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: releng/5.0/sys/kern/vfs_bio.c 108006 2002-12-17 22:20:42Z mckusick $
     */
    
    /*
     * 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/stdint.h>
    #include <sys/bio.h>
    #include <sys/buf.h>
    #include <sys/devicestat.h>
    #include <sys/eventhandler.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mount.h>
    #include <sys/mutex.h>
    #include <sys/kernel.h>
    #include <sys/kthread.h>
    #include <sys/proc.h>
    #include <sys/resourcevar.h>
    #include <sys/sysctl.h>
    #include <sys/vmmeter.h>
    #include <sys/vnode.h>
    #include <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>
    
    static MALLOC_DEFINE(M_BIOBUF, "BIO buffer", "BIO buffer");
    
    struct  bio_ops bioops;         /* I/O operation notification */
    
    struct  buf_ops buf_ops_bio = {
            "buf_ops_bio",
            bwrite
    };
    
    /*
     * XXX buf is global because kern_shutdown.c and ffs_checkoverlap has
     * carnal knowledge of buffers.  This knowledge should be moved to vfs_bio.c.
     */
    struct buf *buf;                /* buffer header pool */
    struct mtx buftimelock;         /* Interlock on setting prio and timo */
    
    static void vm_hold_free_pages(struct buf * bp, vm_offset_t from,
                    vm_offset_t to);
    static void vm_hold_load_pages(struct buf * bp, vm_offset_t from,
                    vm_offset_t to);
    static void vfs_page_set_valid(struct buf *bp, vm_ooffset_t off,
                                   int pageno, vm_page_t m);
    static void vfs_clean_pages(struct buf * bp);
    static void vfs_setdirty(struct buf *bp);
    static void vfs_vmio_release(struct buf *bp);
    static void vfs_backgroundwritedone(struct buf *bp);
    static int flushbufqueues(void);
    static void buf_daemon(void);
    
    int vmiodirenable = TRUE;
    SYSCTL_INT(_vfs, OID_AUTO, vmiodirenable, CTLFLAG_RW, &vmiodirenable, 0,
        "Use the VM system for directory writes");
    int runningbufspace;
    SYSCTL_INT(_vfs, OID_AUTO, runningbufspace, CTLFLAG_RD, &runningbufspace, 0,
        "Amount of presently outstanding async buffer io");
    static int bufspace;
    SYSCTL_INT(_vfs, OID_AUTO, bufspace, CTLFLAG_RD, &bufspace, 0,
        "KVA memory used for bufs");
    static int maxbufspace;
    SYSCTL_INT(_vfs, OID_AUTO, maxbufspace, CTLFLAG_RD, &maxbufspace, 0,
        "Maximum allowed value of bufspace (including buf_daemon)");
    static int bufmallocspace;
    SYSCTL_INT(_vfs, OID_AUTO, bufmallocspace, CTLFLAG_RD, &bufmallocspace, 0,
       "Amount of malloced memory for buffers");
   static int maxbufmallocspace;
   SYSCTL_INT(_vfs, OID_AUTO, maxmallocbufspace, CTLFLAG_RW, &maxbufmallocspace, 0,
       "Maximum amount of malloced memory for buffers");
   static int lobufspace;
   SYSCTL_INT(_vfs, OID_AUTO, lobufspace, CTLFLAG_RD, &lobufspace, 0,
       "Minimum amount of buffers we want to have");
   static int hibufspace;
   SYSCTL_INT(_vfs, OID_AUTO, hibufspace, CTLFLAG_RD, &hibufspace, 0,
       "Maximum allowed value of bufspace (excluding buf_daemon)");
   static int bufreusecnt;
   SYSCTL_INT(_vfs, OID_AUTO, bufreusecnt, CTLFLAG_RW, &bufreusecnt, 0,
       "Number of times we have reused a buffer");
   static int buffreekvacnt;
   SYSCTL_INT(_vfs, OID_AUTO, buffreekvacnt, CTLFLAG_RW, &buffreekvacnt, 0,
       "Number of times we have freed the KVA space from some buffer");
   static int bufdefragcnt;
   SYSCTL_INT(_vfs, OID_AUTO, bufdefragcnt, CTLFLAG_RW, &bufdefragcnt, 0,
       "Number of times we have had to repeat buffer allocation to defragment");
   static int lorunningspace;
   SYSCTL_INT(_vfs, OID_AUTO, lorunningspace, CTLFLAG_RW, &lorunningspace, 0,
       "Minimum preferred space used for in-progress I/O");
   static int hirunningspace;
   SYSCTL_INT(_vfs, OID_AUTO, hirunningspace, CTLFLAG_RW, &hirunningspace, 0,
       "Maximum amount of space to use for in-progress I/O");
   static int numdirtybuffers;
   SYSCTL_INT(_vfs, OID_AUTO, numdirtybuffers, CTLFLAG_RD, &numdirtybuffers, 0,
       "Number of buffers that are dirty (has unwritten changes) at the moment");
   static int lodirtybuffers;
   SYSCTL_INT(_vfs, OID_AUTO, lodirtybuffers, CTLFLAG_RW, &lodirtybuffers, 0,
       "How many buffers we want to have free before bufdaemon can sleep");
   static int hidirtybuffers;
   SYSCTL_INT(_vfs, OID_AUTO, hidirtybuffers, CTLFLAG_RW, &hidirtybuffers, 0,
       "When the number of dirty buffers is considered severe");
   static int numfreebuffers;
   SYSCTL_INT(_vfs, OID_AUTO, numfreebuffers, CTLFLAG_RD, &numfreebuffers, 0,
       "Number of free buffers");
   static int lofreebuffers;
   SYSCTL_INT(_vfs, OID_AUTO, lofreebuffers, CTLFLAG_RW, &lofreebuffers, 0,
      "XXX Unused");
   static int hifreebuffers;
   SYSCTL_INT(_vfs, OID_AUTO, hifreebuffers, CTLFLAG_RW, &hifreebuffers, 0,
      "XXX Complicatedly unused");
   static int getnewbufcalls;
   SYSCTL_INT(_vfs, OID_AUTO, getnewbufcalls, CTLFLAG_RW, &getnewbufcalls, 0,
      "Number of calls to getnewbuf");
   static int getnewbufrestarts;
   SYSCTL_INT(_vfs, OID_AUTO, getnewbufrestarts, CTLFLAG_RW, &getnewbufrestarts, 0,
       "Number of times getnewbuf has had to restart a buffer aquisition");
   static int dobkgrdwrite = 1;
   SYSCTL_INT(_debug, OID_AUTO, dobkgrdwrite, CTLFLAG_RW, &dobkgrdwrite, 0,
       "Do background writes (honoring the BX_BKGRDWRITE flag)?");
   
   /*
    * Wakeup point for bufdaemon, as well as indicator of whether it is already
    * active.  Set to 1 when the bufdaemon is already "on" the queue, 0 when it
    * is idling.
    */
   static int bd_request;
   
   /*
    * 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;
   
   /*
    * Offset for bogus_page.
    * XXX bogus_offset should be local to bufinit
    */
   static vm_offset_t bogus_offset;
   
   /*
    * Synchronization (sleep/wakeup) variable for active buffer space requests.
    * Set when wait starts, cleared prior to wakeup().
    * Used in runningbufwakeup() and waitrunningbufspace().
    */
   static int runningbufreq;
   
   /* 
    * Synchronization (sleep/wakeup) variable for buffer requests.
    * Can contain the VFS_BIO_NEED flags defined below; setting/clearing is done
    * by and/or.
    * Used in numdirtywakeup(), bufspacewakeup(), bufcountwakeup(), bwillwrite(),
    * getnewbuf(), and getblk().
    */
   static int needsbuffer;
   
   #ifdef USE_BUFHASH
   /*
    * Mask for index into the buffer hash table, which needs to be power of 2 in
    * size.  Set in kern_vfs_bio_buffer_alloc.
    */
   static int bufhashmask;
   
   /*
    * Hash table for all buffers, with a linked list hanging from each table
    * entry.  Set in kern_vfs_bio_buffer_alloc, initialized in buf_init.
    */
   static LIST_HEAD(bufhashhdr, buf) *bufhashtbl;
   
   /*
    * Somewhere to store buffers when they are not in another list, to always
    * have them in a list (and thus being able to use the same set of operations
    * on them.)
    */
   static struct bufhashhdr invalhash;
   
   #endif
   
   /*
    * Definitions for the buffer free lists.
    */
   #define BUFFER_QUEUES   6       /* number of free buffer queues */
   
   #define QUEUE_NONE      0       /* on no queue */
   #define QUEUE_LOCKED    1       /* locked buffers */
   #define QUEUE_CLEAN     2       /* non-B_DELWRI buffers */
   #define QUEUE_DIRTY     3       /* B_DELWRI buffers */
   #define QUEUE_EMPTYKVA  4       /* empty buffer headers w/KVA assignment */
   #define QUEUE_EMPTY     5       /* empty buffer headers */
   
   /* Queues for free buffers with various properties */
   static TAILQ_HEAD(bqueues, buf) bufqueues[BUFFER_QUEUES] = { { 0 } };
   /*
    * Single global constant for BUF_WMESG, to avoid getting multiple references.
    * buf_wmesg is referred from macros.
    */
   const char *buf_wmesg = BUF_WMESG;
   
   #define VFS_BIO_NEED_ANY        0x01    /* any freeable buffer */
   #define VFS_BIO_NEED_DIRTYFLUSH 0x02    /* waiting for dirty buffer flush */
   #define VFS_BIO_NEED_FREE       0x04    /* wait for free bufs, hi hysteresis */
   #define VFS_BIO_NEED_BUFSPACE   0x08    /* wait for buf space, lo hysteresis */
   
   #ifdef USE_BUFHASH
   /*
    * Buffer hash table code.  Note that the logical block scans linearly, which
    * gives us some L1 cache locality.
    */
   
   static __inline 
   struct bufhashhdr *
   bufhash(struct vnode *vnp, daddr_t bn)
   {
           return(&bufhashtbl[(((uintptr_t)(vnp) >> 7) + (int)bn) & bufhashmask]);
   }
   
   #endif
   
   /*
    *      numdirtywakeup:
    *
    *      If someone is blocked due to there being too many dirty buffers,
    *      and numdirtybuffers is now reasonable, wake them up.
    */
   
   static __inline void
   numdirtywakeup(int level)
   {
           if (numdirtybuffers <= level) {
                   if (needsbuffer & VFS_BIO_NEED_DIRTYFLUSH) {
                           needsbuffer &= ~VFS_BIO_NEED_DIRTYFLUSH;
                           wakeup(&needsbuffer);
                   }
           }
   }
   
   /*
    *      bufspacewakeup:
    *
    *      Called when buffer space is potentially available for recovery.
    *      getnewbuf() will block on this flag when it is unable to free 
    *      sufficient buffer space.  Buffer space becomes recoverable when 
    *      bp's get placed back in the queues.
    */
   
   static __inline void
   bufspacewakeup(void)
   {
           /*
            * If someone is waiting for BUF space, wake them up.  Even
            * though we haven't freed the kva space yet, the waiting
            * process will be able to now.
            */
           if (needsbuffer & VFS_BIO_NEED_BUFSPACE) {
                   needsbuffer &= ~VFS_BIO_NEED_BUFSPACE;
                   wakeup(&needsbuffer);
           }
   }
   
   /*
    * runningbufwakeup() - in-progress I/O accounting.
    *
    */
   static __inline void
   runningbufwakeup(struct buf *bp)
   {
           if (bp->b_runningbufspace) {
                   runningbufspace -= bp->b_runningbufspace;
                   bp->b_runningbufspace = 0;
                   if (runningbufreq && runningbufspace <= lorunningspace) {
                           runningbufreq = 0;
                           wakeup(&runningbufreq);
                   }
           }
   }
   
   /*
    *      bufcountwakeup:
    *
    *      Called when a buffer has been added to one of the free queues to
    *      account for the buffer and to wakeup anyone waiting for free buffers.
    *      This typically occurs when large amounts of metadata are being handled
    *      by the buffer cache ( else buffer space runs out first, usually ).
    */
   
   static __inline void
   bufcountwakeup(void) 
   {
           ++numfreebuffers;
           if (needsbuffer) {
                   needsbuffer &= ~VFS_BIO_NEED_ANY;
                   if (numfreebuffers >= hifreebuffers)
                           needsbuffer &= ~VFS_BIO_NEED_FREE;
                   wakeup(&needsbuffer);
           }
   }
   
   /*
    *      waitrunningbufspace()
    *
    *      runningbufspace is a measure of the amount of I/O currently
    *      running.  This routine is used in async-write situations to
    *      prevent creating huge backups of pending writes to a device.
    *      Only asynchronous writes are governed by this function.
    *
    *      Reads will adjust runningbufspace, but will not block based on it.
    *      The read load has a side effect of reducing the allowed write load.
    *
    *      This does NOT turn an async write into a sync write.  It waits  
    *      for earlier writes to complete and generally returns before the
    *      caller's write has reached the device.
    */
   static __inline void
   waitrunningbufspace(void)
   {
           /*
            * XXX race against wakeup interrupt, currently
            * protected by Giant.  FIXME!
            */
           while (runningbufspace > hirunningspace) {
                   ++runningbufreq;
                   tsleep(&runningbufreq, PVM, "wdrain", 0);
           }
   }
   
   
   /*
    *      vfs_buf_test_cache:
    *
    *      Called when a buffer is extended.  This function clears the B_CACHE
    *      bit if the newly extended portion of the buffer does not contain
    *      valid data.
    */
   static __inline__
   void
   vfs_buf_test_cache(struct buf *bp,
                     vm_ooffset_t foff, vm_offset_t off, vm_offset_t size,
                     vm_page_t m)
   {
           GIANT_REQUIRED;
   
           if (bp->b_flags & B_CACHE) {
                   int base = (foff + off) & PAGE_MASK;
                   if (vm_page_is_valid(m, base, size) == 0)
                           bp->b_flags &= ~B_CACHE;
           }
   }
   
   /* Wake up the buffer deamon if necessary */
   static __inline__
   void
   bd_wakeup(int dirtybuflevel)
   {
           if (bd_request == 0 && numdirtybuffers >= dirtybuflevel) {
                   bd_request = 1;
                   wakeup(&bd_request);
           }
   }
   
   /*
    * bd_speedup - speedup the buffer cache flushing code
    */
   
   static __inline__
   void
   bd_speedup(void)
   {
           bd_wakeup(1);
   }
   
   /*
    * Calculating buffer cache scaling values and reserve space for buffer
    * headers.  This is called during low level kernel initialization and
    * may be called more then once.  We CANNOT write to the memory area
    * being reserved at this time.
    */
   caddr_t
   kern_vfs_bio_buffer_alloc(caddr_t v, long physmem_est)
   {
           /*
            * physmem_est is in pages.  Convert it to kilobytes (assumes
            * PAGE_SIZE is >= 1K)
            */
           physmem_est = physmem_est * (PAGE_SIZE / 1024);
   
           /*
            * The nominal buffer size (and minimum KVA allocation) is BKVASIZE.
            * For the first 64MB of ram nominally allocate sufficient buffers to
            * cover 1/4 of our ram.  Beyond the first 64MB allocate additional
            * buffers to cover 1/20 of our ram over 64MB.  When auto-sizing
            * the buffer cache we limit the eventual kva reservation to
            * maxbcache bytes.
            *
            * factor represents the 1/4 x ram conversion.
            */
           if (nbuf == 0) {
                   int factor = 4 * BKVASIZE / 1024;
   
                   nbuf = 50;
                   if (physmem_est > 4096)
                           nbuf += min((physmem_est - 4096) / factor,
                               65536 / factor);
                   if (physmem_est > 65536)
                           nbuf += (physmem_est - 65536) * 2 / (factor * 5);
   
                   if (maxbcache && nbuf > maxbcache / BKVASIZE)
                           nbuf = maxbcache / BKVASIZE;
           }
   
