytes, int error, struct devstat *stats)
  {
          struct buf *mbp;
  
          mbp = mbio->bio_buf;    
  
          KKASSERT((int)(intptr_t)mbio->bio_driver_info > 0);
  
          /*
           * If an error occured, propagate it to the master buffer.
           *
           * Several biodone()s may wind up running concurrently so
           * use an atomic op to adjust b_flags.
           */
          if (error) {
                  mbp->b_error = error;
                  atomic_set_int(&mbp->b_flags, B_ERROR);
          }
  
          /*
           * Decrement the operations in progress counter and terminate the
           * I/O if this was the last bit.
           */
          if (atomic_fetchadd_int((int *)&mbio->bio_driver_info, -1) == 1) {
                  mbp->b_resid = 0;
                  if (stats)
                          devstat_end_transaction_buf(stats, mbp);
                  biodone(mbio);
          }
  }
  
  /*
   * Initialize a nestiobuf for use.  Set an initial count of 1 to prevent
   * the mbio from being biodone()'d while we are still adding sub-bios to
   * it.
   */
  void
  nestiobuf_init(struct bio *bio)
  {
          bio->bio_driver_info = (void *)1;
  }
  
  /*
   * The BIOs added to the nestedio have already been started, remove the
   * count that placeheld our mbio and biodone() it if the count would
   * transition to 0.
   */
  void
  nestiobuf_start(struct bio *mbio)
  {
          struct buf *mbp = mbio->bio_buf;
  
          /*
           * Decrement the operations in progress counter and terminate the
           * I/O if this was the last bit.
           */
          if (atomic_fetchadd_int((int *)&mbio->bio_driver_info, -1) == 1) {
                  if (mbp->b_flags & B_ERROR)
                          mbp->b_resid = mbp->b_bcount;
                  else
                          mbp->b_resid = 0;
                  biodone(mbio);
          }
  }
  
  /*
   * Set an intermediate error prior to calling nestiobuf_start()
   */
  void
  nestiobuf_error(struct bio *mbio, int error)
  {
          struct buf *mbp = mbio->bio_buf;
  
          if (error) {
                  mbp->b_error = error;
                  atomic_set_int(&mbp->b_flags, B_ERROR);
          }
  }
  
  /*
   * nestiobuf_add: setup a "nested" buffer.
   *
   * => 'mbp' is a "master" buffer which is being divided into sub pieces.
   * => 'bp' should be a buffer allocated by getiobuf.
   * => 'offset' is a byte offset in the master buffer.
   * => 'size' is a size in bytes of this nested buffer.
   */
  void
  nestiobuf_add(struct bio *mbio, struct buf *bp, int offset, size_t size, struct devstat *stats)
  {
          struct buf *mbp = mbio->bio_buf;
          struct vnode *vp = mbp->b_vp;
  
          KKASSERT(mbp->b_bcount >= offset + size);
  
          atomic_add_int((int *)&mbio->bio_driver_info, 1);
  
          /* kernel needs to own the lock for it to be released in biodone */
          BUF_KERNPROC(bp);
          bp->b_vp = vp;
          bp->b_cmd = mbp->b_cmd;
          bp->b_bio1.bio_done = nestiobuf_iodone;
          bp->b_data = (char *)mbp->b_data + offset;
          bp->b_resid = bp->b_bcount = size;
          bp->b_bufsize = bp->b_bcount;
  
          bp->b_bio1.bio_track = NULL;
          bp->b_bio1.bio_caller_info1.ptr = mbio;
          bp->b_bio1.bio_caller_info2.ptr = stats;
  }
  
  #ifdef DDB
  
  DB_SHOW_COMMAND(buffer, db_show_buffer)
  {
          /* get args */
          struct buf *bp = (struct buf *)addr;
  
          if (!have_addr) {
                  db_printf("usage: show buffer <addr>\n");
                  return;
          }
  
          db_printf("b_flags = 0x%b\n", (u_int)bp->b_flags, PRINT_BUF_FLAGS);
          db_printf("b_cmd = %d\n", bp->b_cmd);
          db_printf("b_error = %d, b_bufsize = %d, b_bcount = %d, "
                    "b_resid = %d\n, b_data = %p, "
                    "bio_offset(disk) = %lld, bio_offset(phys) = %lld\n",
                    bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
                    bp->b_data,
                    (long long)bp->b_bio2.bio_offset,
                    (long long)(bp->b_bio2.bio_next ?
                                  bp->b_bio2.bio_next->bio_offset : (off_t)-1));
          if (bp->b_xio.xio_npages) {
                  int i;
                  db_printf("b_xio.xio_npages = %d, pages(OBJ, IDX, PA): ",
                          bp->b_xio.xio_npages);
                  for (i = 0; i < bp->b_xio.xio_npages; i++) {
                          vm_page_t m;
                          m = bp->b_xio.xio_pages[i];
                          db_printf("(%p, 0x%lx, 0x%lx)", (void *)m->object,
                              (u_long)m->pindex, (u_long)VM_PAGE_TO_PHYS(m));
                          if ((i + 1) < bp->b_xio.xio_npages)
                                  db_printf(",");
                  }
                  db_printf("\n");
          }
  }
  #endif /* DDB */