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

     /*      $NetBSD: vfs_bio.c,v 1.167.2.1 2007/10/24 22:32:39 xtraeme Exp $        */
     
     /*-
      * Copyright (c) 1982, 1986, 1989, 1993
      *      The Regents of the University of California.  All rights reserved.
      * (c) UNIX System Laboratories, Inc.
      * All or some portions of this file are derived from material licensed
      * to the University of California by American Telephone and Telegraph
      * Co. or Unix System Laboratories, Inc. and are reproduced herein with
     * the permission of UNIX System Laboratories, Inc.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)vfs_bio.c   8.6 (Berkeley) 1/11/94
     */
    
    /*-
     * Copyright (c) 1994 Christopher G. Demetriou
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgement:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)vfs_bio.c   8.6 (Berkeley) 1/11/94
     */
    
    /*
     * Some references:
     *      Bach: The Design of the UNIX Operating System (Prentice Hall, 1986)
     *      Leffler, et al.: The Design and Implementation of the 4.3BSD
     *              UNIX Operating System (Addison Welley, 1989)
     */
    
    #include "fs_ffs.h"
    #include "opt_bufcache.h"
    #include "opt_softdep.h"
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: vfs_bio.c,v 1.167.2.1 2007/10/24 22:32:39 xtraeme Exp $");
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/proc.h>
    #include <sys/buf.h>
    #include <sys/vnode.h>
    #include <sys/mount.h>
    #include <sys/malloc.h>
    #include <sys/resourcevar.h>
    #include <sys/sysctl.h>
    #include <sys/conf.h>
    #include <sys/kauth.h>
    
   #include <uvm/uvm.h>
   
   #include <miscfs/specfs/specdev.h>
   
   #ifndef BUFPAGES
   # define BUFPAGES 0
   #endif
   
   #ifdef BUFCACHE
   # if (BUFCACHE < 5) || (BUFCACHE > 95)
   #  error BUFCACHE is not between 5 and 95
   # endif
   #else
   # define BUFCACHE 15
   #endif
   
   u_int   nbuf;                   /* XXX - for softdep_lockedbufs */
   u_int   bufpages = BUFPAGES;    /* optional hardwired count */
   u_int   bufcache = BUFCACHE;    /* max % of RAM to use for buffer cache */
   
   /* Function prototypes */
   struct bqueue;
   
   static void buf_setwm(void);
   static int buf_trim(void);
   static void *bufpool_page_alloc(struct pool *, int);
   static void bufpool_page_free(struct pool *, void *);
   static inline struct buf *bio_doread(struct vnode *, daddr_t, int,
       kauth_cred_t, int);
   static struct buf *getnewbuf(int, int, int);
   static int buf_lotsfree(void);
   static int buf_canrelease(void);
   static inline u_long buf_mempoolidx(u_long);
   static inline u_long buf_roundsize(u_long);
   static inline caddr_t buf_malloc(size_t);
   static void buf_mrelease(caddr_t, size_t);
   static inline void binsheadfree(struct buf *, struct bqueue *);
   static inline void binstailfree(struct buf *, struct bqueue *);
   int count_lock_queue(void); /* XXX */
   #ifdef DEBUG
   static int checkfreelist(struct buf *, struct bqueue *);
   #endif
   
   /*
    * Definitions for the buffer hash lists.
    */
   #define BUFHASH(dvp, lbn)       \
           (&bufhashtbl[(((long)(dvp) >> 8) + (int)(lbn)) & bufhash])
   LIST_HEAD(bufhashhdr, buf) *bufhashtbl, invalhash;
   u_long  bufhash;
   #if !defined(SOFTDEP) || !defined(FFS)
   struct bio_ops bioops;  /* I/O operation notification */
   #endif
   
   /*
    * Insq/Remq for the buffer hash lists.
    */
   #define binshash(bp, dp)        LIST_INSERT_HEAD(dp, bp, b_hash)
   #define bremhash(bp)            LIST_REMOVE(bp, b_hash)
   
   /*
    * Definitions for the buffer free lists.
    */
   #define BQUEUES         3               /* number of free buffer queues */
   
   #define BQ_LOCKED       0               /* super-blocks &c */
   #define BQ_LRU          1               /* lru, useful buffers */
   #define BQ_AGE          2               /* rubbish */
   
   struct bqueue {
           TAILQ_HEAD(, buf) bq_queue;
           uint64_t bq_bytes;
   } bufqueues[BQUEUES];
   int needbuffer;
   
   /*
    * Buffer queue lock.
    * Take this lock first if also taking some buffer's b_interlock.
    */
   struct simplelock bqueue_slock = SIMPLELOCK_INITIALIZER;
   
   /*
    * Buffer pool for I/O buffers.
    */
   static POOL_INIT(bufpool, sizeof(struct buf), 0, 0, 0, "bufpl",
       &pool_allocator_nointr);
   
   
   /* XXX - somewhat gross.. */
   #if MAXBSIZE == 0x2000
   #define NMEMPOOLS 5
   #elif MAXBSIZE == 0x4000
   #define NMEMPOOLS 6
   #elif MAXBSIZE == 0x8000
   #define NMEMPOOLS 7
   #else
   #define NMEMPOOLS 8
   #endif
   
   #define MEMPOOL_INDEX_OFFSET 9  /* smallest pool is 512 bytes */
   #if (1 << (NMEMPOOLS + MEMPOOL_INDEX_OFFSET - 1)) != MAXBSIZE
   #error update vfs_bio buffer memory parameters
   #endif
   
   /* Buffer memory pools */
   static struct pool bmempools[NMEMPOOLS];
   
   struct vm_map *buf_map;
   
   /*
    * Buffer memory pool allocator.
    */
   static void *
   bufpool_page_alloc(struct pool *pp, int flags)
   {
   
           return (void *)uvm_km_alloc(buf_map,
               MAXBSIZE, MAXBSIZE,
               ((flags & PR_WAITOK) ? 0 : UVM_KMF_NOWAIT | UVM_KMF_TRYLOCK)
               | UVM_KMF_WIRED);
   }
   
   static void
   bufpool_page_free(struct pool *pp, void *v)
   {
   
           uvm_km_free(buf_map, (vaddr_t)v, MAXBSIZE, UVM_KMF_WIRED);
   }
   
   static struct pool_allocator bufmempool_allocator = {
           .pa_alloc = bufpool_page_alloc,
           .pa_free = bufpool_page_free,
           .pa_pagesz = MAXBSIZE,
   };
   