   #if 0
           /*
            * Do not allow the buffer_map to be more then 1/2 the size of the
            * kernel_map.
            */
           if (nbuf > (kernel_map->max_offset - kernel_map->min_offset) / 
               (BKVASIZE * 2)) {
                   nbuf = (kernel_map->max_offset - kernel_map->min_offset) / 
                       (BKVASIZE * 2);
                   printf("Warning: nbufs capped at %d\n", nbuf);
           }
   #endif
   
           /*
            * swbufs are used as temporary holders for I/O, such as paging I/O.
            * We have no less then 16 and no more then 256.
            */
           nswbuf = max(min(nbuf/4, 256), 16);
   
           /*
            * Reserve space for the buffer cache buffers
            */
           swbuf = (void *)v;
           v = (caddr_t)(swbuf + nswbuf);
           buf = (void *)v;
           v = (caddr_t)(buf + nbuf);
   
   #ifdef USE_BUFHASH
           /*
            * Calculate the hash table size and reserve space
            */
           for (bufhashmask = 8; bufhashmask < nbuf / 4; bufhashmask <<= 1)
                   ;
           bufhashtbl = (void *)v;
           v = (caddr_t)(bufhashtbl + bufhashmask);
           --bufhashmask;
   #endif
           return(v);
   }
   
   /* Initialize the buffer subsystem.  Called before use of any buffers. */
   void
   bufinit(void)
   {
           struct buf *bp;
           int i;
   
           GIANT_REQUIRED;
   
   #ifdef USE_BUFHASH
           LIST_INIT(&invalhash);
   #endif
           mtx_init(&buftimelock, "buftime lock", NULL, MTX_DEF);
   
   #ifdef USE_BUFHASH
           for (i = 0; i <= bufhashmask; i++)
                   LIST_INIT(&bufhashtbl[i]);
   #endif
   
           /* next, make a null set of free lists */
           for (i = 0; i < BUFFER_QUEUES; i++)
                   TAILQ_INIT(&bufqueues[i]);
   
           /* finally, initialize each buffer header and stick on empty q */
           for (i = 0; i < nbuf; i++) {
                   bp = &buf[i];
                   bzero(bp, sizeof *bp);
                   bp->b_flags = B_INVAL;  /* we're just an empty header */
                   bp->b_dev = NODEV;
                   bp->b_rcred = NOCRED;
                   bp->b_wcred = NOCRED;
                   bp->b_qindex = QUEUE_EMPTY;
                   bp->b_xflags = 0;
                   LIST_INIT(&bp->b_dep);
                   BUF_LOCKINIT(bp);
                   TAILQ_INSERT_TAIL(&bufqueues[QUEUE_EMPTY], bp, b_freelist);
   #ifdef USE_BUFHASH
                   LIST_INSERT_HEAD(&invalhash, bp, b_hash);
   #endif
           }
   
           /*
            * maxbufspace is the absolute maximum amount of buffer space we are 
            * allowed to reserve in KVM and in real terms.  The absolute maximum
            * is nominally used by buf_daemon.  hibufspace is the nominal maximum
            * used by most other processes.  The differential is required to 
            * ensure that buf_daemon is able to run when other processes might 
            * be blocked waiting for buffer space.
            *
            * maxbufspace is based on BKVASIZE.  Allocating buffers larger then
            * this may result in KVM fragmentation which is not handled optimally
            * by the system.
            */
           maxbufspace = nbuf * BKVASIZE;
           hibufspace = imax(3 * maxbufspace / 4, maxbufspace - MAXBSIZE * 10);
           lobufspace = hibufspace - MAXBSIZE;
   
           lorunningspace = 512 * 1024;
           hirunningspace = 1024 * 1024;
   
   /*
    * Limit the amount of malloc memory since it is wired permanently into
    * the kernel space.  Even though this is accounted for in the buffer
    * allocation, we don't want the malloced region to grow uncontrolled.
    * The malloc scheme improves memory utilization significantly on average
    * (small) directories.
    */
           maxbufmallocspace = hibufspace / 20;
   
   /*
    * Reduce the chance of a deadlock occuring by limiting the number
    * of delayed-write dirty buffers we allow to stack up.
    */
           hidirtybuffers = nbuf / 4 + 20;
           numdirtybuffers = 0;
   /*
    * To support extreme low-memory systems, make sure hidirtybuffers cannot
    * eat up all available buffer space.  This occurs when our minimum cannot
    * be met.  We try to size hidirtybuffers to 3/4 our buffer space assuming
    * BKVASIZE'd (8K) buffers.
    */
           while (hidirtybuffers * BKVASIZE > 3 * hibufspace / 4) {
                   hidirtybuffers >>= 1;
           }
           lodirtybuffers = hidirtybuffers / 2;
   
   /*
    * Try to keep the number of free buffers in the specified range,
    * and give special processes (e.g. like buf_daemon) access to an 
    * emergency reserve.
    */
           lofreebuffers = nbuf / 18 + 5;
           hifreebuffers = 2 * lofreebuffers;
           numfreebuffers = nbuf;
   
   /*
    * Maximum number of async ops initiated per buf_daemon loop.  This is
    * somewhat of a hack at the moment, we really need to limit ourselves
    * based on the number of bytes of I/O in-transit that were initiated
    * from buf_daemon.
    */
   
           bogus_offset = kmem_alloc_pageable(kernel_map, PAGE_SIZE);
           bogus_page = vm_page_alloc(kernel_object,
                           ((bogus_offset - VM_MIN_KERNEL_ADDRESS) >> PAGE_SHIFT),
                           VM_ALLOC_NORMAL);
           cnt.v_wire_count++;
   }
   
   /*
    * bfreekva() - free the kva allocation for a buffer.
    *
    *      Must be called at splbio() or higher as this is the only locking for
    *      buffer_map.
    *
    *      Since this call frees up buffer space, we call bufspacewakeup().
    */
   static void
   bfreekva(struct buf * bp)
   {
           GIANT_REQUIRED;
   
           if (bp->b_kvasize) {
                   ++buffreekvacnt;
                   bufspace -= bp->b_kvasize;
                   vm_map_delete(buffer_map,
                       (vm_offset_t) bp->b_kvabase,
                       (vm_offset_t) bp->b_kvabase + bp->b_kvasize
                   );
                   bp->b_kvasize = 0;
                   bufspacewakeup();
           }
   }
   
   /*
    *      bremfree:
    *
    *      Remove the buffer from the appropriate free list.
    */
   void
   bremfree(struct buf * bp)
   {
           int s = splbio();
           int old_qindex = bp->b_qindex;
   
           GIANT_REQUIRED;
   
           if (bp->b_qindex != QUEUE_NONE) {
                   KASSERT(BUF_REFCNT(bp) == 1, ("bremfree: bp %p not locked",bp));
                   TAILQ_REMOVE(&bufqueues[bp->b_qindex], bp, b_freelist);
                   bp->b_qindex = QUEUE_NONE;
           } else {
                   if (BUF_REFCNT(bp) <= 1)
                           panic("bremfree: removing a buffer not on a queue");
           }
   
           /*
            * Fixup numfreebuffers count.  If the buffer is invalid or not
            * delayed-write, and it was on the EMPTY, LRU, or AGE queues,
            * the buffer was free and we must decrement numfreebuffers.
            */
           if ((bp->b_flags & B_INVAL) || (bp->b_flags & B_DELWRI) == 0) {
                   switch(old_qindex) {
                   case QUEUE_DIRTY:
                   case QUEUE_CLEAN:
                   case QUEUE_EMPTY:
                   case QUEUE_EMPTYKVA:
                           --numfreebuffers;
                           break;
                   default:
                           break;
                   }
           }
           splx(s);
   }
   
   
   /*
    * Get a buffer with the specified data.  Look in the cache first.  We
    * must clear BIO_ERROR and B_INVAL prior to initiating I/O.  If B_CACHE
    * is set, the buffer is valid and we do not have to do anything ( see
    * getblk() ).  This is really just a special case of breadn().
    */
   int
   bread(struct vnode * vp, daddr_t blkno, int size, struct ucred * cred,
       struct buf ** bpp)
   {
   
           return (breadn(vp, blkno, size, 0, 0, 0, cred, bpp));
   }
   
   /*
    * Operates like bread, but also starts asynchronous I/O on
    * read-ahead blocks.  We must clear BIO_ERROR and B_INVAL prior
    * to initiating I/O . If B_CACHE is set, the buffer is valid 
    * and we do not have to do anything.
    */
   int
   breadn(struct vnode * vp, daddr_t blkno, int size,
       daddr_t * rablkno, int *rabsize,
       int cnt, struct ucred * cred, struct buf ** bpp)
   {
           struct buf *bp, *rabp;
           int i;
           int rv = 0, readwait = 0;
   
           *bpp = bp = getblk(vp, blkno, size, 0, 0);
   
           /* if not found in cache, do some I/O */
           if ((bp->b_flags & B_CACHE) == 0) {
                   if (curthread != PCPU_GET(idlethread))
                           curthread->td_proc->p_stats->p_ru.ru_inblock++;
                   bp->b_iocmd = BIO_READ;
                   bp->b_flags &= ~B_INVAL;
                   bp->b_ioflags &= ~BIO_ERROR;
                   if (bp->b_rcred == NOCRED && cred != NOCRED)
                           bp->b_rcred = crhold(cred);
                   vfs_busy_pages(bp, 0);
                   VOP_STRATEGY(vp, bp);
                   ++readwait;
           }
   
           for (i = 0; i < cnt; i++, rablkno++, rabsize++) {
                   if (inmem(vp, *rablkno))
                           continue;
                   rabp = getblk(vp, *rablkno, *rabsize, 0, 0);
   
                   if ((rabp->b_flags & B_CACHE) == 0) {
                           if (curthread != PCPU_GET(idlethread))
                                   curthread->td_proc->p_stats->p_ru.ru_inblock++;
                           rabp->b_flags |= B_ASYNC;
                           rabp->b_flags &= ~B_INVAL;
                           rabp->b_ioflags &= ~BIO_ERROR;
                           rabp->b_iocmd = BIO_READ;
                           if (rabp->b_rcred == NOCRED && cred != NOCRED)
                                   rabp->b_rcred = crhold(cred);
                           vfs_busy_pages(rabp, 0);
                           BUF_KERNPROC(rabp);
                           VOP_STRATEGY(vp, rabp);
                   } else {
                           brelse(rabp);
                   }
           }
   
           if (readwait) {
                   rv = bufwait(bp);
           }
           return (rv);
   }
   
   /*
    * Write, release buffer on completion.  (Done by iodone
    * if async).  Do not bother writing anything if the buffer
    * is invalid.
    *
    * Note that we set B_CACHE here, indicating that buffer is
    * fully valid and thus cacheable.  This is true even of NFS
    * now so we set it generally.  This could be set either here 
    * or in biodone() since the I/O is synchronous.  We put it
    * here.
    */
   
   int
   bwrite(struct buf * bp)
   {
           int oldflags, s;
           struct buf *newbp;
   
           if (bp->b_flags & B_INVAL) {
                   brelse(bp);
                   return (0);
           }
   
           oldflags = bp->b_flags;
   
           if (BUF_REFCNT(bp) == 0)
                   panic("bwrite: buffer is not busy???");
           s = splbio();
           /*
            * If a background write is already in progress, delay
            * writing this block if it is asynchronous. Otherwise
            * wait for the background write to complete.
            */
           if (bp->b_xflags & BX_BKGRDINPROG) {
                   if (bp->b_flags & B_ASYNC) {
                           splx(s);
                           bdwrite(bp);
                           return (0);
                   }
                   bp->b_xflags |= BX_BKGRDWAIT;
                   tsleep(&bp->b_xflags, PRIBIO, "bwrbg", 0);
                   if (bp->b_xflags & BX_BKGRDINPROG)
                           panic("bwrite: still writing");
           }
   
           /* Mark the buffer clean */
           bundirty(bp);
   
           /*
            * If this buffer is marked for background writing and we
            * do not have to wait for it, make a copy and write the
            * copy so as to leave this buffer ready for further use.
            *
            * This optimization eats a lot of memory.  If we have a page
            * or buffer shortfall we can't do it.
            */
           if (dobkgrdwrite && (bp->b_xflags & BX_BKGRDWRITE) && 
               (bp->b_flags & B_ASYNC) &&
               !vm_page_count_severe() &&
               !buf_dirty_count_severe()) {
                   if (bp->b_iodone != NULL) {
                           printf("bp->b_iodone = %p\n", bp->b_iodone);
                           panic("bwrite: need chained iodone");
                   }
   
                   /* get a new block */
                   newbp = geteblk(bp->b_bufsize);
   
                   /*
                    * set it to be identical to the old block.  We have to
                    * set b_lblkno and BKGRDMARKER before calling bgetvp()
                    * to avoid confusing the splay tree and gbincore().
                    */
                   memcpy(newbp->b_data, bp->b_data, bp->b_bufsize);
                   newbp->b_lblkno = bp->b_lblkno;
                   newbp->b_xflags |= BX_BKGRDMARKER;
                   bgetvp(bp->b_vp, newbp);
                   newbp->b_blkno = bp->b_blkno;
                   newbp->b_offset = bp->b_offset;
                   newbp->b_iodone = vfs_backgroundwritedone;
                   newbp->b_flags |= B_ASYNC;
                   newbp->b_flags &= ~B_INVAL;
   
                   /* move over the dependencies */
                   if (LIST_FIRST(&bp->b_dep) != NULL)
                           buf_movedeps(bp, newbp);
   
                   /*
                    * Initiate write on the copy, release the original to
                    * the B_LOCKED queue so that it cannot go away until
                    * the background write completes. If not locked it could go
                    * away and then be reconstituted while it was being written.
                    * If the reconstituted buffer were written, we could end up
                    * with two background copies being written at the same time.
                    */
                   bp->b_xflags |= BX_BKGRDINPROG;
                   bp->b_flags |= B_LOCKED;
                   bqrelse(bp);
                   bp = newbp;
           }
   
           bp->b_flags &= ~B_DONE;
           bp->b_ioflags &= ~BIO_ERROR;
           bp->b_flags |= B_WRITEINPROG | B_CACHE;
           bp->b_iocmd = BIO_WRITE;
   
           VI_LOCK(bp->b_vp);
           bp->b_vp->v_numoutput++;
           VI_UNLOCK(bp->b_vp);
           vfs_busy_pages(bp, 1);
   
           /*
            * Normal bwrites pipeline writes
            */
           bp->b_runningbufspace = bp->b_bufsize;
           runningbufspace += bp->b_runningbufspace;
   
           if (curthread != PCPU_GET(idlethread))
                   curthread->td_proc->p_stats->p_ru.ru_oublock++;
           splx(s);
           if (oldflags & B_ASYNC)
                   BUF_KERNPROC(bp);
           BUF_STRATEGY(bp);
   
           if ((oldflags & B_ASYNC) == 0) {
                   int rtval = bufwait(bp);
                   brelse(bp);
                   return (rtval);
           } else if ((oldflags & B_NOWDRAIN) == 0) {
                   /*
                    * don't allow the async write to saturate the I/O
                    * system.  Deadlocks can occur only if a device strategy
                    * routine (like in MD) turns around and issues another
                    * high-level write, in which case B_NOWDRAIN is expected
                    * to be set.  Otherwise we will not deadlock here because
                    * we are blocking waiting for I/O that is already in-progress
                    * to complete.
                    */
                   waitrunningbufspace();
           }
   
           return (0);
   }
   
   /*
    * Complete a background write started from bwrite.
    */
   static void
   vfs_backgroundwritedone(bp)
           struct buf *bp;
   {
           struct buf *origbp;
   
           /*
            * Find the original buffer that we are writing.
            */
           VI_LOCK(bp->b_vp);
           if ((origbp = gbincore(bp->b_vp, bp->b_lblkno)) == NULL)
                   panic("backgroundwritedone: lost buffer");
           VI_UNLOCK(bp->b_vp);
           /*
            * Process dependencies then return any unfinished ones.
            */
           if (LIST_FIRST(&bp->b_dep) != NULL)
                   buf_complete(bp);
           if (LIST_FIRST(&bp->b_dep) != NULL)
                   buf_movedeps(bp, origbp);
           /*
            * Clear the BX_BKGRDINPROG flag in the original buffer
            * and awaken it if it is waiting for the write to complete.
            * If BX_BKGRDINPROG is not set in the original buffer it must
            * have been released and re-instantiated - which is not legal.
            */
           KASSERT((origbp->b_xflags & BX_BKGRDINPROG),
               ("backgroundwritedone: lost buffer2"));
           origbp->b_xflags &= ~BX_BKGRDINPROG;
           if (origbp->b_xflags & BX_BKGRDWAIT) {
                   origbp->b_xflags &= ~BX_BKGRDWAIT;
                   wakeup(&origbp->b_xflags);
           }
           /*
            * Clear the B_LOCKED flag and remove it from the locked
            * queue if it currently resides there.
            */
           origbp->b_flags &= ~B_LOCKED;
           if (BUF_LOCK(origbp, LK_EXCLUSIVE | LK_NOWAIT) == 0) {
                   bremfree(origbp);
                   bqrelse(origbp);
           }
           /*
            * This buffer is marked B_NOCACHE, so when it is released
            * by biodone, it will be tossed. We mark it with BIO_READ
            * to avoid biodone doing a second vwakeup.
            */
           bp->b_flags |= B_NOCACHE;
           bp->b_iocmd = BIO_READ;
           bp->b_flags &= ~(B_CACHE | B_DONE);
           bp->b_iodone = 0;
           bufdone(bp);
   }
   
   /*
    * Delayed write. (Buffer is marked dirty).  Do not bother writing
    * anything if the buffer is marked invalid.
    *
    * Note that since the buffer must be completely valid, we can safely
    * set B_CACHE.  In fact, we have to set B_CACHE here rather then in
    * biodone() in order to prevent getblk from writing the buffer
    * out synchronously.
    */
   void
   bdwrite(struct buf * bp)
   {
           GIANT_REQUIRED;
   
           if (BUF_REFCNT(bp) == 0)
                   panic("bdwrite: buffer is not busy");
   
           if (bp->b_flags & B_INVAL) {
                   brelse(bp);
                   return;
           }
           bdirty(bp);
   
           /*
            * Set B_CACHE, indicating that the buffer is fully valid.  This is
            * true even of NFS now.
            */
           bp->b_flags |= B_CACHE;
   
           /*
            * This bmap keeps the system from needing to do the bmap later,
            * perhaps when the system is attempting to do a sync.  Since it
            * is likely that the indirect block -- or whatever other datastructure
            * that the filesystem needs is still in memory now, it is a good
            * thing to do this.  Note also, that if the pageout daemon is
            * requesting a sync -- there might not be enough memory to do
            * the bmap then...  So, this is important to do.
            */
           if (bp->b_lblkno == bp->b_blkno) {
                   VOP_BMAP(bp->b_vp, bp->b_lblkno, NULL, &bp->b_blkno, NULL, NULL);
           }
   
           /*
            * Set the *dirty* buffer range based upon the VM system dirty pages.
            */
           vfs_setdirty(bp);
   
           /*
            * We need to do this here to satisfy the vnode_pager and the
            * pageout daemon, so that it thinks that the pages have been
            * "cleaned".  Note that since the pages are in a delayed write
            * buffer -- the VFS layer "will" see that the pages get written
            * out on the next sync, or perhaps the cluster will be completed.
            */
           vfs_clean_pages(bp);
           bqrelse(bp);
   
           /*
            * Wakeup the buffer flushing daemon if we have a lot of dirty
            * buffers (midpoint between our recovery point and our stall
            * point).
            */
           bd_wakeup((lodirtybuffers + hidirtybuffers) / 2);
   
           /*
           * note: we cannot initiate I/O from a bdwrite even if we wanted to,
           * due to the softdep code.
           */
  }
  
  /*
   *      bdirty:
   *
   *      Turn buffer into delayed write request.  We must clear BIO_READ and
   *      B_RELBUF, and we must set B_DELWRI.  We reassign the buffer to 
   *      itself to properly update it in the dirty/clean lists.  We mark it
   *      B_DONE to ensure that any asynchronization of the buffer properly
   *      clears B_DONE ( else a panic will occur later ).  
   *
   *      bdirty() is kinda like bdwrite() - we have to clear B_INVAL which
   *      might have been set pre-getblk().  Unlike bwrite/bdwrite, bdirty()
   *      should only be called if the buffer is known-good.
   *
   *      Since the buffer is not on a queue, we do not update the numfreebuffers
   *      count.
   *
   *      Must be called at splbio().
   *      The buffer must be on QUEUE_NONE.
   */
  void
  bdirty(bp)
          struct buf *bp;
  {
          KASSERT(bp->b_qindex == QUEUE_NONE,
              ("bdirty: buffer %p still on queue %d", bp, bp->b_qindex));
          bp->b_flags &= ~(B_RELBUF);
          bp->b_iocmd = BIO_WRITE;
  
          if ((bp->b_flags & B_DELWRI) == 0) {
                  bp->b_flags |= B_DONE | B_DELWRI;
                  reassignbuf(bp, bp->b_vp);
                  ++numdirtybuffers;
                  bd_wakeup((lodirtybuffers + hidirtybuffers) / 2);
          }
  }
  
  /*
   *      bundirty:
   *
   *      Clear B_DELWRI for buffer.
   *
   *      Since the buffer is not on a queue, we do not update the numfreebuffers
   *      count.
   *      
   *      Must be called at splbio().
   *      The buffer must be on QUEUE_NONE.
   */
  
  void
  bundirty(bp)
          struct buf *bp;
  {
          KASSERT(bp->b_qindex == QUEUE_NONE,
              ("bundirty: buffer %p still on queue %d", bp, bp->b_qindex));
  
          if (bp->b_flags & B_DELWRI) {
                  bp->b_flags &= ~B_DELWRI;
                  reassignbuf(bp, bp->b_vp);
                  --numdirtybuffers;
                  numdirtywakeup(lodirtybuffers);
          }
          /*
           * Since it is now being written, we can clear its deferred write flag.
           */
          bp->b_flags &= ~B_DEFERRED;
  }
  
  /*
   *      bawrite:
   *
   *      Asynchronous write.  Start output on a buffer, but do not wait for
   *      it to complete.  The buffer is released when the output completes.
   *
   *      bwrite() ( or the VOP routine anyway ) is responsible for handling 
   *      B_INVAL buffers.  Not us.
   */
  void
  bawrite(struct buf * bp)
  {
          bp->b_flags |= B_ASYNC;
          (void) BUF_WRITE(bp);
  }
  
  /*
   *      bwillwrite:
   *
   *      Called prior to the locking of any vnodes when we are expecting to
   *      write.  We do not want to starve the buffer cache with too many
   *      dirty buffers so we block here.  By blocking prior to the locking
   *      of any vnodes we attempt to avoid the situation where a locked vnode
   *      prevents the various system daemons from flushing related buffers.
   */
  
  void
  bwillwrite(void)
  {
          if (numdirtybuffers >= hidirtybuffers) {
                  int s;
  
                  mtx_lock(&Giant);
                  s = splbio();
                  while (numdirtybuffers >= hidirtybuffers) {
                          bd_wakeup(1);
                          needsbuffer |= VFS_BIO_NEED_DIRTYFLUSH;
                          tsleep(&needsbuffer, (PRIBIO + 4), "flswai", 0);
                  }
                  splx(s);
                  mtx_unlock(&Giant);
          }
  }
  
  /*
   * Return true if we have too many dirty buffers.
   */
  int
  buf_dirty_count_severe(void)
  {
          return(numdirtybuffers >= hidirtybuffers);
  }
  
  /*
   *      brelse:
   *
   *      Release a busy buffer and, if requested, free its resources.  The
   *      buffer will be stashed in the appropriate bufqueue[] allowing it
   *      to be accessed later as a cache entity or reused for other purposes.
   */
  void
  brelse(struct buf * bp)
  {
          int s;
  
          GIANT_REQUIRED;
  
          KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)),
              ("brelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
  
          s = splbio();
  
          if (bp->b_flags & B_LOCKED)
                  bp->b_ioflags &= ~BIO_ERROR;
  
          if (bp->b_iocmd == BIO_WRITE &&
              (bp->b_ioflags & BIO_ERROR) &&
              !(bp->b_flags & B_INVAL)) {
                  /*
                   * Failed write, redirty.  Must clear BIO_ERROR to prevent
                   * pages from being scrapped.  If B_INVAL is set then
                   * this case is not run and the next case is run to 
                   * destroy the buffer.  B_INVAL can occur if the buffer
                   * is outside the range supported by the underlying device.
                   */
                  bp->b_ioflags &= ~BIO_ERROR;
                  bdirty(bp);
          } else if ((bp->b_flags & (B_NOCACHE | B_INVAL)) ||
              (bp->b_ioflags & BIO_ERROR) ||
              bp->b_iocmd == BIO_DELETE || (bp->b_bufsize <= 0)) {
                  /*
                   * Either a failed I/O or we were asked to free or not
                   * cache the buffer.
                   */
                  bp->b_flags |= B_INVAL;
                  if (LIST_FIRST(&bp->b_dep) != NULL)
                          buf_deallocate(bp);
                  if (bp->b_flags & B_DELWRI) {
                          --numdirtybuffers;
                          numdirtywakeup(lodirtybuffers);
                  }
                  bp->b_flags &= ~(B_DELWRI | B_CACHE);
                  if ((bp->b_flags & B_VMIO) == 0) {
                          if (bp->b_bufsize)
                                  allocbuf(bp, 0);
                          if (bp->b_vp)
                                  brelvp(bp);
                  }
          }
  
          /*
           * We must clear B_RELBUF if B_DELWRI is set.  If vfs_vmio_release() 
           * is called with B_DELWRI set, the underlying pages may wind up
           * getting freed causing a previous write (bdwrite()) to get 'lost'
           * because pages associated with a B_DELWRI bp are marked clean.
           * 
           * We still allow the B_INVAL case to call vfs_vmio_release(), even
           * if B_DELWRI is set.
           *
           * If B_DELWRI is not set we may have to set B_RELBUF if we are low
           * on pages to return pages to the VM page queues.
           */
          if (bp->b_flags & B_DELWRI)
                  bp->b_flags &= ~B_RELBUF;
          else if (vm_page_count_severe() && !(bp->b_xflags & BX_BKGRDINPROG))
                  bp->b_flags |= B_RELBUF;
  
          /*
           * VMIO buffer rundown.  It is not very necessary to keep a VMIO buffer
           * constituted, not even NFS buffers now.  Two flags effect this.  If
           * B_INVAL, the struct buf is invalidated but the VM object is kept
           * around ( i.e. so it is trivial to reconstitute the buffer later ).
           *
           * If BIO_ERROR or B_NOCACHE is set, pages in the VM object will be
           * invalidated.  BIO_ERROR cannot be set for a failed write unless the
           * buffer is also B_INVAL because it hits the re-dirtying code above.
           *
           * Normally we can do this whether a buffer is B_DELWRI or not.  If
           * the buffer is an NFS buffer, it is tracking piecemeal writes or
           * the commit state and we cannot afford to lose the buffer. If the
           * buffer has a background write in progress, we need to keep it
           * around to prevent it from being reconstituted and starting a second
           * background write.
           */
          if ((bp->b_flags & B_VMIO)
              && !(bp->b_vp->v_mount != NULL &&
                   (bp->b_vp->v_mount->mnt_vfc->vfc_flags & VFCF_NETWORK) != 0 &&
                   !vn_isdisk(bp->b_vp, NULL) &&
                   (bp->b_flags & B_DELWRI))
              ) {
  
                  int i, j, resid;
                  vm_page_t m;
                  off_t foff;
                  vm_pindex_t poff;
                  vm_object_t obj;
                  struct vnode *vp;
  
                  vp = bp->b_vp;
                  obj = bp->b_object;
  
                  /*
                   * Get the base offset and length of the buffer.  Note that 
                   * in the VMIO case if the buffer block size is not
                   * page-aligned then b_data pointer may not be page-aligned.
                   * But our b_pages[] array *IS* page aligned.
                   *
                   * block sizes less then DEV_BSIZE (usually 512) are not 
                   * supported due to the page granularity bits (m->valid,
                   * m->dirty, etc...). 
                   *
                   * See man buf(9) for more information
                   */
                  resid = bp->b_bufsize;
                  foff = bp->b_offset;
  
                  for (i = 0; i < bp->b_npages; i++) {
                          int had_bogus = 0;
  
                          m = bp->b_pages[i];
                          vm_page_flag_clear(m, PG_ZERO);
  
                          /*
                           * If we hit a bogus page, fixup *all* the bogus pages
                           * now.
                           */
                          if (m == bogus_page) {
                                  poff = OFF_TO_IDX(bp->b_offset);
                                  had_bogus = 1;
  
                                  for (j = i; j < bp->b_npages; j++) {
                                          vm_page_t mtmp;
                                          mtmp = bp->b_pages[j];
                                          if (mtmp == bogus_page) {
                                                  mtmp = vm_page_lookup(obj, poff + j);
                                                  if (!mtmp) {
                                                          panic("brelse: page missing\n");
                                                  }
                                                  bp->b_pages[j] = mtmp;
                                          }
                                  }
  
                                  if ((bp->b_flags & B_INVAL) == 0) {
                                          pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_npages);
                                  }
                                  m = bp->b_pages[i];
                          }
                          if ((bp->b_flags & B_NOCACHE) || (bp->b_ioflags & BIO_ERROR)) {
                                  int poffset = foff & PAGE_MASK;
                                  int presid = resid > (PAGE_SIZE - poffset) ?
                                          (PAGE_SIZE - poffset) : resid;
  
                                  KASSERT(presid >= 0, ("brelse: extra page"));
                                  vm_page_set_invalid(m, poffset, presid);
                                  if (had_bogus)
                                          printf("avoided corruption bug in bogus_page/brelse code\n");
                          }
                          resid -= PAGE_SIZE - (foff & PAGE_MASK);
                          foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                  }
  
                  if (bp->b_flags & (B_INVAL | B_RELBUF))
                          vfs_vmio_release(bp);
  
          } else if (bp->b_flags & B_VMIO) {
  
                  if (bp->b_flags & (B_INVAL | B_RELBUF)) {
                          vfs_vmio_release(bp);
                  }
  