   /* Buffer memory management variables */
   uint64_t bufmem_valimit;
   uint64_t bufmem_hiwater;
   uint64_t bufmem_lowater;
   uint64_t bufmem;
   
   /*
    * MD code can call this to set a hard limit on the amount
    * of virtual memory used by the buffer cache.
    */
   int
   buf_setvalimit(vsize_t sz)
   {
   
           /* We need to accommodate at least NMEMPOOLS of MAXBSIZE each */
           if (sz < NMEMPOOLS * MAXBSIZE)
                   return EINVAL;
   
           bufmem_valimit = sz;
           return 0;
   }
   
   static void
   buf_setwm(void)
   {
   
           bufmem_hiwater = buf_memcalc();
           /* lowater is approx. 2% of memory (with bufcache = 15) */
   #define BUFMEM_WMSHIFT  3
   #define BUFMEM_HIWMMIN  (64 * 1024 << BUFMEM_WMSHIFT)
           if (bufmem_hiwater < BUFMEM_HIWMMIN)
                   /* Ensure a reasonable minimum value */
                   bufmem_hiwater = BUFMEM_HIWMMIN;
           bufmem_lowater = bufmem_hiwater >> BUFMEM_WMSHIFT;
   }
   
   #ifdef DEBUG
   int debug_verify_freelist = 0;
   static int
   checkfreelist(struct buf *bp, struct bqueue *dp)
   {
           struct buf *b;
   
           TAILQ_FOREACH(b, &dp->bq_queue, b_freelist) {
                   if (b == bp)
                           return 1;
           }
           return 0;
   }
   #endif
   
   /*
    * Insq/Remq for the buffer hash lists.
    * Call with buffer queue locked.
    */
   static inline void
   binsheadfree(struct buf *bp, struct bqueue *dp)
   {
   
           KASSERT(bp->b_freelistindex == -1);
           TAILQ_INSERT_HEAD(&dp->bq_queue, bp, b_freelist);
           dp->bq_bytes += bp->b_bufsize;
           bp->b_freelistindex = dp - bufqueues;
   }
   
   static inline void
   binstailfree(struct buf *bp, struct bqueue *dp)
   {
   
           KASSERT(bp->b_freelistindex == -1);
           TAILQ_INSERT_TAIL(&dp->bq_queue, bp, b_freelist);
           dp->bq_bytes += bp->b_bufsize;
           bp->b_freelistindex = dp - bufqueues;
   }
   
   void
   bremfree(struct buf *bp)
   {
           struct bqueue *dp;
           int bqidx = bp->b_freelistindex;
   
           LOCK_ASSERT(simple_lock_held(&bqueue_slock));
   
           KASSERT(bqidx != -1);
           dp = &bufqueues[bqidx];
           KDASSERT(!debug_verify_freelist || checkfreelist(bp, dp));
           KASSERT(dp->bq_bytes >= bp->b_bufsize);
           TAILQ_REMOVE(&dp->bq_queue, bp, b_freelist);
           dp->bq_bytes -= bp->b_bufsize;
   #if defined(DIAGNOSTIC)
           bp->b_freelistindex = -1;
   #endif /* defined(DIAGNOSTIC) */
   }
   
   u_long
   buf_memcalc(void)
   {
           u_long n;
   
           /*
            * Determine the upper bound of memory to use for buffers.
            *
            *      - If bufpages is specified, use that as the number
            *        pages.
            *
            *      - Otherwise, use bufcache as the percentage of
            *        physical memory.
            */
           if (bufpages != 0) {
                   n = bufpages;
           } else {
                   if (bufcache < 5) {
                           printf("forcing bufcache %d -> 5", bufcache);
                           bufcache = 5;
                   }
                   if (bufcache > 95) {
                           printf("forcing bufcache %d -> 95", bufcache);
                           bufcache = 95;
                   }
                   n = physmem / 100 * bufcache;
           }
   
           n <<= PAGE_SHIFT;
           if (bufmem_valimit != 0 && n > bufmem_valimit)
                   n = bufmem_valimit;
   
           return (n);
   }
   
   /*
    * Initialize buffers and hash links for buffers.
    */
   void
   bufinit(void)
   {
           struct bqueue *dp;
           int use_std;
           u_int i;
   
           /*
            * Initialize buffer cache memory parameters.
            */
           bufmem = 0;
           buf_setwm();
   
           if (bufmem_valimit != 0) {
                   vaddr_t minaddr = 0, maxaddr;
                   buf_map = uvm_km_suballoc(kernel_map, &minaddr, &maxaddr,
                                             bufmem_valimit, 0, FALSE, 0);
                   if (buf_map == NULL)
                           panic("bufinit: cannot allocate submap");
           } else
                   buf_map = kernel_map;
   
           /* On "small" machines use small pool page sizes where possible */
           use_std = (physmem < atop(16*1024*1024));
   
           /*
            * Also use them on systems that can map the pool pages using
            * a direct-mapped segment.
            */
   #ifdef PMAP_MAP_POOLPAGE
           use_std = 1;
   #endif
   
           bufmempool_allocator.pa_backingmap = buf_map;
           for (i = 0; i < NMEMPOOLS; i++) {
                   struct pool_allocator *pa;
                   struct pool *pp = &bmempools[i];
                   u_int size = 1 << (i + MEMPOOL_INDEX_OFFSET);
                   char *name = malloc(8, M_TEMP, M_WAITOK);
                   if (__predict_true(size >= 1024))
                           (void)snprintf(name, 8, "buf%dk", size / 1024);
                   else
                           (void)snprintf(name, 8, "buf%db", size);
                   pa = (size <= PAGE_SIZE && use_std)
                           ? &pool_allocator_nointr
                           : &bufmempool_allocator;
                   pool_init(pp, size, 0, 0, 0, name, pa);
                   pool_setlowat(pp, 1);
                   pool_sethiwat(pp, 1);
           }
   
           /* Initialize the buffer queues */
           for (dp = bufqueues; dp < &bufqueues[BQUEUES]; dp++) {
                   TAILQ_INIT(&dp->bq_queue);
                   dp->bq_bytes = 0;
           }
   
           /*
            * Estimate hash table size based on the amount of memory we
            * intend to use for the buffer cache. The average buffer
            * size is dependent on our clients (i.e. filesystems).
            *
            * For now, use an empirical 3K per buffer.
            */
           nbuf = (bufmem_hiwater / 1024) / 3;
           bufhashtbl = hashinit(nbuf, HASH_LIST, M_CACHE, M_WAITOK, &bufhash);
   }
   
   static int
   buf_lotsfree(void)
   {
           int try, thresh;
           struct lwp *l = curlwp;
   