          }
                          
          if (bp->b_qindex != QUEUE_NONE)
                  panic("brelse: free buffer onto another queue???");
          if (BUF_REFCNT(bp) > 1) {
                  /* do not release to free list */
                  BUF_UNLOCK(bp);
                  splx(s);
                  return;
          }
  
          /* enqueue */
  
          /* buffers with no memory */
          if (bp->b_bufsize == 0) {
                  bp->b_flags |= B_INVAL;
                  bp->b_xflags &= ~(BX_BKGRDWRITE | BX_ALTDATA);
                  if (bp->b_xflags & BX_BKGRDINPROG)
                          panic("losing buffer 1");
                  if (bp->b_kvasize) {
                          bp->b_qindex = QUEUE_EMPTYKVA;
                  } else {
                          bp->b_qindex = QUEUE_EMPTY;
                  }
                  TAILQ_INSERT_HEAD(&bufqueues[bp->b_qindex], bp, b_freelist);
  #ifdef USE_BUFHASH
                  LIST_REMOVE(bp, b_hash);
                  LIST_INSERT_HEAD(&invalhash, bp, b_hash);
  #endif
                  bp->b_dev = NODEV;
          /* buffers with junk contents */
          } else if (bp->b_flags & (B_INVAL | B_NOCACHE | B_RELBUF) ||
              (bp->b_ioflags & BIO_ERROR)) {
                  bp->b_flags |= B_INVAL;
                  bp->b_xflags &= ~(BX_BKGRDWRITE | BX_ALTDATA);
                  if (bp->b_xflags & BX_BKGRDINPROG)
                          panic("losing buffer 2");
                  bp->b_qindex = QUEUE_CLEAN;
                  TAILQ_INSERT_HEAD(&bufqueues[QUEUE_CLEAN], bp, b_freelist);
  #ifdef USE_BUFHASH
                  LIST_REMOVE(bp, b_hash);
                  LIST_INSERT_HEAD(&invalhash, bp, b_hash);
  #endif
                  bp->b_dev = NODEV;
  
          /* buffers that are locked */
          } else if (bp->b_flags & B_LOCKED) {
                  bp->b_qindex = QUEUE_LOCKED;
                  TAILQ_INSERT_TAIL(&bufqueues[QUEUE_LOCKED], bp, b_freelist);
  
          /* remaining buffers */
          } else {
                  if (bp->b_flags & B_DELWRI)
                          bp->b_qindex = QUEUE_DIRTY;
                  else
                          bp->b_qindex = QUEUE_CLEAN;
                  if (bp->b_flags & B_AGE)
                          TAILQ_INSERT_HEAD(&bufqueues[bp->b_qindex], bp, b_freelist);
                  else
                          TAILQ_INSERT_TAIL(&bufqueues[bp->b_qindex], bp, b_freelist);
          }
  
          /*
           * If B_INVAL and B_DELWRI is set, clear B_DELWRI.  We have already
           * placed the buffer on the correct queue.  We must also disassociate
           * the device and vnode for a B_INVAL buffer so gbincore() doesn't
           * find it.
           */
          if (bp->b_flags & B_INVAL) {
                  if (bp->b_flags & B_DELWRI)
                          bundirty(bp);
                  if (bp->b_vp)
                          brelvp(bp);
          }
  
          /*
           * Fixup numfreebuffers count.  The bp is on an appropriate queue
           * unless locked.  We then bump numfreebuffers if it is not B_DELWRI.
           * We've already handled the B_INVAL case ( B_DELWRI will be clear
           * if B_INVAL is set ).
           */
  
          if ((bp->b_flags & B_LOCKED) == 0 && !(bp->b_flags & B_DELWRI))
                  bufcountwakeup();
  
          /*
           * Something we can maybe free or reuse
           */
          if (bp->b_bufsize || bp->b_kvasize)
                  bufspacewakeup();
  
          /* unlock */
          BUF_UNLOCK(bp);
          bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF | 
                          B_DIRECT | B_NOWDRAIN);
          if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
                  panic("brelse: not dirty");
          splx(s);
  }
  
  /*
   * Release a buffer back to the appropriate queue but do not try to free
   * it.  The buffer is expected to be used again soon.
   *
   * bqrelse() is used by bdwrite() to requeue a delayed write, and used by
   * biodone() to requeue an async I/O on completion.  It is also used when
   * known good buffers need to be requeued but we think we may need the data
   * again soon.
   *
   * XXX we should be able to leave the B_RELBUF hint set on completion.
   */
  void
  bqrelse(struct buf * bp)
  {
          int s;
  
          s = splbio();
  
          KASSERT(!(bp->b_flags & (B_CLUSTER|B_PAGING)), ("bqrelse: inappropriate B_PAGING or B_CLUSTER bp %p", bp));
  
          if (bp->b_qindex != QUEUE_NONE)
                  panic("bqrelse: free buffer onto another queue???");
          if (BUF_REFCNT(bp) > 1) {
                  /* do not release to free list */
                  BUF_UNLOCK(bp);
                  splx(s);
                  return;
          }
          if (bp->b_flags & B_LOCKED) {
                  bp->b_ioflags &= ~BIO_ERROR;
                  bp->b_qindex = QUEUE_LOCKED;
                  TAILQ_INSERT_TAIL(&bufqueues[QUEUE_LOCKED], bp, b_freelist);
                  /* buffers with stale but valid contents */
          } else if (bp->b_flags & B_DELWRI) {
                  bp->b_qindex = QUEUE_DIRTY;
                  TAILQ_INSERT_TAIL(&bufqueues[QUEUE_DIRTY], bp, b_freelist);
          } else if (vm_page_count_severe()) {
                  /*
                   * We are too low on memory, we have to try to free the
                   * buffer (most importantly: the wired pages making up its
                   * backing store) *now*.
                   */
                  splx(s);
                  brelse(bp);
                  return;
          } else {
                  bp->b_qindex = QUEUE_CLEAN;
                  TAILQ_INSERT_TAIL(&bufqueues[QUEUE_CLEAN], bp, b_freelist);
          }
  
          if ((bp->b_flags & B_LOCKED) == 0 &&
              ((bp->b_flags & B_INVAL) || !(bp->b_flags & B_DELWRI))) {
                  bufcountwakeup();
          }
  
          /*
           * Something we can maybe free or reuse.
           */
          if (bp->b_bufsize && !(bp->b_flags & B_DELWRI))
                  bufspacewakeup();
  
          /* unlock */
          BUF_UNLOCK(bp);
          bp->b_flags &= ~(B_ASYNC | B_NOCACHE | B_AGE | B_RELBUF);
          if ((bp->b_flags & B_DELWRI) == 0 && (bp->b_xflags & BX_VNDIRTY))
                  panic("bqrelse: not dirty");
          splx(s);
  }
  
  /* Give pages used by the bp back to the VM system (where possible) */
  static void
  vfs_vmio_release(bp)
          struct buf *bp;
  {
          int i;
          vm_page_t m;
  
          GIANT_REQUIRED;
          vm_page_lock_queues();
          for (i = 0; i < bp->b_npages; i++) {
                  m = bp->b_pages[i];
                  bp->b_pages[i] = NULL;
                  /*
                   * In order to keep page LRU ordering consistent, put
                   * everything on the inactive queue.
                   */
                  vm_page_unwire(m, 0);
                  /*
                   * We don't mess with busy pages, it is
                   * the responsibility of the process that
                   * busied the pages to deal with them.
                   */
                  if ((m->flags & PG_BUSY) || (m->busy != 0))
                          continue;
                          
                  if (m->wire_count == 0) {
                          vm_page_flag_clear(m, PG_ZERO);
                          /*
                           * Might as well free the page if we can and it has
                           * no valid data.  We also free the page if the
                           * buffer was used for direct I/O
                           */
                          if ((bp->b_flags & B_ASYNC) == 0 && !m->valid &&
                              m->hold_count == 0) {
                                  vm_page_busy(m);
                                  pmap_remove_all(m);
                                  vm_page_free(m);
                          } else if (bp->b_flags & B_DIRECT) {
                                  vm_page_try_to_free(m);
                          } else if (vm_page_count_severe()) {
                                  vm_page_try_to_cache(m);
                          }
                  }
          }
          vm_page_unlock_queues();
          pmap_qremove(trunc_page((vm_offset_t) bp->b_data), bp->b_npages);
          
          if (bp->b_bufsize) {
                  bufspacewakeup();
                  bp->b_bufsize = 0;
          }
          bp->b_npages = 0;
          bp->b_flags &= ~B_VMIO;
          if (bp->b_vp)
                  brelvp(bp);
  }
  
  #ifdef USE_BUFHASH
  /*
   * XXX MOVED TO VFS_SUBR.C
   *
   * Check to see if a block is currently memory resident.
   */
  struct buf *
  gbincore(struct vnode * vp, daddr_t blkno)
  {
          struct buf *bp;
          struct bufhashhdr *bh;
  
          bh = bufhash(vp, blkno);
  
          /* Search hash chain */
          LIST_FOREACH(bp, bh, b_hash) {
                  /* hit */
                  if (bp->b_vp == vp && bp->b_lblkno == blkno &&
                      (bp->b_flags & B_INVAL) == 0) {
                          break;
                  }
          }
          return (bp);
  }
  #endif
  
  /*
   *      vfs_bio_awrite:
   *
   *      Implement clustered async writes for clearing out B_DELWRI buffers.
   *      This is much better then the old way of writing only one buffer at
   *      a time.  Note that we may not be presented with the buffers in the 
   *      correct order, so we search for the cluster in both directions.
   */
  int
  vfs_bio_awrite(struct buf * bp)
  {
          int i;
          int j;
          daddr_t lblkno = bp->b_lblkno;
          struct vnode *vp = bp->b_vp;
          int s;
          int ncl;
          struct buf *bpa;
          int nwritten;
          int size;
          int maxcl;
  
          s = splbio();
          /*
           * right now we support clustered writing only to regular files.  If
           * we find a clusterable block we could be in the middle of a cluster
           * rather then at the beginning.
           */
          if ((vp->v_type == VREG) && 
              (vp->v_mount != 0) && /* Only on nodes that have the size info */
              (bp->b_flags & (B_CLUSTEROK | B_INVAL)) == B_CLUSTEROK) {
  
                  size = vp->v_mount->mnt_stat.f_iosize;
                  maxcl = MAXPHYS / size;
  
                  VI_LOCK(vp);
                  for (i = 1; i < maxcl; i++) {
                          if ((bpa = gbincore(vp, lblkno + i)) &&
                              BUF_REFCNT(bpa) == 0 &&
                              ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) ==
                              (B_DELWRI | B_CLUSTEROK)) &&
                              (bpa->b_bufsize == size)) {
                                  if ((bpa->b_blkno == bpa->b_lblkno) ||
                                      (bpa->b_blkno !=
                                       bp->b_blkno + ((i * size) >> DEV_BSHIFT)))
                                          break;
                          } else {
                                  break;
                          }
                  }
                  for (j = 1; i + j <= maxcl && j <= lblkno; j++) {
                          if ((bpa = gbincore(vp, lblkno - j)) &&
                              BUF_REFCNT(bpa) == 0 &&
                              ((bpa->b_flags & (B_DELWRI | B_CLUSTEROK | B_INVAL)) ==
                              (B_DELWRI | B_CLUSTEROK)) &&
                              (bpa->b_bufsize == size)) {
                                  if ((bpa->b_blkno == bpa->b_lblkno) ||
                                      (bpa->b_blkno !=
                                       bp->b_blkno - ((j * size) >> DEV_BSHIFT)))
                                          break;
                          } else {
                                  break;
                          }
                  }
                  VI_UNLOCK(vp);
                  --j;
                  ncl = i + j;
                  /*
                   * this is a possible cluster write
                   */
                  if (ncl != 1) {
                          nwritten = cluster_wbuild(vp, size, lblkno - j, ncl);
                          splx(s);
                          return nwritten;
                  }
          }
  
          BUF_LOCK(bp, LK_EXCLUSIVE);
          bremfree(bp);
          bp->b_flags |= B_ASYNC;
  
          splx(s);
          /*
           * default (old) behavior, writing out only one block
           *
           * XXX returns b_bufsize instead of b_bcount for nwritten?
           */
          nwritten = bp->b_bufsize;
          (void) BUF_WRITE(bp);
  
          return nwritten;
  }
  
  /*
   *      getnewbuf:
   *
   *      Find and initialize a new buffer header, freeing up existing buffers 
   *      in the bufqueues as necessary.  The new buffer is returned locked.
   *
   *      Important:  B_INVAL is not set.  If the caller wishes to throw the
   *      buffer away, the caller must set B_INVAL prior to calling brelse().
   *
   *      We block if:
   *              We have insufficient buffer headers
   *              We have insufficient buffer space
   *              buffer_map is too fragmented ( space reservation fails )
   *              If we have to flush dirty buffers ( but we try to avoid this )
   *
   *      To avoid VFS layer recursion we do not flush dirty buffers ourselves.
   *      Instead we ask the buf daemon to do it for us.  We attempt to
   *      avoid piecemeal wakeups of the pageout daemon.
   */
  
  static struct buf *
  getnewbuf(int slpflag, int slptimeo, int size, int maxsize)
  {
          struct buf *bp;
          struct buf *nbp;
          int defrag = 0;
          int nqindex;
          static int flushingbufs;
  
          GIANT_REQUIRED;
  
          /*
           * We can't afford to block since we might be holding a vnode lock,
           * which may prevent system daemons from running.  We deal with
           * low-memory situations by proactively returning memory and running
           * async I/O rather then sync I/O.
           */
  
          ++getnewbufcalls;
          --getnewbufrestarts;
  restart:
          ++getnewbufrestarts;
  
          /*
           * Setup for scan.  If we do not have enough free buffers,
           * we setup a degenerate case that immediately fails.  Note
           * that if we are specially marked process, we are allowed to
           * dip into our reserves.
           *
           * The scanning sequence is nominally:  EMPTY->EMPTYKVA->CLEAN
           *
           * We start with EMPTYKVA.  If the list is empty we backup to EMPTY.
           * However, there are a number of cases (defragging, reusing, ...)
           * where we cannot backup.
           */
          nqindex = QUEUE_EMPTYKVA;
          nbp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTYKVA]);
  
          if (nbp == NULL) {
                  /*
                   * If no EMPTYKVA buffers and we are either
                   * defragging or reusing, locate a CLEAN buffer
                   * to free or reuse.  If bufspace useage is low
                   * skip this step so we can allocate a new buffer.
                   */
                  if (defrag || bufspace >= lobufspace) {
                          nqindex = QUEUE_CLEAN;
                          nbp = TAILQ_FIRST(&bufqueues[QUEUE_CLEAN]);
                  }
  
                  /*
                   * If we could not find or were not allowed to reuse a
                   * CLEAN buffer, check to see if it is ok to use an EMPTY
                   * buffer.  We can only use an EMPTY buffer if allocating
                   * its KVA would not otherwise run us out of buffer space.
                   */
                  if (nbp == NULL && defrag == 0 &&
                      bufspace + maxsize < hibufspace) {
                          nqindex = QUEUE_EMPTY;
                          nbp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTY]);
                  }
          }
  
          /*
           * Run scan, possibly freeing data and/or kva mappings on the fly
           * depending.
           */
  
          while ((bp = nbp) != NULL) {
                  int qindex = nqindex;
  
                  /*
                   * Calculate next bp ( we can only use it if we do not block
                   * or do other fancy things ).
                   */
                  if ((nbp = TAILQ_NEXT(bp, b_freelist)) == NULL) {
                          switch(qindex) {
                          case QUEUE_EMPTY:
                                  nqindex = QUEUE_EMPTYKVA;
                                  if ((nbp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTYKVA])))
                                          break;
                                  /* FALLTHROUGH */
                          case QUEUE_EMPTYKVA:
                                  nqindex = QUEUE_CLEAN;
                                  if ((nbp = TAILQ_FIRST(&bufqueues[QUEUE_CLEAN])))
                                          break;
                                  /* FALLTHROUGH */
                          case QUEUE_CLEAN:
                                  /*
                                   * nbp is NULL. 
                                   */
                                  break;
                          }
                  }
  
                  /*
                   * Sanity Checks
                   */
                  KASSERT(bp->b_qindex == qindex, ("getnewbuf: inconsistant queue %d bp %p", qindex, bp));
  
                  /*
                   * Note: we no longer distinguish between VMIO and non-VMIO
                   * buffers.
                   */
  
                  KASSERT((bp->b_flags & B_DELWRI) == 0, ("delwri buffer %p found in queue %d", bp, qindex));
  
                  /*
                   * If we are defragging then we need a buffer with 
                   * b_kvasize != 0.  XXX this situation should no longer
                   * occur, if defrag is non-zero the buffer's b_kvasize
                   * should also be non-zero at this point.  XXX
                   */
                  if (defrag && bp->b_kvasize == 0) {
                          printf("Warning: defrag empty buffer %p\n", bp);
                          continue;
                  }
  
                  /*
                   * Start freeing the bp.  This is somewhat involved.  nbp
                   * remains valid only for QUEUE_EMPTY[KVA] bp's.
                   */
  
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0)
                          panic("getnewbuf: locked buf");
                  bremfree(bp);
  
                  if (qindex == QUEUE_CLEAN) {
                          if (bp->b_flags & B_VMIO) {
                                  bp->b_flags &= ~B_ASYNC;
                                  vfs_vmio_release(bp);
                          }
                          if (bp->b_vp)
                                  brelvp(bp);
                  }
  
                  /*
                   * NOTE:  nbp is now entirely invalid.  We can only restart
                   * the scan from this point on.
                   *
                   * Get the rest of the buffer freed up.  b_kva* is still
                   * valid after this operation.
                   */
  
                  if (bp->b_rcred != NOCRED) {
                          crfree(bp->b_rcred);
                          bp->b_rcred = NOCRED;
                  }
                  if (bp->b_wcred != NOCRED) {
                          crfree(bp->b_wcred);
                          bp->b_wcred = NOCRED;
                  }
                  if (LIST_FIRST(&bp->b_dep) != NULL)
                          buf_deallocate(bp);
                  if (bp->b_xflags & BX_BKGRDINPROG)
                          panic("losing buffer 3");
  #ifdef USE_BUFHASH
                  LIST_REMOVE(bp, b_hash);
                  LIST_INSERT_HEAD(&invalhash, bp, b_hash);
  #endif
  
                  if (bp->b_bufsize)
                          allocbuf(bp, 0);
  
                  bp->b_flags = 0;
                  bp->b_ioflags = 0;
                  bp->b_xflags = 0;
                  bp->b_dev = NODEV;
                  bp->b_vp = NULL;
                  bp->b_blkno = bp->b_lblkno = 0;
                  bp->b_offset = NOOFFSET;
                  bp->b_iodone = 0;
                  bp->b_error = 0;
                  bp->b_resid = 0;
                  bp->b_bcount = 0;
                  bp->b_npages = 0;
                  bp->b_dirtyoff = bp->b_dirtyend = 0;
                  bp->b_magic = B_MAGIC_BIO;
                  bp->b_op = &buf_ops_bio;
                  bp->b_object = NULL;
  
                  LIST_INIT(&bp->b_dep);
  
                  /*
                   * If we are defragging then free the buffer.
                   */
                  if (defrag) {
                          bp->b_flags |= B_INVAL;
                          bfreekva(bp);
                          brelse(bp);
                          defrag = 0;
                          goto restart;
                  }
  
                  /*
                   * If we are overcomitted then recover the buffer and its
                   * KVM space.  This occurs in rare situations when multiple
                   * processes are blocked in getnewbuf() or allocbuf().
                   */
                  if (bufspace >= hibufspace)
                          flushingbufs = 1;
                  if (flushingbufs && bp->b_kvasize != 0) {
                          bp->b_flags |= B_INVAL;
                          bfreekva(bp);
                          brelse(bp);
                          goto restart;
                  }
                  if (bufspace < lobufspace)
                          flushingbufs = 0;
                  break;
          }
  
          /*
           * If we exhausted our list, sleep as appropriate.  We may have to
           * wakeup various daemons and write out some dirty buffers.
           *
           * Generally we are sleeping due to insufficient buffer space.
           */
  
          if (bp == NULL) {
                  int flags;
                  char *waitmsg;
  
                  if (defrag) {
                          flags = VFS_BIO_NEED_BUFSPACE;
                          waitmsg = "nbufkv";
                  } else if (bufspace >= hibufspace) {
                          waitmsg = "nbufbs";
                          flags = VFS_BIO_NEED_BUFSPACE;
                  } else {
                          waitmsg = "newbuf";
                          flags = VFS_BIO_NEED_ANY;
                  }
  
                  bd_speedup();   /* heeeelp */
  
                  needsbuffer |= flags;
                  while (needsbuffer & flags) {
                          if (tsleep(&needsbuffer, (PRIBIO + 4) | slpflag,
                              waitmsg, slptimeo))
                                  return (NULL);
                  }
          } else {
                  /*
                   * We finally have a valid bp.  We aren't quite out of the
                   * woods, we still have to reserve kva space.  In order
                   * to keep fragmentation sane we only allocate kva in
                   * BKVASIZE chunks.
                   */
                  maxsize = (maxsize + BKVAMASK) & ~BKVAMASK;
  
                  if (maxsize != bp->b_kvasize) {
                          vm_offset_t addr = 0;
  
                          bfreekva(bp);
  
                          if (vm_map_findspace(buffer_map,
                                  vm_map_min(buffer_map), maxsize, &addr)) {
                                  /*
                                   * Uh oh.  Buffer map is to fragmented.  We
                                   * must defragment the map.
                                   */
                                  ++bufdefragcnt;
                                  defrag = 1;
                                  bp->b_flags |= B_INVAL;
                                  brelse(bp);
                                  goto restart;
                          }
                          if (addr) {
                                  vm_map_insert(buffer_map, NULL, 0,
                                          addr, addr + maxsize,
                                          VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
  
                                  bp->b_kvabase = (caddr_t) addr;
                                  bp->b_kvasize = maxsize;
                                  bufspace += bp->b_kvasize;
                                  ++bufreusecnt;
                          }
                  }
                  bp->b_data = bp->b_kvabase;
          }
          return(bp);
  }
  
  /*
   *      buf_daemon:
   *
   *      buffer flushing daemon.  Buffers are normally flushed by the
   *      update daemon but if it cannot keep up this process starts to
   *      take the load in an attempt to prevent getnewbuf() from blocking.
   */
  
  static struct proc *bufdaemonproc;
  
  static struct kproc_desc buf_kp = {
          "bufdaemon",
          buf_daemon,
          &bufdaemonproc
  };
  SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start, &buf_kp)
  
  static void
  buf_daemon()
  {
          int s;
  
          mtx_lock(&Giant);
  
          /*
           * This process needs to be suspended prior to shutdown sync.
           */
          EVENTHANDLER_REGISTER(shutdown_pre_sync, kproc_shutdown, bufdaemonproc,
              SHUTDOWN_PRI_LAST);
  
          /*
           * This process is allowed to take the buffer cache to the limit
           */
          s = splbio();
  
          for (;;) {
                  kthread_suspend_check(bufdaemonproc);
  
                  bd_request = 0;
  
                  /*
                   * Do the flush.  Limit the amount of in-transit I/O we
                   * allow to build up, otherwise we would completely saturate
                   * the I/O system.  Wakeup any waiting processes before we
                   * normally would so they can run in parallel with our drain.
                   */
                  while (numdirtybuffers > lodirtybuffers) {
                          if (flushbufqueues() == 0)
                                  break;
                          waitrunningbufspace();
                          numdirtywakeup((lodirtybuffers + hidirtybuffers) / 2);
                  }
  
                  /*
                   * Only clear bd_request if we have reached our low water
                   * mark.  The buf_daemon normally waits 1 second and
                   * then incrementally flushes any dirty buffers that have
                   * built up, within reason.
                   *
                   * If we were unable to hit our low water mark and couldn't
                   * find any flushable buffers, we sleep half a second.
                   * Otherwise we loop immediately.
                   */
                  if (numdirtybuffers <= lodirtybuffers) {
                          /*
                           * We reached our low water mark, reset the
                           * request and sleep until we are needed again.
                           * The sleep is just so the suspend code works.
                           */
                          bd_request = 0;
                          tsleep(&bd_request, PVM, "psleep", hz);
                  } else {
                          /*
                           * We couldn't find any flushable dirty buffers but
                           * still have too many dirty buffers, we
                           * have to sleep and try again.  (rare)
                           */
                          tsleep(&bd_request, PVM, "qsleep", hz / 10);
                  }
          }
  }
  
  /*
   *      flushbufqueues:
   *
   *      Try to flush a buffer in the dirty queue.  We must be careful to
   *      free up B_INVAL buffers instead of write them, which NFS is 
   *      particularly sensitive to.
   */
  int flushwithdeps = 0;
  SYSCTL_INT(_vfs, OID_AUTO, flushwithdeps, CTLFLAG_RW, &flushwithdeps,
      0, "Number of buffers flushed with dependecies that require rollbacks");
  static int
  flushbufqueues(void)
  {
          struct thread *td = curthread;
          struct vnode *vp;
          struct buf *bp;
  
          TAILQ_FOREACH(bp, &bufqueues[QUEUE_DIRTY], b_freelist) {
                  KASSERT((bp->b_flags & B_DELWRI),
                      ("unexpected clean buffer %p", bp));
                  if ((bp->b_xflags & BX_BKGRDINPROG) != 0)
                          continue;
                  if (bp->b_flags & B_INVAL) {
                          if (BUF_LOCK(bp, LK_EXCLUSIVE) != 0)
                                  panic("flushbufqueues: locked buf");
                          bremfree(bp);
                          brelse(bp);
                          return (1);
                  }
                  if (LIST_FIRST(&bp->b_dep) != NULL && buf_countdeps(bp, 0))
                          continue;
                  /*
                   * We must hold the lock on a vnode before writing
                   * one of its buffers. Otherwise we may confuse, or
                   * in the case of a snapshot vnode, deadlock the
                   * system.
                   */
                  if ((vp = bp->b_vp) == NULL ||
                      vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT, td) == 0) {
                          vfs_bio_awrite(bp);
                          if (vp != NULL)
                                  VOP_UNLOCK(vp, 0, td);
                          return (1);
                  }
          }
          /*
           * Could not find any buffers without rollback dependencies,
           * so just write the first one in the hopes of eventually
           * making progress.
           */
          TAILQ_FOREACH(bp, &bufqueues[QUEUE_DIRTY], b_freelist) {
                  KASSERT((bp->b_flags & B_DELWRI),
                      ("unexpected clean buffer %p", bp));
                  if ((bp->b_xflags & BX_BKGRDINPROG) != 0)
                          continue;
                  if (bp->b_flags & B_INVAL) {
                          if (BUF_LOCK(bp, LK_EXCLUSIVE) != 0)
                                  panic("flushbufqueues: locked buf");
                          bremfree(bp);
                          brelse(bp);
                          return (1);
                  }
                  /*
                   * We must hold the lock on a vnode before writing
                   * one of its buffers. Otherwise we may confuse, or
                   * in the case of a snapshot vnode, deadlock the
                   * system.
                   */
                  if ((vp = bp->b_vp) == NULL ||
                      vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT, td) == 0) {
                          vfs_bio_awrite(bp);
                          if (vp != NULL)
                                  VOP_UNLOCK(vp, 0, td);
                          flushwithdeps += 1;
                          return (0);
                  }
          }
          return (0);
  }
  
  /*
   * Check to see if a block is currently memory resident.
   */
  struct buf *
  incore(struct vnode * vp, daddr_t blkno)
  {
          struct buf *bp;
  
          int s = splbio();
          VI_LOCK(vp);
          bp = gbincore(vp, blkno);
          VI_UNLOCK(vp);
          splx(s);
          return (bp);
  }
  
  /*
   * Returns true if no I/O is needed to access the
   * associated VM object.  This is like incore except
   * it also hunts around in the VM system for the data.
   */
  
  int
  inmem(struct vnode * vp, daddr_t blkno)
  {
          vm_object_t obj;
          vm_offset_t toff, tinc, size;
          vm_page_t m;
          vm_ooffset_t off;
  
          GIANT_REQUIRED;
          ASSERT_VOP_LOCKED(vp, "inmem");
  
          if (incore(vp, blkno))
                  return 1;
          if (vp->v_mount == NULL)
                  return 0;
          if (VOP_GETVOBJECT(vp, &obj) != 0 || (vp->v_vflag & VV_OBJBUF) == 0)
                  return 0;
  
          size = PAGE_SIZE;
          if (size > vp->v_mount->mnt_stat.f_iosize)
                  size = vp->v_mount->mnt_stat.f_iosize;
          off = (vm_ooffset_t)blkno * (vm_ooffset_t)vp->v_mount->mnt_stat.f_iosize;
  
          for (toff = 0; toff < vp->v_mount->mnt_stat.f_iosize; toff += tinc) {
                  m = vm_page_lookup(obj, OFF_TO_IDX(off + toff));
                  if (!m)
                          goto notinmem;
                  tinc = size;
                  if (tinc > PAGE_SIZE - ((toff + off) & PAGE_MASK))
                          tinc = PAGE_SIZE - ((toff + off) & PAGE_MASK);
                  if (vm_page_is_valid(m,
                      (vm_offset_t) ((toff + off) & PAGE_MASK), tinc) == 0)
                          goto notinmem;
          }
          return 1;
  
  notinmem:
          return (0);
  }
  
  /*
   *      vfs_setdirty:
   *
   *      Sets the dirty range for a buffer based on the status of the dirty
   *      bits in the pages comprising the buffer.
   *
   *      The range is limited to the size of the buffer.
   *
   *      This routine is primarily used by NFS, but is generalized for the
   *      B_VMIO case.
   */
  static void
  vfs_setdirty(struct buf *bp) 
  {
          int i;
          vm_object_t object;
  
          GIANT_REQUIRED;
          /*
           * Degenerate case - empty buffer
           */
  
          if (bp->b_bufsize == 0)
                  return;
  
          /*
           * We qualify the scan for modified pages on whether the
           * object has been flushed yet.  The OBJ_WRITEABLE flag
           * is not cleared simply by protecting pages off.
           */
  
          if ((bp->b_flags & B_VMIO) == 0)
                  return;
  
          object = bp->b_pages[0]->object;
  
          if ((object->flags & OBJ_WRITEABLE) && !(object->flags & OBJ_MIGHTBEDIRTY))
                  printf("Warning: object %p writeable but not mightbedirty\n", object);
          if (!(object->flags & OBJ_WRITEABLE) && (object->flags & OBJ_MIGHTBEDIRTY))
                  printf("Warning: object %p mightbedirty but not writeable\n", object);
  
          if (object->flags & (OBJ_MIGHTBEDIRTY|OBJ_CLEANING)) {
                  vm_offset_t boffset;
                  vm_offset_t eoffset;
  