           /* Always allocate if doing copy on write */
           if (l->l_flag & L_COWINPROGRESS)
                   return 1;
   
           /* Always allocate if less than the low water mark. */
           if (bufmem < bufmem_lowater)
                   return 1;
   
           /* Never allocate if greater than the high water mark. */
           if (bufmem > bufmem_hiwater)
                   return 0;
   
           /* If there's anything on the AGE list, it should be eaten. */
           if (TAILQ_FIRST(&bufqueues[BQ_AGE].bq_queue) != NULL)
                   return 0;
   
           /*
            * The probabily of getting a new allocation is inversely
            * proportional to the current size of the cache, using
            * a granularity of 16 steps.
            */
           try = random() & 0x0000000fL;
   
           /* Don't use "16 * bufmem" here to avoid a 32-bit overflow. */
           thresh = (bufmem - bufmem_lowater) /
               ((bufmem_hiwater - bufmem_lowater) / 16);
   
           if (try >= thresh)
                   return 1;
   
           /* Otherwise don't allocate. */
           return 0;
   }
   
   /*
    * Return estimate of bytes we think need to be
    * released to help resolve low memory conditions.
    *
    * => called at splbio.
    * => called with bqueue_slock held.
    */
   static int
   buf_canrelease(void)
   {
           int pagedemand, ninvalid = 0;
   
           LOCK_ASSERT(simple_lock_held(&bqueue_slock));
   
           if (bufmem < bufmem_lowater)
                   return 0;
   
           if (bufmem > bufmem_hiwater)
                   return bufmem - bufmem_hiwater;
   
           ninvalid += bufqueues[BQ_AGE].bq_bytes;
   
           pagedemand = uvmexp.freetarg - uvmexp.free;
           if (pagedemand < 0)
                   return ninvalid;
           return MAX(ninvalid, MIN(2 * MAXBSIZE,
               MIN((bufmem - bufmem_lowater) / 16, pagedemand * PAGE_SIZE)));
   }
   
   /*
    * Buffer memory allocation helper functions
    */
   static inline u_long
   buf_mempoolidx(u_long size)
   {
           u_int n = 0;
   
           size -= 1;
           size >>= MEMPOOL_INDEX_OFFSET;
           while (size) {
                   size >>= 1;
                   n += 1;
           }
           if (n >= NMEMPOOLS)
                   panic("buf mem pool index %d", n);
           return n;
   }
   
   static inline u_long
   buf_roundsize(u_long size)
   {
           /* Round up to nearest power of 2 */
           return (1 << (buf_mempoolidx(size) + MEMPOOL_INDEX_OFFSET));
   }
   
   static inline caddr_t
   buf_malloc(size_t size)
   {
           u_int n = buf_mempoolidx(size);
           caddr_t addr;
           int s;
   
           while (1) {
                   addr = pool_get(&bmempools[n], PR_NOWAIT);
                   if (addr != NULL)
                           break;
   
                   /* No memory, see if we can free some. If so, try again */
                   if (buf_drain(1) > 0)
                           continue;
   
                   /* Wait for buffers to arrive on the LRU queue */
                   s = splbio();
                   simple_lock(&bqueue_slock);
                   needbuffer = 1;
                   ltsleep(&needbuffer, PNORELOCK | (PRIBIO + 1),
                           "buf_malloc", 0, &bqueue_slock);
                   splx(s);
           }
   
           return addr;
   }
   
   static void
   buf_mrelease(caddr_t addr, size_t size)
   {
   
           pool_put(&bmempools[buf_mempoolidx(size)], addr);
   }
   
   /*
    * bread()/breadn() helper.
    */
   static inline struct buf *
   bio_doread(struct vnode *vp, daddr_t blkno, int size, kauth_cred_t cred,
       int async)
   {
           struct buf *bp;
           struct lwp *l  = (curlwp != NULL ? curlwp : &lwp0);     /* XXX */
           struct proc *p = l->l_proc;
           struct mount *mp;
   
           bp = getblk(vp, blkno, size, 0, 0);
   
   #ifdef DIAGNOSTIC
           if (bp == NULL) {
                   panic("bio_doread: no such buf");
           }
   #endif
   
           /*
            * If buffer does not have data valid, start a read.
            * Note that if buffer is B_INVAL, getblk() won't return it.
            * Therefore, it's valid if its I/O has completed or been delayed.
            */
           if (!ISSET(bp->b_flags, (B_DONE | B_DELWRI))) {
                   /* Start I/O for the buffer. */
                   SET(bp->b_flags, B_READ | async);
                   if (async)
                           BIO_SETPRIO(bp, BPRIO_TIMELIMITED);
                   else
                           BIO_SETPRIO(bp, BPRIO_TIMECRITICAL);
                   VOP_STRATEGY(vp, bp);
   
                   /* Pay for the read. */
                   p->p_stats->p_ru.ru_inblock++;
           } else if (async) {
                   brelse(bp);
           }
   
           if (vp->v_type == VBLK)
                   mp = vp->v_specmountpoint;
           else
                   mp = vp->v_mount;
   
           /*
            * Collect statistics on synchronous and asynchronous reads.
            * Reads from block devices are charged to their associated
            * filesystem (if any).
            */
           if (mp != NULL) {
                   if (async == 0)
                           mp->mnt_stat.f_syncreads++;
                   else
                           mp->mnt_stat.f_asyncreads++;
           }
   
           return (bp);
   }
   
   /*
    * Read a disk block.
    * This algorithm described in Bach (p.54).
    */
   int
   bread(struct vnode *vp, daddr_t blkno, int size, kauth_cred_t cred,
       struct buf **bpp)
   {
           struct buf *bp;
   
           /* Get buffer for block. */
           bp = *bpp = bio_doread(vp, blkno, size, cred, 0);
   
           /* Wait for the read to complete, and return result. */
           return (biowait(bp));
   }
   
   /*
    * Read-ahead multiple disk blocks. The first is sync, the rest async.
    * Trivial modification to the breada algorithm presented in Bach (p.55).
    */
   int
   breadn(struct vnode *vp, daddr_t blkno, int size, daddr_t *rablks,
       int *rasizes, int nrablks, kauth_cred_t cred, struct buf **bpp)
   {
           struct buf *bp;
           int i;
   
           bp = *bpp = bio_doread(vp, blkno, size, cred, 0);
   
           /*
            * For each of the read-ahead blocks, start a read, if necessary.
            */
           for (i = 0; i < nrablks; i++) {
                   /* If it's in the cache, just go on to next one. */
                   if (incore(vp, rablks[i]))
                           continue;
   