                  /*
                   * test the pages to see if they have been modified directly
                   * by users through the VM system.
                   */
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_flag_clear(bp->b_pages[i], PG_ZERO);
                          vm_page_test_dirty(bp->b_pages[i]);
                  }
  
                  /*
                   * Calculate the encompassing dirty range, boffset and eoffset,
                   * (eoffset - boffset) bytes.
                   */
  
                  for (i = 0; i < bp->b_npages; i++) {
                          if (bp->b_pages[i]->dirty)
                                  break;
                  }
                  boffset = (i << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);
  
                  for (i = bp->b_npages - 1; i >= 0; --i) {
                          if (bp->b_pages[i]->dirty) {
                                  break;
                          }
                  }
                  eoffset = ((i + 1) << PAGE_SHIFT) - (bp->b_offset & PAGE_MASK);
  
                  /*
                   * Fit it to the buffer.
                   */
  
                  if (eoffset > bp->b_bcount)
                          eoffset = bp->b_bcount;
  
                  /*
                   * If we have a good dirty range, merge with the existing
                   * dirty range.
                   */
  
                  if (boffset < eoffset) {
                          if (bp->b_dirtyoff > boffset)
                                  bp->b_dirtyoff = boffset;
                          if (bp->b_dirtyend < eoffset)
                                  bp->b_dirtyend = eoffset;
                  }
          }
  }
  
  /*
   *      getblk:
   *
   *      Get a block given a specified block and offset into a file/device.
   *      The buffers B_DONE bit will be cleared on return, making it almost
   *      ready for an I/O initiation.  B_INVAL may or may not be set on 
   *      return.  The caller should clear B_INVAL prior to initiating a
   *      READ.
   *
   *      For a non-VMIO buffer, B_CACHE is set to the opposite of B_INVAL for
   *      an existing buffer.
   *
   *      For a VMIO buffer, B_CACHE is modified according to the backing VM.
   *      If getblk()ing a previously 0-sized invalid buffer, B_CACHE is set
   *      and then cleared based on the backing VM.  If the previous buffer is
   *      non-0-sized but invalid, B_CACHE will be cleared.
   *
   *      If getblk() must create a new buffer, the new buffer is returned with
   *      both B_INVAL and B_CACHE clear unless it is a VMIO buffer, in which
   *      case it is returned with B_INVAL clear and B_CACHE set based on the
   *      backing VM.
   *
   *      getblk() also forces a BUF_WRITE() for any B_DELWRI buffer whos
   *      B_CACHE bit is clear.
   *      
   *      What this means, basically, is that the caller should use B_CACHE to
   *      determine whether the buffer is fully valid or not and should clear
   *      B_INVAL prior to issuing a read.  If the caller intends to validate
   *      the buffer by loading its data area with something, the caller needs
   *      to clear B_INVAL.  If the caller does this without issuing an I/O, 
   *      the caller should set B_CACHE ( as an optimization ), else the caller
   *      should issue the I/O and biodone() will set B_CACHE if the I/O was
   *      a write attempt or if it was a successfull read.  If the caller 
   *      intends to issue a READ, the caller must clear B_INVAL and BIO_ERROR
   *      prior to issuing the READ.  biodone() will *not* clear B_INVAL.
   */
  struct buf *
  getblk(struct vnode * vp, daddr_t blkno, int size, int slpflag, int slptimeo)
  {
          struct buf *bp;
          int s;
  #ifdef USE_BUFHASH
          struct bufhashhdr *bh;
  #endif
          ASSERT_VOP_LOCKED(vp, "getblk");
  
          if (size > MAXBSIZE)
                  panic("getblk: size(%d) > MAXBSIZE(%d)\n", size, MAXBSIZE);
  
          s = splbio();
  loop:
          /*
           * Block if we are low on buffers.   Certain processes are allowed
           * to completely exhaust the buffer cache.
           *
           * If this check ever becomes a bottleneck it may be better to
           * move it into the else, when gbincore() fails.  At the moment
           * it isn't a problem.
           *
           * XXX remove if 0 sections (clean this up after its proven)
           */
          if (numfreebuffers == 0) {
                  if (curthread == PCPU_GET(idlethread))
                          return NULL;
                  needsbuffer |= VFS_BIO_NEED_ANY;
          }
  
          VI_LOCK(vp);
          if ((bp = gbincore(vp, blkno))) {
                  VI_UNLOCK(vp);
                  /*
                   * Buffer is in-core.  If the buffer is not busy, it must
                   * be on a queue.
                   */
  
                  if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
                          if (BUF_TIMELOCK(bp, LK_EXCLUSIVE | LK_SLEEPFAIL,
                              "getblk", slpflag, slptimeo) == ENOLCK)
                                  goto loop;
                          splx(s);
                          return (struct buf *) NULL;
                  }
  
                  /*
                   * The buffer is locked.  B_CACHE is cleared if the buffer is 
                   * invalid.  Otherwise, for a non-VMIO buffer, B_CACHE is set
                   * and for a VMIO buffer B_CACHE is adjusted according to the
                   * backing VM cache.
                   */
                  if (bp->b_flags & B_INVAL)
                          bp->b_flags &= ~B_CACHE;
                  else if ((bp->b_flags & (B_VMIO | B_INVAL)) == 0)
                          bp->b_flags |= B_CACHE;
                  bremfree(bp);
  
                  /*
                   * check for size inconsistancies for non-VMIO case.
                   */
  
                  if (bp->b_bcount != size) {
                          if ((bp->b_flags & B_VMIO) == 0 ||
                              (size > bp->b_kvasize)) {
                                  if (bp->b_flags & B_DELWRI) {
                                          bp->b_flags |= B_NOCACHE;
                                          BUF_WRITE(bp);
                                  } else {
                                          if ((bp->b_flags & B_VMIO) &&
                                             (LIST_FIRST(&bp->b_dep) == NULL)) {
                                                  bp->b_flags |= B_RELBUF;
                                                  brelse(bp);
                                          } else {
                                                  bp->b_flags |= B_NOCACHE;
                                                  BUF_WRITE(bp);
                                          }
                                  }
                                  goto loop;
                          }
                  }
  
                  /*
                   * If the size is inconsistant in the VMIO case, we can resize
                   * the buffer.  This might lead to B_CACHE getting set or
                   * cleared.  If the size has not changed, B_CACHE remains
                   * unchanged from its previous state.
                   */
  
                  if (bp->b_bcount != size)
                          allocbuf(bp, size);
  
                  KASSERT(bp->b_offset != NOOFFSET, 
                      ("getblk: no buffer offset"));
  
                  /*
                   * A buffer with B_DELWRI set and B_CACHE clear must
                   * be committed before we can return the buffer in
                   * order to prevent the caller from issuing a read
                   * ( due to B_CACHE not being set ) and overwriting
                   * it.
                   *
                   * Most callers, including NFS and FFS, need this to
                   * operate properly either because they assume they
                   * can issue a read if B_CACHE is not set, or because
                   * ( for example ) an uncached B_DELWRI might loop due 
                   * to softupdates re-dirtying the buffer.  In the latter
                   * case, B_CACHE is set after the first write completes,
                   * preventing further loops.
                   * NOTE!  b*write() sets B_CACHE.  If we cleared B_CACHE
                   * above while extending the buffer, we cannot allow the
                   * buffer to remain with B_CACHE set after the write
                   * completes or it will represent a corrupt state.  To
                   * deal with this we set B_NOCACHE to scrap the buffer
                   * after the write.
                   *
                   * We might be able to do something fancy, like setting
                   * B_CACHE in bwrite() except if B_DELWRI is already set,
                   * so the below call doesn't set B_CACHE, but that gets real
                   * confusing.  This is much easier.
                   */
  
                  if ((bp->b_flags & (B_CACHE|B_DELWRI)) == B_DELWRI) {
                          bp->b_flags |= B_NOCACHE;
                          BUF_WRITE(bp);
                          goto loop;
                  }
  
                  splx(s);
                  bp->b_flags &= ~B_DONE;
          } else {
                  int bsize, maxsize, vmio;
                  off_t offset;
  
                  /*
                   * Buffer is not in-core, create new buffer.  The buffer
                   * returned by getnewbuf() is locked.  Note that the returned
                   * buffer is also considered valid (not marked B_INVAL).
                   */
                  VI_UNLOCK(vp);
                  if (vn_isdisk(vp, NULL))
                          bsize = DEV_BSIZE;
                  else if (vp->v_mountedhere)
                          bsize = vp->v_mountedhere->mnt_stat.f_iosize;
                  else if (vp->v_mount)
                          bsize = vp->v_mount->mnt_stat.f_iosize;
                  else
                          bsize = size;
  
                  offset = blkno * bsize;
                  vmio = (VOP_GETVOBJECT(vp, NULL) == 0) &&
                      (vp->v_vflag & VV_OBJBUF);
                  maxsize = vmio ? size + (offset & PAGE_MASK) : size;
                  maxsize = imax(maxsize, bsize);
  
                  if ((bp = getnewbuf(slpflag, slptimeo, size, maxsize)) == NULL) {
                          if (slpflag || slptimeo) {
                                  splx(s);
                                  return NULL;
                          }
                          goto loop;
                  }
  
                  /*
                   * This code is used to make sure that a buffer is not
                   * created while the getnewbuf routine is blocked.
                   * This can be a problem whether the vnode is locked or not.
                   * If the buffer is created out from under us, we have to
                   * throw away the one we just created.  There is now window
                   * race because we are safely running at splbio() from the
                   * point of the duplicate buffer creation through to here,
                   * and we've locked the buffer.
                   *
                   * Note: this must occur before we associate the buffer
                   * with the vp especially considering limitations in
                   * the splay tree implementation when dealing with duplicate
                   * lblkno's.
                   */
                  VI_LOCK(vp);
                  if (gbincore(vp, blkno)) {
                          VI_UNLOCK(vp);
                          bp->b_flags |= B_INVAL;
                          brelse(bp);
                          goto loop;
                  }
                  VI_UNLOCK(vp);
  
                  /*
                   * Insert the buffer into the hash, so that it can
                   * be found by incore.
                   */
                  bp->b_blkno = bp->b_lblkno = blkno;
                  bp->b_offset = offset;
  
                  bgetvp(vp, bp);
  #ifdef USE_BUFHASH
                  LIST_REMOVE(bp, b_hash);
                  bh = bufhash(vp, blkno);
                  LIST_INSERT_HEAD(bh, bp, b_hash);
  #endif
  
                  /*
                   * set B_VMIO bit.  allocbuf() the buffer bigger.  Since the
                   * buffer size starts out as 0, B_CACHE will be set by
                   * allocbuf() for the VMIO case prior to it testing the
                   * backing store for validity.
                   */
  
                  if (vmio) {
                          bp->b_flags |= B_VMIO;
  #if defined(VFS_BIO_DEBUG)
                          if (vp->v_type != VREG)
                                  printf("getblk: vmioing file type %d???\n", vp->v_type);
  #endif
                          VOP_GETVOBJECT(vp, &bp->b_object);
                  } else {
                          bp->b_flags &= ~B_VMIO;
                          bp->b_object = NULL;
                  }
  
                  allocbuf(bp, size);
  
                  splx(s);
                  bp->b_flags &= ~B_DONE;
          }
          KASSERT(BUF_REFCNT(bp) == 1, ("getblk: bp %p not locked",bp));
          return (bp);
  }
  
  /*
   * Get an empty, disassociated buffer of given size.  The buffer is initially
   * set to B_INVAL.
   */
  struct buf *
  geteblk(int size)
  {
          struct buf *bp;
          int s;
          int maxsize;
  
          maxsize = (size + BKVAMASK) & ~BKVAMASK;
  
          s = splbio();
          while ((bp = getnewbuf(0, 0, size, maxsize)) == 0)
                  continue;
          splx(s);
          allocbuf(bp, size);
          bp->b_flags |= B_INVAL; /* b_dep cleared by getnewbuf() */
          KASSERT(BUF_REFCNT(bp) == 1, ("geteblk: bp %p not locked",bp));
          return (bp);
  }
  
  
  /*
   * This code constitutes the buffer memory from either anonymous system
   * memory (in the case of non-VMIO operations) or from an associated
   * VM object (in the case of VMIO operations).  This code is able to
   * resize a buffer up or down.
   *
   * Note that this code is tricky, and has many complications to resolve
   * deadlock or inconsistant data situations.  Tread lightly!!! 
   * There are B_CACHE and B_DELWRI interactions that must be dealt with by 
   * the caller.  Calling this code willy nilly can result in the loss of data.
   *
   * allocbuf() only adjusts B_CACHE for VMIO buffers.  getblk() deals with
   * B_CACHE for the non-VMIO case.
   */
  
  int
  allocbuf(struct buf *bp, int size)
  {
          int newbsize, mbsize;
          int i;
  
          GIANT_REQUIRED;
  
          if (BUF_REFCNT(bp) == 0)
                  panic("allocbuf: buffer not busy");
  
          if (bp->b_kvasize < size)
                  panic("allocbuf: buffer too small");
  
          if ((bp->b_flags & B_VMIO) == 0) {
                  caddr_t origbuf;
                  int origbufsize;
                  /*
                   * Just get anonymous memory from the kernel.  Don't
                   * mess with B_CACHE.
                   */
                  mbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                  if (bp->b_flags & B_MALLOC)
                          newbsize = mbsize;
                  else
                          newbsize = round_page(size);
  
                  if (newbsize < bp->b_bufsize) {
                          /*
                           * malloced buffers are not shrunk
                           */
                          if (bp->b_flags & B_MALLOC) {
                                  if (newbsize) {
                                          bp->b_bcount = size;
                                  } else {
                                          free(bp->b_data, M_BIOBUF);
                                          if (bp->b_bufsize) {
                                                  bufmallocspace -= bp->b_bufsize;
                                                  bufspacewakeup();
                                                  bp->b_bufsize = 0;
                                          }
                                          bp->b_data = bp->b_kvabase;
                                          bp->b_bcount = 0;
                                          bp->b_flags &= ~B_MALLOC;
                                  }
                                  return 1;
                          }               
                          vm_hold_free_pages(
                              bp,
                              (vm_offset_t) bp->b_data + newbsize,
                              (vm_offset_t) bp->b_data + bp->b_bufsize);
                  } else if (newbsize > bp->b_bufsize) {
                          /*
                           * We only use malloced memory on the first allocation.
                           * and revert to page-allocated memory when the buffer
                           * grows.
                           */
                          if ( (bufmallocspace < maxbufmallocspace) &&
                                  (bp->b_bufsize == 0) &&
                                  (mbsize <= PAGE_SIZE/2)) {
  
                                  bp->b_data = malloc(mbsize, M_BIOBUF, M_WAITOK);
                                  bp->b_bufsize = mbsize;
                                  bp->b_bcount = size;
                                  bp->b_flags |= B_MALLOC;
                                  bufmallocspace += mbsize;
                                  return 1;
                          }
                          origbuf = NULL;
                          origbufsize = 0;
                          /*
                           * If the buffer is growing on its other-than-first allocation,
                           * then we revert to the page-allocation scheme.
                           */
                          if (bp->b_flags & B_MALLOC) {
                                  origbuf = bp->b_data;
                                  origbufsize = bp->b_bufsize;
                                  bp->b_data = bp->b_kvabase;
                                  if (bp->b_bufsize) {
                                          bufmallocspace -= bp->b_bufsize;
                                          bufspacewakeup();
                                          bp->b_bufsize = 0;
                                  }
                                  bp->b_flags &= ~B_MALLOC;
                                  newbsize = round_page(newbsize);
                          }
                          vm_hold_load_pages(
                              bp,
                              (vm_offset_t) bp->b_data + bp->b_bufsize,
                              (vm_offset_t) bp->b_data + newbsize);
                          if (origbuf) {
                                  bcopy(origbuf, bp->b_data, origbufsize);
                                  free(origbuf, M_BIOBUF);
                          }
                  }
          } else {
                  int desiredpages;
  
                  newbsize = (size + DEV_BSIZE - 1) & ~(DEV_BSIZE - 1);
                  desiredpages = (size == 0) ? 0 :
                          num_pages((bp->b_offset & PAGE_MASK) + newbsize);
  
                  if (bp->b_flags & B_MALLOC)
                          panic("allocbuf: VMIO buffer can't be malloced");
                  /*
                   * Set B_CACHE initially if buffer is 0 length or will become
                   * 0-length.
                   */
                  if (size == 0 || bp->b_bufsize == 0)
                          bp->b_flags |= B_CACHE;
  
                  if (newbsize < bp->b_bufsize) {
                          /*
                           * DEV_BSIZE aligned new buffer size is less then the
                           * DEV_BSIZE aligned existing buffer size.  Figure out
                           * if we have to remove any pages.
                           */
                          if (desiredpages < bp->b_npages) {
                                  vm_page_t m;
  
                                  vm_page_lock_queues();
                                  for (i = desiredpages; i < bp->b_npages; i++) {
                                          /*
                                           * the page is not freed here -- it
                                           * is the responsibility of 
                                           * vnode_pager_setsize
                                           */
                                          m = bp->b_pages[i];
                                          KASSERT(m != bogus_page,
                                              ("allocbuf: bogus page found"));
                                          while (vm_page_sleep_if_busy(m, TRUE, "biodep"))
                                                  vm_page_lock_queues();
  
                                          bp->b_pages[i] = NULL;
                                          vm_page_unwire(m, 0);
                                  }
                                  vm_page_unlock_queues();
                                  pmap_qremove((vm_offset_t) trunc_page((vm_offset_t)bp->b_data) +
                                      (desiredpages << PAGE_SHIFT), (bp->b_npages - desiredpages));
                                  bp->b_npages = desiredpages;
                          }
                  } else if (size > bp->b_bcount) {
                          /*
                           * We are growing the buffer, possibly in a 
                           * byte-granular fashion.
                           */
                          struct vnode *vp;
                          vm_object_t obj;
                          vm_offset_t toff;
                          vm_offset_t tinc;
  
                          /*
                           * 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.
                           */
  
                          vp = bp->b_vp;
                          obj = bp->b_object;
  
                          while (bp->b_npages < desiredpages) {
                                  vm_page_t m;
                                  vm_pindex_t pi;
  
                                  pi = OFF_TO_IDX(bp->b_offset) + bp->b_npages;
                                  if ((m = vm_page_lookup(obj, pi)) == NULL) {
                                          /*
                                           * note: must allocate system pages
                                           * since blocking here could intefere
                                           * with paging I/O, no matter which
                                           * process we are.
                                           */
                                          m = vm_page_alloc(obj, pi,
                                              VM_ALLOC_SYSTEM | VM_ALLOC_WIRED);
                                          if (m == NULL) {
                                                  VM_WAIT;
                                                  vm_pageout_deficit += desiredpages - bp->b_npages;
                                          } else {
                                                  vm_page_lock_queues();
                                                  vm_page_wakeup(m);
                                                  vm_page_unlock_queues();
                                                  bp->b_flags &= ~B_CACHE;
                                                  bp->b_pages[bp->b_npages] = m;
                                                  ++bp->b_npages;
                                          }
                                          continue;
                                  }
  
                                  /*
                                   * We found a page.  If we have to sleep on it,
                                   * retry because it might have gotten freed out
                                   * from under us.
                                   *
                                   * We can only test PG_BUSY here.  Blocking on
                                   * m->busy might lead to a deadlock:
                                   *
                                   *  vm_fault->getpages->cluster_read->allocbuf
                                   *
                                   */
                                  vm_page_lock_queues();
                                  if (vm_page_sleep_if_busy(m, FALSE, "pgtblk"))
                                          continue;
  
                                  /*
                                   * We have a good page.  Should we wakeup the
                                   * page daemon?
                                   */
                                  if ((curproc != pageproc) &&
                                      ((m->queue - m->pc) == PQ_CACHE) &&
                                      ((cnt.v_free_count + cnt.v_cache_count) <
                                          (cnt.v_free_min + cnt.v_cache_min))) {
                                          pagedaemon_wakeup();
                                  }
                                  vm_page_flag_clear(m, PG_ZERO);
                                  vm_page_wire(m);
                                  vm_page_unlock_queues();
                                  bp->b_pages[bp->b_npages] = m;
                                  ++bp->b_npages;
                          }
  
                          /*
                           * Step 2.  We've loaded the pages into the buffer,
                           * we have to figure out if we can still have B_CACHE
                           * set.  Note that B_CACHE is set according to the
                           * byte-granular range ( bcount and size ), new the
                           * aligned range ( newbsize ).
                           *
                           * The VM test is against m->valid, which is DEV_BSIZE
                           * aligned.  Needless to say, the validity of the data
                           * needs to also be DEV_BSIZE aligned.  Note that this
                           * fails with NFS if the server or some other client
                           * extends the file's EOF.  If our buffer is resized, 
                           * B_CACHE may remain set! XXX
                           */
  
                          toff = bp->b_bcount;
                          tinc = PAGE_SIZE - ((bp->b_offset + toff) & PAGE_MASK);
  
                          while ((bp->b_flags & B_CACHE) && toff < size) {
                                  vm_pindex_t pi;
  
                                  if (tinc > (size - toff))
                                          tinc = size - toff;
  
                                  pi = ((bp->b_offset & PAGE_MASK) + toff) >> 
                                      PAGE_SHIFT;
  
                                  vfs_buf_test_cache(
                                      bp, 
                                      bp->b_offset,
                                      toff, 
                                      tinc, 
                                      bp->b_pages[pi]
                                  );
                                  toff += tinc;
                                  tinc = PAGE_SIZE;
                          }
  
                          /*
                           * Step 3, fixup the KVM pmap.  Remember that
                           * bp->b_data is relative to bp->b_offset, but 
                           * bp->b_offset may be offset into the first page.
                           */
  
                          bp->b_data = (caddr_t)
                              trunc_page((vm_offset_t)bp->b_data);
                          pmap_qenter(
                              (vm_offset_t)bp->b_data,
                              bp->b_pages, 
                              bp->b_npages
                          );
                          
                          bp->b_data = (caddr_t)((vm_offset_t)bp->b_data | 
                              (vm_offset_t)(bp->b_offset & PAGE_MASK));
                  }
          }
          if (newbsize < bp->b_bufsize)
                  bufspacewakeup();
          bp->b_bufsize = newbsize;       /* actual buffer allocation     */
          bp->b_bcount = size;            /* requested buffer size        */
          return 1;
  }
  
  void
  biodone(struct bio *bp)
  {
          bp->bio_flags |= BIO_DONE;
          if (bp->bio_done != NULL)
                  bp->bio_done(bp);
          else
                  wakeup(bp);
  }
  
  /*
   * Wait for a BIO to finish.
   *
   * XXX: resort to a timeout for now.  The optimal locking (if any) for this
   * case is not yet clear.
   */
  int
  biowait(struct bio *bp, const char *wchan)
  {
  
          while ((bp->bio_flags & BIO_DONE) == 0)
                  msleep(bp, NULL, PRIBIO, wchan, hz / 10);
          if (bp->bio_error != 0)
                  return (bp->bio_error);
          if (!(bp->bio_flags & BIO_ERROR))
                  return (0);
          return (EIO);
  }
  
  void
  biofinish(struct bio *bp, struct devstat *stat, int error)
  {
          
          if (error) {
                  bp->bio_error = error;
                  bp->bio_flags |= BIO_ERROR;
          }
          if (stat != NULL)
                  devstat_end_transaction_bio(stat, bp);
          biodone(bp);
  }
  
  void
  bioq_init(struct bio_queue_head *head)
  {
          TAILQ_INIT(&head->queue);
          head->last_pblkno = 0;
          head->insert_point = NULL;
          head->switch_point = NULL;
  }
  
  void
  bioq_remove(struct bio_queue_head *head, struct bio *bp)
  {
          if (bp == head->switch_point)
                  head->switch_point = TAILQ_NEXT(bp, bio_queue);
          if (bp == head->insert_point) {
                  head->insert_point = TAILQ_PREV(bp, bio_queue, bio_queue);
                  if (head->insert_point == NULL)
                          head->last_pblkno = 0;
          } else if (bp == TAILQ_FIRST(&head->queue))
                  head->last_pblkno = bp->bio_pblkno;
          TAILQ_REMOVE(&head->queue, bp, bio_queue);
          if (TAILQ_FIRST(&head->queue) == head->switch_point)
                  head->switch_point = NULL;
  }
  
  /*
   *      bufwait:
   *
   *      Wait for buffer I/O completion, returning error status.  The buffer
   *      is left locked and B_DONE on return.  B_EINTR is converted into a EINTR
   *      error and cleared.
   */
  int
  bufwait(register struct buf * bp)
  {
          int s;
  
          s = splbio();
          while ((bp->b_flags & B_DONE) == 0) {
                  if (bp->b_iocmd == BIO_READ)
                          tsleep(bp, PRIBIO, "biord", 0);
                  else
                          tsleep(bp, PRIBIO, "biowr", 0);
          }
          splx(s);
          if (bp->b_flags & B_EINTR) {
                  bp->b_flags &= ~B_EINTR;
                  return (EINTR);
          }
          if (bp->b_ioflags & BIO_ERROR) {
                  return (bp->b_error ? bp->b_error : EIO);
          } else {
                  return (0);
          }
  }
  
   /*
    * Call back function from struct bio back up to struct buf.
    * The corresponding initialization lives in sys/conf.h:DEV_STRATEGY().
    */
  void
  bufdonebio(struct bio *bp)
  {
          bufdone(bp->bio_caller2);
  }
  
  /*
   *      bufdone:
   *
   *      Finish I/O on a buffer, optionally calling a completion function.
   *      This is usually called from an interrupt so process blocking is
   *      not allowed.
   *
   *      biodone is also responsible for setting B_CACHE in a B_VMIO bp.
   *      In a non-VMIO bp, B_CACHE will be set on the next getblk() 
   *      assuming B_INVAL is clear.
   *
   *      For the VMIO case, we set B_CACHE if the op was a read and no
   *      read error occured, or if the op was a write.  B_CACHE is never
   *      set if the buffer is invalid or otherwise uncacheable.
   *
   *      biodone does not mess with B_INVAL, allowing the I/O routine or the
   *      initiator to leave B_INVAL set to brelse the buffer out of existance
   *      in the biodone routine.
   */
  void
  bufdone(struct buf *bp)
  {
          int s;
          void    (*biodone)(struct buf *);
  
          GIANT_REQUIRED;
  
          s = splbio();
  
          KASSERT(BUF_REFCNT(bp) > 0, ("biodone: bp %p not busy %d", bp, BUF_REFCNT(bp)));
          KASSERT(!(bp->b_flags & B_DONE), ("biodone: bp %p already done", bp));
  
          bp->b_flags |= B_DONE;
          runningbufwakeup(bp);
  
          if (bp->b_iocmd == BIO_DELETE) {
                  brelse(bp);
                  splx(s);
                  return;
          }
  
          if (bp->b_iocmd == BIO_WRITE) {
                  vwakeup(bp);
          }
  
          /* call optional completion function if requested */
          if (bp->b_iodone != NULL) {
                  biodone = bp->b_iodone;
                  bp->b_iodone = NULL;
                  (*biodone) (bp);
                  splx(s);
                  return;
          }
          if (LIST_FIRST(&bp->b_dep) != NULL)
                  buf_complete(bp);
  
          if (bp->b_flags & B_VMIO) {
                  int i;
                  vm_ooffset_t foff;
                  vm_page_t m;
                  vm_object_t obj;
                  int iosize;
                  struct vnode *vp = bp->b_vp;
  
                  obj = bp->b_object;
  
  #if defined(VFS_BIO_DEBUG)
                  mp_fixme("usecount and vflag accessed without locks.");
                  if (vp->v_usecount == 0) {
                          panic("biodone: zero vnode ref count");
                  }
  
                  if ((vp->v_vflag & VV_OBJBUF) == 0) {
                          panic("biodone: vnode is not setup for merged cache");
                  }
  #endif
  
                  foff = bp->b_offset;
                  KASSERT(bp->b_offset != NOOFFSET,
                      ("biodone: no buffer offset"));
  
  #if defined(VFS_BIO_DEBUG)
                  if (obj->paging_in_progress < bp->b_npages) {
                          printf("biodone: paging in progress(%d) < bp->b_npages(%d)\n",
                              obj->paging_in_progress, bp->b_npages);
                  }
  #endif
  
                  /*
                   * Set B_CACHE if the op was a normal read and no error
                   * occured.  B_CACHE is set for writes in the b*write()
                   * routines.
                   */
                  iosize = bp->b_bcount - bp->b_resid;
                  if (bp->b_iocmd == BIO_READ &&
                      !(bp->b_flags & (B_INVAL|B_NOCACHE)) &&
                      !(bp->b_ioflags & BIO_ERROR)) {
                          bp->b_flags |= B_CACHE;
                  }
                  vm_page_lock_queues();
                  for (i = 0; i < bp->b_npages; i++) {
                          int bogusflag = 0;
                          int resid;
  
                          resid = ((foff + PAGE_SIZE) & ~(off_t)PAGE_MASK) - foff;
                          if (resid > iosize)
                                  resid = iosize;
  
                          /*
                           * cleanup bogus pages, restoring the originals
                           */
                          m = bp->b_pages[i];
                          if (m == bogus_page) {
                                  bogusflag = 1;
                                  m = vm_page_lookup(obj, OFF_TO_IDX(foff));
                                  if (m == NULL)
                                          panic("biodone: page disappeared!");
                                  bp->b_pages[i] = m;
                                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_npages);
                          }
  #if defined(VFS_BIO_DEBUG)
                          if (OFF_TO_IDX(foff) != m->pindex) {
                                  printf(
  "biodone: foff(%jd)/m->pindex(%ju) mismatch\n",
                                      (intmax_t)foff, (uintmax_t)m->pindex);
                          }
  #endif
  
                          /*
                           * In the write case, the valid and clean bits are
                           * already changed correctly ( see bdwrite() ), so we 
                           * only need to do this here in the read case.
                           */
                          if ((bp->b_iocmd == BIO_READ) && !bogusflag && resid > 0) {
                                  vfs_page_set_valid(bp, foff, i, m);
                          }
                          vm_page_flag_clear(m, PG_ZERO);
  