                   /* Get a buffer for the read-ahead block */
                   (void) bio_doread(vp, rablks[i], rasizes[i], cred, B_ASYNC);
           }
   
           /* Otherwise, we had to start a read for it; wait until it's valid. */
           return (biowait(bp));
   }
   
   /*
    * Read with single-block read-ahead.  Defined in Bach (p.55), but
    * implemented as a call to breadn().
    * XXX for compatibility with old file systems.
    */
   int
   breada(struct vnode *vp, daddr_t blkno, int size, daddr_t rablkno,
       int rabsize, kauth_cred_t cred, struct buf **bpp)
   {
   
           return (breadn(vp, blkno, size, &rablkno, &rabsize, 1, cred, bpp));
   }
   
   /*
    * Block write.  Described in Bach (p.56)
    */
   int
   bwrite(struct buf *bp)
   {
           int rv, sync, wasdelayed, s;
           struct lwp *l  = (curlwp != NULL ? curlwp : &lwp0);     /* XXX */
           struct proc *p = l->l_proc;
           struct vnode *vp;
           struct mount *mp;
   
           KASSERT(ISSET(bp->b_flags, B_BUSY));
   
           vp = bp->b_vp;
           if (vp != NULL) {
                   if (vp->v_type == VBLK)
                           mp = vp->v_specmountpoint;
                   else
                           mp = vp->v_mount;
           } else {
                   mp = NULL;
           }
   
           /*
            * Remember buffer type, to switch on it later.  If the write was
            * synchronous, but the file system was mounted with MNT_ASYNC,
            * convert it to a delayed write.
            * XXX note that this relies on delayed tape writes being converted
            * to async, not sync writes (which is safe, but ugly).
            */
           sync = !ISSET(bp->b_flags, B_ASYNC);
           if (sync && mp != NULL && ISSET(mp->mnt_flag, MNT_ASYNC)) {
                   bdwrite(bp);
                   return (0);
           }
   
           /*
            * Collect statistics on synchronous and asynchronous writes.
            * Writes to block devices are charged to their associated
            * filesystem (if any).
            */
           if (mp != NULL) {
                   if (sync)
                           mp->mnt_stat.f_syncwrites++;
                   else
                           mp->mnt_stat.f_asyncwrites++;
           }
   
           s = splbio();
           simple_lock(&bp->b_interlock);
   
           wasdelayed = ISSET(bp->b_flags, B_DELWRI);
   
           CLR(bp->b_flags, (B_READ | B_DONE | B_ERROR | B_DELWRI));
   
           /*
            * Pay for the I/O operation and make sure the buf is on the correct
            * vnode queue.
            */
           if (wasdelayed)
                   reassignbuf(bp, bp->b_vp);
           else
                   p->p_stats->p_ru.ru_oublock++;
   
           /* Initiate disk write.  Make sure the appropriate party is charged. */
           V_INCR_NUMOUTPUT(bp->b_vp);
           simple_unlock(&bp->b_interlock);
           splx(s);
   
           if (sync)
                   BIO_SETPRIO(bp, BPRIO_TIMECRITICAL);
           else
                   BIO_SETPRIO(bp, BPRIO_TIMELIMITED);
   
           VOP_STRATEGY(vp, bp);
   
           if (sync) {
                   /* If I/O was synchronous, wait for it to complete. */
                   rv = biowait(bp);
   
                   /* Release the buffer. */
                   brelse(bp);
   
                   return (rv);
           } else {
                   return (0);
           }
   }
   
   int
   vn_bwrite(void *v)
   {
           struct vop_bwrite_args *ap = v;
   
           return (bwrite(ap->a_bp));
   }
   
   /*
    * Delayed write.
    *
    * The buffer is marked dirty, but is not queued for I/O.
    * This routine should be used when the buffer is expected
    * to be modified again soon, typically a small write that
    * partially fills a buffer.
    *
    * NB: magnetic tapes cannot be delayed; they must be
    * written in the order that the writes are requested.
    *
    * Described in Leffler, et al. (pp. 208-213).
    */
   void
   bdwrite(struct buf *bp)
   {
           struct lwp *l  = (curlwp != NULL ? curlwp : &lwp0);     /* XXX */
           struct proc *p = l->l_proc;
           const struct bdevsw *bdev;
           int s;
   
           /* If this is a tape block, write the block now. */
           bdev = bdevsw_lookup(bp->b_dev);
           if (bdev != NULL && bdev->d_type == D_TAPE) {
                   bawrite(bp);
                   return;
           }
   
           /*
            * If the block hasn't been seen before:
            *      (1) Mark it as having been seen,
            *      (2) Charge for the write,
            *      (3) Make sure it's on its vnode's correct block list.
            */
           s = splbio();
           simple_lock(&bp->b_interlock);
   
           KASSERT(ISSET(bp->b_flags, B_BUSY));
   
           if (!ISSET(bp->b_flags, B_DELWRI)) {
                   SET(bp->b_flags, B_DELWRI);
                   p->p_stats->p_ru.ru_oublock++;
                   reassignbuf(bp, bp->b_vp);
           }
   
           /* Otherwise, the "write" is done, so mark and release the buffer. */
           CLR(bp->b_flags, B_DONE);
           simple_unlock(&bp->b_interlock);
           splx(s);
   
           brelse(bp);
   }
   
   /*
    * Asynchronous block write; just an asynchronous bwrite().
    */
   void
   bawrite(struct buf *bp)
   {
           int s;
   
           s = splbio();
           simple_lock(&bp->b_interlock);
   
           KASSERT(ISSET(bp->b_flags, B_BUSY));
   
           SET(bp->b_flags, B_ASYNC);
           simple_unlock(&bp->b_interlock);
           splx(s);
           VOP_BWRITE(bp);
   }
   
   /*
    * Same as first half of bdwrite, mark buffer dirty, but do not release it.
    * Call at splbio() and with the buffer interlock locked.
    * Note: called only from biodone() through ffs softdep's bioops.io_complete()
    */
   void
   bdirty(struct buf *bp)
   {
           struct lwp *l  = (curlwp != NULL ? curlwp : &lwp0);     /* XXX */
           struct proc *p = l->l_proc;
   
           LOCK_ASSERT(simple_lock_held(&bp->b_interlock));
           KASSERT(ISSET(bp->b_flags, B_BUSY));
   
           CLR(bp->b_flags, B_AGE);
   
           if (!ISSET(bp->b_flags, B_DELWRI)) {
                   SET(bp->b_flags, B_DELWRI);
                   p->p_stats->p_ru.ru_oublock++;
                   reassignbuf(bp, bp->b_vp);
           }
   }
   