                          /*
                           * when debugging new filesystems or buffer I/O methods, this
                           * is the most common error that pops up.  if you see this, you
                           * have not set the page busy flag correctly!!!
                           */
                          if (m->busy == 0) {
                                  printf("biodone: page busy < 0, "
                                      "pindex: %d, foff: 0x(%x,%x), "
                                      "resid: %d, index: %d\n",
                                      (int) m->pindex, (int)(foff >> 32),
                                                  (int) foff & 0xffffffff, resid, i);
                                  if (!vn_isdisk(vp, NULL))
                                          printf(" iosize: %ld, lblkno: %jd, flags: 0x%lx, npages: %d\n",
                                              bp->b_vp->v_mount->mnt_stat.f_iosize,
                                              (intmax_t) bp->b_lblkno,
                                              bp->b_flags, bp->b_npages);
                                  else
                                          printf(" VDEV, lblkno: %jd, flags: 0x%lx, npages: %d\n",
                                              (intmax_t) bp->b_lblkno,
                                              bp->b_flags, bp->b_npages);
                                  printf(" valid: 0x%x, dirty: 0x%x, wired: %d\n",
                                      m->valid, m->dirty, m->wire_count);
                                  panic("biodone: page busy < 0\n");
                          }
                          vm_page_io_finish(m);
                          vm_object_pip_subtract(obj, 1);
                          foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                          iosize -= resid;
                  }
                  vm_page_unlock_queues();
                  if (obj)
                          vm_object_pip_wakeupn(obj, 0);
          }
  
          /*
           * For asynchronous completions, release the buffer now. The brelse
           * will do a wakeup there if necessary - so no need to do a wakeup
           * here in the async case. The sync case always needs to do a wakeup.
           */
  
          if (bp->b_flags & B_ASYNC) {
                  if ((bp->b_flags & (B_NOCACHE | B_INVAL | B_RELBUF)) || (bp->b_ioflags & BIO_ERROR))
                          brelse(bp);
                  else
                          bqrelse(bp);
          } else {
                  wakeup(bp);
          }
          splx(s);
  }
  
  /*
   * This routine is called in lieu of iodone in the case of
   * incomplete I/O.  This keeps the busy status for pages
   * consistant.
   */
  void
  vfs_unbusy_pages(struct buf * bp)
  {
          int i;
  
          GIANT_REQUIRED;
  
          runningbufwakeup(bp);
          if (bp->b_flags & B_VMIO) {
                  vm_object_t obj;
  
                  obj = bp->b_object;
                  vm_page_lock_queues();
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_t m = bp->b_pages[i];
  
                          if (m == bogus_page) {
                                  m = vm_page_lookup(obj, OFF_TO_IDX(bp->b_offset) + i);
                                  if (!m) {
                                          panic("vfs_unbusy_pages: page missing\n");
                                  }
                                  bp->b_pages[i] = m;
                                  pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_npages);
                          }
                          vm_object_pip_subtract(obj, 1);
                          vm_page_flag_clear(m, PG_ZERO);
                          vm_page_io_finish(m);
                  }
                  vm_page_unlock_queues();
                  vm_object_pip_wakeupn(obj, 0);
          }
  }
  
  /*
   * vfs_page_set_valid:
   *
   *      Set the valid bits in a page based on the supplied offset.   The
   *      range is restricted to the buffer's size.
   *
   *      This routine is typically called after a read completes.
   */
  static void
  vfs_page_set_valid(struct buf *bp, vm_ooffset_t off, int pageno, vm_page_t m)
  {
          vm_ooffset_t soff, eoff;
  
          mtx_assert(&vm_page_queue_mtx, MA_OWNED);
          /*
           * Start and end offsets in buffer.  eoff - soff may not cross a
           * page boundry or cross the end of the buffer.  The end of the
           * buffer, in this case, is our file EOF, not the allocation size
           * of the buffer.
           */
          soff = off;
          eoff = (off + PAGE_SIZE) & ~(off_t)PAGE_MASK;
          if (eoff > bp->b_offset + bp->b_bcount)
                  eoff = bp->b_offset + bp->b_bcount;
  
          /*
           * Set valid range.  This is typically the entire buffer and thus the
           * entire page.
           */
          if (eoff > soff) {
                  vm_page_set_validclean(
                      m,
                     (vm_offset_t) (soff & PAGE_MASK),
                     (vm_offset_t) (eoff - soff)
                  );
          }
  }
  
  /*
   * This routine is called before a device strategy routine.
   * It is used to tell the VM system that paging I/O is in
   * progress, and treat the pages associated with the buffer
   * almost as being PG_BUSY.  Also the object paging_in_progress
   * flag is handled to make sure that the object doesn't become
   * inconsistant.
   *
   * Since I/O has not been initiated yet, certain buffer flags
   * such as BIO_ERROR or B_INVAL may be in an inconsistant state
   * and should be ignored.
   */
  void
  vfs_busy_pages(struct buf * bp, int clear_modify)
  {
          int i, bogus;
  
          if (bp->b_flags & B_VMIO) {
                  vm_object_t obj;
                  vm_ooffset_t foff;
  
                  obj = bp->b_object;
                  foff = bp->b_offset;
                  KASSERT(bp->b_offset != NOOFFSET,
                      ("vfs_busy_pages: no buffer offset"));
                  vfs_setdirty(bp);
  retry:
                  vm_page_lock_queues();
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_t m = bp->b_pages[i];
  
                          if (vm_page_sleep_if_busy(m, FALSE, "vbpage"))
                                  goto retry;
                  }
                  bogus = 0;
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_t m = bp->b_pages[i];
  
                          vm_page_flag_clear(m, PG_ZERO);
                          if ((bp->b_flags & B_CLUSTER) == 0) {
                                  vm_object_pip_add(obj, 1);
                                  vm_page_io_start(m);
                          }
                          /*
                           * When readying a buffer for a read ( i.e
                           * clear_modify == 0 ), it is important to do
                           * bogus_page replacement for valid pages in 
                           * partially instantiated buffers.  Partially 
                           * instantiated buffers can, in turn, occur when
                           * reconstituting a buffer from its VM backing store
                           * base.  We only have to do this if B_CACHE is
                           * clear ( which causes the I/O to occur in the
                           * first place ).  The replacement prevents the read
                           * I/O from overwriting potentially dirty VM-backed
                           * pages.  XXX bogus page replacement is, uh, bogus.
                           * It may not work properly with small-block devices.
                           * We need to find a better way.
                           */
                          pmap_remove_all(m);
                          if (clear_modify)
                                  vfs_page_set_valid(bp, foff, i, m);
                          else if (m->valid == VM_PAGE_BITS_ALL &&
                                  (bp->b_flags & B_CACHE) == 0) {
                                  bp->b_pages[i] = bogus_page;
                                  bogus++;
                          }
                          foff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                  }
                  vm_page_unlock_queues();
                  if (bogus)
                          pmap_qenter(trunc_page((vm_offset_t)bp->b_data), bp->b_pages, bp->b_npages);
          }
  }
  
  /*
   * Tell the VM system that the pages associated with this buffer
   * are clean.  This is used for delayed writes where the data is
   * going to go to disk eventually without additional VM intevention.
   *
   * Note that while we only really need to clean through to b_bcount, we
   * just go ahead and clean through to b_bufsize.
   */
  static void
  vfs_clean_pages(struct buf * bp)
  {
          int i;
  
          GIANT_REQUIRED;
  
          if (bp->b_flags & B_VMIO) {
                  vm_ooffset_t foff;
  
                  foff = bp->b_offset;
                  KASSERT(bp->b_offset != NOOFFSET,
                      ("vfs_clean_pages: no buffer offset"));
                  vm_page_lock_queues();
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_t m = bp->b_pages[i];
                          vm_ooffset_t noff = (foff + PAGE_SIZE) & ~(off_t)PAGE_MASK;
                          vm_ooffset_t eoff = noff;
  
                          if (eoff > bp->b_offset + bp->b_bufsize)
                                  eoff = bp->b_offset + bp->b_bufsize;
                          vfs_page_set_valid(bp, foff, i, m);
                          /* vm_page_clear_dirty(m, foff & PAGE_MASK, eoff - foff); */
                          foff = noff;
                  }
                  vm_page_unlock_queues();
          }
  }
  
  /*
   *      vfs_bio_set_validclean:
   *
   *      Set the range within the buffer to valid and clean.  The range is 
   *      relative to the beginning of the buffer, b_offset.  Note that b_offset
   *      itself may be offset from the beginning of the first page.
   *
   */
  
  void   
  vfs_bio_set_validclean(struct buf *bp, int base, int size)
  {
          if (bp->b_flags & B_VMIO) {
                  int i;
                  int n;
  
                  /*
                   * Fixup base to be relative to beginning of first page.
                   * Set initial n to be the maximum number of bytes in the
                   * first page that can be validated.
                   */
  
                  base += (bp->b_offset & PAGE_MASK);
                  n = PAGE_SIZE - (base & PAGE_MASK);
  
                  vm_page_lock_queues();
                  for (i = base / PAGE_SIZE; size > 0 && i < bp->b_npages; ++i) {
                          vm_page_t m = bp->b_pages[i];
  
                          if (n > size)
                                  n = size;
  
                          vm_page_set_validclean(m, base & PAGE_MASK, n);
                          base += n;
                          size -= n;
                          n = PAGE_SIZE;
                  }
                  vm_page_unlock_queues();
          }
  }
  
  /*
   *      vfs_bio_clrbuf:
   *
   *      clear a buffer.  This routine essentially fakes an I/O, so we need
   *      to clear BIO_ERROR and B_INVAL.
   *
   *      Note that while we only theoretically need to clear through b_bcount,
   *      we go ahead and clear through b_bufsize.
   */
  
  void
  vfs_bio_clrbuf(struct buf *bp) 
  {
          int i, mask = 0;
          caddr_t sa, ea;
  
          GIANT_REQUIRED;
  
          if ((bp->b_flags & (B_VMIO | B_MALLOC)) == B_VMIO) {
                  bp->b_flags &= ~B_INVAL;
                  bp->b_ioflags &= ~BIO_ERROR;
                  if( (bp->b_npages == 1) && (bp->b_bufsize < PAGE_SIZE) &&
                      (bp->b_offset & PAGE_MASK) == 0) {
                          mask = (1 << (bp->b_bufsize / DEV_BSIZE)) - 1;
                          if ((bp->b_pages[0]->valid & mask) == mask) {
                                  bp->b_resid = 0;
                                  return;
                          }
                          if (((bp->b_pages[0]->flags & PG_ZERO) == 0) &&
                              ((bp->b_pages[0]->valid & mask) == 0)) {
                                  bzero(bp->b_data, bp->b_bufsize);
                                  bp->b_pages[0]->valid |= mask;
                                  bp->b_resid = 0;
                                  return;
                          }
                  }
                  ea = sa = bp->b_data;
                  for(i=0;i<bp->b_npages;i++,sa=ea) {
                          int j = ((vm_offset_t)sa & PAGE_MASK) / DEV_BSIZE;
                          ea = (caddr_t)trunc_page((vm_offset_t)sa + PAGE_SIZE);
                          ea = (caddr_t)(vm_offset_t)ulmin(
                              (u_long)(vm_offset_t)ea,
                              (u_long)(vm_offset_t)bp->b_data + bp->b_bufsize);
                          mask = ((1 << ((ea - sa) / DEV_BSIZE)) - 1) << j;
                          if ((bp->b_pages[i]->valid & mask) == mask)
                                  continue;
                          if ((bp->b_pages[i]->valid & mask) == 0) {
                                  if ((bp->b_pages[i]->flags & PG_ZERO) == 0) {
                                          bzero(sa, ea - sa);
                                  }
                          } else {
                                  for (; sa < ea; sa += DEV_BSIZE, j++) {
                                          if (((bp->b_pages[i]->flags & PG_ZERO) == 0) &&
                                                  (bp->b_pages[i]->valid & (1<<j)) == 0)
                                                  bzero(sa, DEV_BSIZE);
                                  }
                          }
                          bp->b_pages[i]->valid |= mask;
                          vm_page_flag_clear(bp->b_pages[i], PG_ZERO);
                  }
                  bp->b_resid = 0;
          } else {
                  clrbuf(bp);
          }
  }
  
  /*
   * vm_hold_load_pages and vm_hold_free_pages get pages into
   * a buffers address space.  The pages are anonymous and are
   * not associated with a file object.
   */
  static void
  vm_hold_load_pages(struct buf * bp, vm_offset_t from, vm_offset_t to)
  {
          vm_offset_t pg;
          vm_page_t p;
          int index;
  
          GIANT_REQUIRED;
  
          to = round_page(to);
          from = round_page(from);
          index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
  
          for (pg = from; pg < to; pg += PAGE_SIZE, index++) {
  tryagain:
                  /*
                   * note: must allocate system pages since blocking here
                   * could intefere with paging I/O, no matter which
                   * process we are.
                   */
                  p = vm_page_alloc(kernel_object,
                          ((pg - VM_MIN_KERNEL_ADDRESS) >> PAGE_SHIFT),
                      VM_ALLOC_SYSTEM | VM_ALLOC_WIRED);
                  if (!p) {
                          vm_pageout_deficit += (to - from) >> PAGE_SHIFT;
                          VM_WAIT;
                          goto tryagain;
                  }
                  vm_page_lock_queues();
                  p->valid = VM_PAGE_BITS_ALL;
                  vm_page_flag_clear(p, PG_ZERO);
                  vm_page_unlock_queues();
                  pmap_qenter(pg, &p, 1);
                  bp->b_pages[index] = p;
                  vm_page_wakeup(p);
          }
          bp->b_npages = index;
  }
  
  /* Return pages associated with this buf to the vm system */
  static 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;
  
          GIANT_REQUIRED;
  
          from = round_page(from);
          to = round_page(to);
          newnpages = index = (from - trunc_page((vm_offset_t)bp->b_data)) >> PAGE_SHIFT;
  
          for (pg = from; pg < to; pg += PAGE_SIZE, index++) {
                  p = bp->b_pages[index];
                  if (p && (index < bp->b_npages)) {
                          if (p->busy) {
                                  printf(
                              "vm_hold_free_pages: blkno: %jd, lblkno: %jd\n",
                                      (intmax_t)bp->b_blkno,
                                      (intmax_t)bp->b_lblkno);
                          }
                          bp->b_pages[index] = NULL;
                          pmap_qremove(pg, 1);
                          vm_page_lock_queues();
                          vm_page_busy(p);
                          vm_page_unwire(p, 0);
                          vm_page_free(p);
                          vm_page_unlock_queues();
                  }
          }
          bp->b_npages = newnpages;
  }
  
  
  #include "opt_ddb.h"
  #ifdef DDB
  #include <ddb/ddb.h>
  
  /* DDB command to show buffer data */
  DB_SHOW_COMMAND(buffer, db_show_buffer)
  {
          /* get args */
          struct buf *bp = (struct buf *)addr;
  
          if (!have_addr) {
                  db_printf("usage: show buffer <addr>\n");
                  return;
          }
  
          db_printf("b_flags = 0x%b\n", (u_int)bp->b_flags, PRINT_BUF_FLAGS);
          db_printf(
              "b_error = %d, b_bufsize = %ld, b_bcount = %ld, b_resid = %ld\n"
              "b_dev = (%d,%d), b_data = %p, b_blkno = %jd, b_pblkno = %jd\n",
              bp->b_error, bp->b_bufsize, bp->b_bcount, bp->b_resid,
              major(bp->b_dev), minor(bp->b_dev), bp->b_data,
              (intmax_t)bp->b_blkno, (intmax_t)bp->b_pblkno);
          if (bp->b_npages) {
                  int i;
                  db_printf("b_npages = %d, pages(OBJ, IDX, PA): ", bp->b_npages);
                  for (i = 0; i < bp->b_npages; i++) {
                          vm_page_t m;
                          m = bp->b_pages[i];
                          db_printf("(%p, 0x%lx, 0x%lx)", (void *)m->object,
                              (u_long)m->pindex, (u_long)VM_PAGE_TO_PHYS(m));
                          if ((i + 1) < bp->b_npages)
                                  db_printf(",");
                  }
                  db_printf("\n");
          }
  }
  #endif /* DDB */