   /*
    * Release a buffer on to the free lists.
    * Described in Bach (p. 46).
    */
   void
   brelse(struct buf *bp)
   {
           struct bqueue *bufq;
           int s;
   
           /* Block disk interrupts. */
           s = splbio();
           simple_lock(&bqueue_slock);
           simple_lock(&bp->b_interlock);
   
           KASSERT(ISSET(bp->b_flags, B_BUSY));
           KASSERT(!ISSET(bp->b_flags, B_CALL));
   
           /* Wake up any processes waiting for any buffer to become free. */
           if (needbuffer) {
                   needbuffer = 0;
                   wakeup(&needbuffer);
           }
   
           /* Wake up any proceeses waiting for _this_ buffer to become free. */
           if (ISSET(bp->b_flags, B_WANTED)) {
                   CLR(bp->b_flags, B_WANTED|B_AGE);
                   wakeup(bp);
           }
   
           /*
            * Determine which queue the buffer should be on, then put it there.
            */
   
           /* If it's locked, don't report an error; try again later. */
           if (ISSET(bp->b_flags, (B_LOCKED|B_ERROR)) == (B_LOCKED|B_ERROR))
                   CLR(bp->b_flags, B_ERROR);
   
           /* If it's not cacheable, or an error, mark it invalid. */
           if (ISSET(bp->b_flags, (B_NOCACHE|B_ERROR)))
                   SET(bp->b_flags, B_INVAL);
   
           if (ISSET(bp->b_flags, B_VFLUSH)) {
                   /*
                    * This is a delayed write buffer that was just flushed to
                    * disk.  It is still on the LRU queue.  If it's become
                    * invalid, then we need to move it to a different queue;
                    * otherwise leave it in its current position.
                    */
                   CLR(bp->b_flags, B_VFLUSH);
                   if (!ISSET(bp->b_flags, B_ERROR|B_INVAL|B_LOCKED|B_AGE)) {
                           KDASSERT(!debug_verify_freelist || checkfreelist(bp, &bufqueues[BQ_LRU]));
                           goto already_queued;
                   } else {
                           bremfree(bp);
                   }
           }
   
     KDASSERT(!debug_verify_freelist || !checkfreelist(bp, &bufqueues[BQ_AGE]));
     KDASSERT(!debug_verify_freelist || !checkfreelist(bp, &bufqueues[BQ_LRU]));
     KDASSERT(!debug_verify_freelist || !checkfreelist(bp, &bufqueues[BQ_LOCKED]));
   
           if ((bp->b_bufsize <= 0) || ISSET(bp->b_flags, B_INVAL)) {
                   /*
                    * If it's invalid or empty, dissociate it from its vnode
                    * and put on the head of the appropriate queue.
                    */
                   if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_deallocate)
                           (*bioops.io_deallocate)(bp);
                   CLR(bp->b_flags, B_DONE|B_DELWRI);
                   if (bp->b_vp) {
                           reassignbuf(bp, bp->b_vp);
                           brelvp(bp);
                   }
                   if (bp->b_bufsize <= 0)
                           /* no data */
                           goto already_queued;
                   else
                           /* invalid data */
                           bufq = &bufqueues[BQ_AGE];
                   binsheadfree(bp, bufq);
           } else {
                   /*
                    * It has valid data.  Put it on the end of the appropriate
                    * queue, so that it'll stick around for as long as possible.
                    * If buf is AGE, but has dependencies, must put it on last
                    * bufqueue to be scanned, ie LRU. This protects against the
                    * livelock where BQ_AGE only has buffers with dependencies,
                    * and we thus never get to the dependent buffers in BQ_LRU.
                    */
                   if (ISSET(bp->b_flags, B_LOCKED))
                           /* locked in core */
                           bufq = &bufqueues[BQ_LOCKED];
                   else if (!ISSET(bp->b_flags, B_AGE))
                           /* valid data */
                           bufq = &bufqueues[BQ_LRU];
                   else {
                           /* stale but valid data */
                           int has_deps;
   
                           if (LIST_FIRST(&bp->b_dep) != NULL &&
                               bioops.io_countdeps)
                                   has_deps = (*bioops.io_countdeps)(bp, 0);
                           else
                                   has_deps = 0;
                           bufq = has_deps ? &bufqueues[BQ_LRU] :
                               &bufqueues[BQ_AGE];
                   }
                   binstailfree(bp, bufq);
           }
   
   already_queued:
           /* Unlock the buffer. */
           CLR(bp->b_flags, B_AGE|B_ASYNC|B_BUSY|B_NOCACHE);
           SET(bp->b_flags, B_CACHE);
   
           /* Allow disk interrupts. */
           simple_unlock(&bp->b_interlock);
           simple_unlock(&bqueue_slock);
           splx(s);
           if (bp->b_bufsize <= 0) {
   #ifdef DEBUG
                   memset((char *)bp, 0, sizeof(*bp));
  #endif
                  pool_put(&bufpool, bp);
          }
  }
  
  /*
   * Determine if a block is in the cache.
   * Just look on what would be its hash chain.  If it's there, return
   * a pointer to it, unless it's marked invalid.  If it's marked invalid,
   * we normally don't return the buffer, unless the caller explicitly
   * wants us to.
   */
  struct buf *
  incore(struct vnode *vp, daddr_t blkno)
  {
          struct buf *bp;
  
          /* Search hash chain */
          LIST_FOREACH(bp, BUFHASH(vp, blkno), b_hash) {
                  if (bp->b_lblkno == blkno && bp->b_vp == vp &&
                      !ISSET(bp->b_flags, B_INVAL))
                  return (bp);
          }
  
          return (NULL);
  }
  
  /*
   * Get a block of requested size that is associated with
   * a given vnode and block offset. If it is found in the
   * block cache, mark it as having been found, make it busy
   * and return it. Otherwise, return an empty block of the
   * correct size. It is up to the caller to insure that the
   * cached blocks be of the correct size.
   */
  struct buf *
  getblk(struct vnode *vp, daddr_t blkno, int size, int slpflag, int slptimeo)
  {
          struct buf *bp;
          int s, err;
          int preserve;
  
  start:
          s = splbio();
          simple_lock(&bqueue_slock);
          bp = incore(vp, blkno);
          if (bp != NULL) {
                  simple_lock(&bp->b_interlock);
                  if (ISSET(bp->b_flags, B_BUSY)) {
                          simple_unlock(&bqueue_slock);
                          if (curproc == uvm.pagedaemon_proc) {
                                  simple_unlock(&bp->b_interlock);
                                  splx(s);
                                  return NULL;
                          }
                          SET(bp->b_flags, B_WANTED);
                          err = ltsleep(bp, slpflag | (PRIBIO + 1) | PNORELOCK,
                                          "getblk", slptimeo, &bp->b_interlock);
                          splx(s);
                          if (err)
                                  return (NULL);
                          goto start;
                  }
  #ifdef DIAGNOSTIC
                  if (ISSET(bp->b_flags, B_DONE|B_DELWRI) &&
                      bp->b_bcount < size && vp->v_type != VBLK)
                          panic("getblk: block size invariant failed");
  #endif
                  SET(bp->b_flags, B_BUSY);
                  bremfree(bp);
                  preserve = 1;
          } else {
                  if ((bp = getnewbuf(slpflag, slptimeo, 0)) == NULL) {
                          simple_unlock(&bqueue_slock);
                          splx(s);
                          goto start;
                  }
  
                  binshash(bp, BUFHASH(vp, blkno));
                  bp->b_blkno = bp->b_lblkno = bp->b_rawblkno = blkno;
                  bgetvp(vp, bp);
                  preserve = 0;
          }
          simple_unlock(&bp->b_interlock);
          simple_unlock(&bqueue_slock);
          splx(s);
          /*
           * LFS can't track total size of B_LOCKED buffer (locked_queue_bytes)
           * if we re-size buffers here.
           */
          if (ISSET(bp->b_flags, B_LOCKED)) {
                  KASSERT(bp->b_bufsize >= size);
          } else {
                  allocbuf(bp, size, preserve);
          }
          BIO_SETPRIO(bp, BPRIO_DEFAULT);
          return (bp);
  }
  
  /*
   * Get an empty, disassociated buffer of given size.
   */
  struct buf *
  geteblk(int size)
  {
          struct buf *bp;
          int s;
  
          s = splbio();
          simple_lock(&bqueue_slock);
          while ((bp = getnewbuf(0, 0, 0)) == 0)
                  ;
  
          SET(bp->b_flags, B_INVAL);
          binshash(bp, &invalhash);
          simple_unlock(&bqueue_slock);
          simple_unlock(&bp->b_interlock);
          splx(s);
          BIO_SETPRIO(bp, BPRIO_DEFAULT);
          allocbuf(bp, size, 0);
          return (bp);
  }
  
  /*
   * Expand or contract the actual memory allocated to a buffer.
   *
   * If the buffer shrinks, data is lost, so it's up to the
   * caller to have written it out *first*; this routine will not
   * start a write.  If the buffer grows, it's the callers
   * responsibility to fill out the buffer's additional contents.
   */
  void
  allocbuf(struct buf *bp, int size, int preserve)
  {
          vsize_t oldsize, desired_size;
          caddr_t addr;
          int s, delta;
  
          desired_size = buf_roundsize(size);
          if (desired_size > MAXBSIZE)
                  printf("allocbuf: buffer larger than MAXBSIZE requested");
  
          bp->b_bcount = size;
  
          oldsize = bp->b_bufsize;
          if (oldsize == desired_size)
                  return;
  
          /*
           * If we want a buffer of a different size, re-allocate the
           * buffer's memory; copy old content only if needed.
           */
          addr = buf_malloc(desired_size);
          if (preserve)
                  memcpy(addr, bp->b_data, MIN(oldsize,desired_size));
          if (bp->b_data != NULL)
                  buf_mrelease(bp->b_data, oldsize);
          bp->b_data = addr;
          bp->b_bufsize = desired_size;
  
          /*
           * Update overall buffer memory counter (protected by bqueue_slock)
           */
          delta = (long)desired_size - (long)oldsize;
  
          s = splbio();
          simple_lock(&bqueue_slock);
          if ((bufmem += delta) > bufmem_hiwater) {
                  /*
                   * Need to trim overall memory usage.
                   */
                  while (buf_canrelease()) {
                          if (curcpu()->ci_schedstate.spc_flags &
                              SPCF_SHOULDYIELD) {
                                  simple_unlock(&bqueue_slock);
                                  splx(s);
                                  preempt(1);
                                  s = splbio();
                                  simple_lock(&bqueue_slock);
                          }
  
                          if (buf_trim() == 0)
                                  break;
                  }
          }
  
          simple_unlock(&bqueue_slock);
          splx(s);
  }
  
  /*
   * Find a buffer which is available for use.
   * Select something from a free list.
   * Preference is to AGE list, then LRU list.
   *
   * Called at splbio and with buffer queues locked.
   * Return buffer locked.
   */
  struct buf *
  getnewbuf(int slpflag, int slptimeo, int from_bufq)
  {
          struct buf *bp;
  
  start:
          LOCK_ASSERT(simple_lock_held(&bqueue_slock));
  
          /*
           * Get a new buffer from the pool; but use NOWAIT because
           * we have the buffer queues locked.
           */
          if (!from_bufq && buf_lotsfree() &&
              (bp = pool_get(&bufpool, PR_NOWAIT)) != NULL) {
                  memset((char *)bp, 0, sizeof(*bp));
                  BUF_INIT(bp);
                  bp->b_dev = NODEV;
                  bp->b_vnbufs.le_next = NOLIST;
                  bp->b_flags = B_BUSY;
                  simple_lock(&bp->b_interlock);
  #if defined(DIAGNOSTIC)
                  bp->b_freelistindex = -1;
  #endif /* defined(DIAGNOSTIC) */
                  return (bp);
          }
  
          if ((bp = TAILQ_FIRST(&bufqueues[BQ_AGE].bq_queue)) != NULL ||
              (bp = TAILQ_FIRST(&bufqueues[BQ_LRU].bq_queue)) != NULL) {
                  simple_lock(&bp->b_interlock);
                  bremfree(bp);
          } else {
                  /*
                   * XXX: !from_bufq should be removed.
                   */
                  if (!from_bufq || curproc != uvm.pagedaemon_proc) {
                          /* wait for a free buffer of any kind */
                          needbuffer = 1;
                          ltsleep(&needbuffer, slpflag|(PRIBIO + 1),
                              "getnewbuf", slptimeo, &bqueue_slock);
                  }
                  return (NULL);
          }
  
  #ifdef DIAGNOSTIC
          if (bp->b_bufsize <= 0)
                  panic("buffer %p: on queue but empty", bp);
  #endif
  
          if (ISSET(bp->b_flags, B_VFLUSH)) {
                  /*
                   * This is a delayed write buffer being flushed to disk.  Make
                   * sure it gets aged out of the queue when it's finished, and
                   * leave it off the LRU queue.
                   */
                  CLR(bp->b_flags, B_VFLUSH);
                  SET(bp->b_flags, B_AGE);
                  simple_unlock(&bp->b_interlock);
                  goto start;
          }
  
          /* Buffer is no longer on free lists. */
          SET(bp->b_flags, B_BUSY);
  
          /*
           * If buffer was a delayed write, start it and return NULL
           * (since we might sleep while starting the write).
           */
          if (ISSET(bp->b_flags, B_DELWRI)) {
                  /*
                   * This buffer has gone through the LRU, so make sure it gets
                   * reused ASAP.
                   */
                  SET(bp->b_flags, B_AGE);
                  simple_unlock(&bp->b_interlock);
                  simple_unlock(&bqueue_slock);
                  bawrite(bp);
                  simple_lock(&bqueue_slock);
                  return (NULL);
          }
  
          /* disassociate us from our vnode, if we had one... */
          if (bp->b_vp)
                  brelvp(bp);
  
          if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_deallocate)
                  (*bioops.io_deallocate)(bp);
  
          /* clear out various other fields */
          bp->b_flags = B_BUSY;
          bp->b_dev = NODEV;
          bp->b_blkno = bp->b_lblkno = bp->b_rawblkno = 0;
          bp->b_iodone = 0;
          bp->b_error = 0;
          bp->b_resid = 0;
          bp->b_bcount = 0;
  
          bremhash(bp);
          return (bp);
  }
  
  /*
   * Attempt to free an aged buffer off the queues.
   * Called at splbio and with queue lock held.
   * Returns the amount of buffer memory freed.
   */
  static int
  buf_trim(void)
  {
          struct buf *bp;
          long size = 0;
  
          /* Instruct getnewbuf() to get buffers off the queues */
          if ((bp = getnewbuf(PCATCH, 1, 1)) == NULL)
                  return 0;
  
          KASSERT(!ISSET(bp->b_flags, B_WANTED));
          simple_unlock(&bp->b_interlock);
          size = bp->b_bufsize;
          bufmem -= size;
          simple_unlock(&bqueue_slock);
          if (size > 0) {
                  buf_mrelease(bp->b_data, size);
                  bp->b_bcount = bp->b_bufsize = 0;
          }
          /* brelse() will return the buffer to the global buffer pool */
          brelse(bp);
          simple_lock(&bqueue_slock);
          return size;
  }
  
  int
  buf_drain(int n)
  {
          int s, size = 0, sz;
  
          s = splbio();
          simple_lock(&bqueue_slock);
  
          while (size < n && bufmem > bufmem_lowater) {
                  sz = buf_trim();
                  if (sz <= 0)
                          break;
                  size += sz;
          }
  
          simple_unlock(&bqueue_slock);
          splx(s);
          return size;
  }
  
  /*
   * Wait for operations on the buffer to complete.
   * When they do, extract and return the I/O's error value.
   */
  int
  biowait(struct buf *bp)
  {
          int s, error;
  
          s = splbio();
          simple_lock(&bp->b_interlock);
          while (!ISSET(bp->b_flags, B_DONE | B_DELWRI))
                  ltsleep(bp, PRIBIO + 1, "biowait", 0, &bp->b_interlock);
  
          /* check errors. */
          if (ISSET(bp->b_flags, B_ERROR))
                  error = bp->b_error ? bp->b_error : EIO;
          else
                  error = 0;
  
          simple_unlock(&bp->b_interlock);
          splx(s);
          return (error);
  }
  
  /*
   * Mark I/O complete on a buffer.
   *
   * If a callback has been requested, e.g. the pageout
   * daemon, do so. Otherwise, awaken waiting processes.
   *
   * [ Leffler, et al., says on p.247:
   *      "This routine wakes up the blocked process, frees the buffer
   *      for an asynchronous write, or, for a request by the pagedaemon
   *      process, invokes a procedure specified in the buffer structure" ]
   *
   * In real life, the pagedaemon (or other system processes) wants
   * to do async stuff to, and doesn't want the buffer brelse()'d.
   * (for swap pager, that puts swap buffers on the free lists (!!!),
   * for the vn device, that puts malloc'd buffers on the free lists!)
   */
  void
  biodone(struct buf *bp)
  {
          int s = splbio();
  
          simple_lock(&bp->b_interlock);
          if (ISSET(bp->b_flags, B_DONE))
                  panic("biodone already");
          SET(bp->b_flags, B_DONE);               /* note that it's done */
          BIO_SETPRIO(bp, BPRIO_DEFAULT);
  
          if (LIST_FIRST(&bp->b_dep) != NULL && bioops.io_complete)
                  (*bioops.io_complete)(bp);
  
          if (!ISSET(bp->b_flags, B_READ))        /* wake up reader */
                  vwakeup(bp);
  
          /*
           * If necessary, call out.  Unlock the buffer before calling
           * iodone() as the buffer isn't valid any more when it return.
           */
          if (ISSET(bp->b_flags, B_CALL)) {
                  CLR(bp->b_flags, B_CALL);       /* but note callout done */
                  simple_unlock(&bp->b_interlock);
                  (*bp->b_iodone)(bp);
          } else {
                  if (ISSET(bp->b_flags, B_ASYNC)) {      /* if async, release */
                          simple_unlock(&bp->b_interlock);
                          brelse(bp);
                  } else {                        /* or just wakeup the buffer */
                          CLR(bp->b_flags, B_WANTED);
                          wakeup(bp);
                          simple_unlock(&bp->b_interlock);
                  }
          }
  
          splx(s);
  }
  
  /*
   * Return a count of buffers on the "locked" queue.
   */
  int
  count_lock_queue(void)
  {
          struct buf *bp;
          int n = 0;
  
          simple_lock(&bqueue_slock);
          TAILQ_FOREACH(bp, &bufqueues[BQ_LOCKED].bq_queue, b_freelist)
                  n++;
          simple_unlock(&bqueue_slock);
          return (n);
  }
  
  /*
   * Wait for all buffers to complete I/O
   * Return the number of "stuck" buffers.
   */
  int
  buf_syncwait(void)
  {
          struct buf *bp;
          int iter, nbusy, nbusy_prev = 0, dcount, s, ihash;
  
          dcount = 10000;
          for (iter = 0; iter < 20;) {
                  s = splbio();
                  simple_lock(&bqueue_slock);
                  nbusy = 0;
                  for (ihash = 0; ihash < bufhash+1; ihash++) {
                      LIST_FOREACH(bp, &bufhashtbl[ihash], b_hash) {
                          if ((bp->b_flags & (B_BUSY|B_INVAL|B_READ)) == B_BUSY)
                                  nbusy++;
                          /*
                           * With soft updates, some buffers that are
                           * written will be remarked as dirty until other
                           * buffers are written.
                           */
                          if (bp->b_vp && bp->b_vp->v_mount
                              && (bp->b_vp->v_mount->mnt_flag & MNT_SOFTDEP)
                              && (bp->b_flags & B_DELWRI)) {
                                  simple_lock(&bp->b_interlock);
                                  bremfree(bp);
                                  bp->b_flags |= B_BUSY;
                                  nbusy++;
                                  simple_unlock(&bp->b_interlock);
                                  simple_unlock(&bqueue_slock);
                                  bawrite(bp);
                                  if (dcount-- <= 0) {
                                          printf("softdep ");
                                          splx(s);
                                          goto fail;
                                  }
                                  simple_lock(&bqueue_slock);
                          }
                      }
                  }
  
                  simple_unlock(&bqueue_slock);
                  splx(s);
  
                  if (nbusy == 0)
                          break;
                  if (nbusy_prev == 0)
                          nbusy_prev = nbusy;
                  printf("%d ", nbusy);
                  tsleep(&nbusy, PRIBIO, "bflush",
                      (iter == 0) ? 1 : hz / 25 * iter);
                  if (nbusy >= nbusy_prev) /* we didn't flush anything */
                          iter++;
                  else
                          nbusy_prev = nbusy;
          }
  
          if (nbusy) {
  fail:;
  #if defined(DEBUG) || defined(DEBUG_HALT_BUSY)
                  printf("giving up\nPrinting vnodes for busy buffers\n");
                  s = splbio();
                  for (ihash = 0; ihash < bufhash+1; ihash++) {
                      LIST_FOREACH(bp, &bufhashtbl[ihash], b_hash) {
                          if ((bp->b_flags & (B_BUSY|B_INVAL|B_READ)) == B_BUSY)
                                  vprint(NULL, bp->b_vp);
                      }
                  }
                  splx(s);
  #endif
          }
  
          return nbusy;
  }
  
  static void
  sysctl_fillbuf(struct buf *i, struct buf_sysctl *o)
  {
  
          o->b_flags = i->b_flags;
          o->b_error = i->b_error;
          o->b_prio = i->b_prio;
          o->b_dev = i->b_dev;
          o->b_bufsize = i->b_bufsize;
          o->b_bcount = i->b_bcount;
          o->b_resid = i->b_resid;
          o->b_addr = PTRTOUINT64(i->b_un.b_addr);
          o->b_blkno = i->b_blkno;
          o->b_rawblkno = i->b_rawblkno;
          o->b_iodone = PTRTOUINT64(i->b_iodone);
          o->b_proc = PTRTOUINT64(i->b_proc);
          o->b_vp = PTRTOUINT64(i->b_vp);
          o->b_saveaddr = PTRTOUINT64(i->b_saveaddr);
          o->b_lblkno = i->b_lblkno;
  }
  
  #define KERN_BUFSLOP 20
  static int
  sysctl_dobuf(SYSCTLFN_ARGS)
  {
          struct buf *bp;
          struct buf_sysctl bs;
          char *dp;
          u_int i, op, arg;
          size_t len, needed, elem_size, out_size;
          int error, s, elem_count;
  
          if (namelen == 1 && name[0] == CTL_QUERY)
                  return (sysctl_query(SYSCTLFN_CALL(rnode)));
  
          if (namelen != 4)
                  return (EINVAL);
  
          dp = oldp;
          len = (oldp != NULL) ? *oldlenp : 0;
          op = name[0];
          arg = name[1];
          elem_size = name[2];
          elem_count = name[3];
          out_size = MIN(sizeof(bs), elem_size);
  
          /*
           * at the moment, these are just "placeholders" to make the
           * API for retrieving kern.buf data more extensible in the
           * future.
           *
           * XXX kern.buf currently has "netbsd32" issues.  hopefully
           * these will be resolved at a later point.
           */
          if (op != KERN_BUF_ALL || arg != KERN_BUF_ALL ||
              elem_size < 1 || elem_count < 0)
                  return (EINVAL);
  
          error = 0;
          needed = 0;
          s = splbio();
          simple_lock(&bqueue_slock);
          for (i = 0; i < BQUEUES; i++) {
                  TAILQ_FOREACH(bp, &bufqueues[i].bq_queue, b_freelist) {
                          if (len >= elem_size && elem_count > 0) {
                                  sysctl_fillbuf(bp, &bs);
                                  error = copyout(&bs, dp, out_size);
                                  if (error)
                                          goto cleanup;
                                  dp += elem_size;
                                  len -= elem_size;
                          }
                          if (elem_count > 0) {
                                  needed += elem_size;
                                  if (elem_count != INT_MAX)
                                          elem_count--;
                          }
                  }
          }
  cleanup:
          simple_unlock(&bqueue_slock);
          splx(s);
  
          *oldlenp = needed;
          if (oldp == NULL)
                  *oldlenp += KERN_BUFSLOP * sizeof(struct buf);
  
          return (error);
  }
  
  static void
  sysctl_bufvm_common(void)
  {
          int64_t t;
  
          /* Drain until below new high water mark */
          while ((t = (int64_t)bufmem - (int64_t)bufmem_hiwater) >= 0) {
                  if (buf_drain(t / (2 * 1024)) <= 0)
                          break;
          }
  }
  
  static int
  sysctl_bufcache_update(SYSCTLFN_ARGS)
  {
          int t, error;
          struct sysctlnode node;
  
          node = *rnode;
          node.sysctl_data = &t;
          t = *(int *)rnode->sysctl_data;
          error = sysctl_lookup(SYSCTLFN_CALL(&node));
          if (error || newp == NULL)
                  return (error);
  
          if (t < 0 || t > 100)
                  return EINVAL;
          bufcache = t;
          buf_setwm();
  
          sysctl_bufvm_common();
          return 0;
  }
  
  static int
  sysctl_bufvm_update(SYSCTLFN_ARGS)
  {
          int64_t t;
          int error;
          struct sysctlnode node;
  
          node = *rnode;
          node.sysctl_data = &t;
          t = *(int64_t *)rnode->sysctl_data;
          error = sysctl_lookup(SYSCTLFN_CALL(&node));
          if (error || newp == NULL)
                  return (error);
  
          if (t < 0)
                  return EINVAL;
          if (rnode->sysctl_data == &bufmem_lowater) {
                  if (bufmem_hiwater - t < 16)
                          return (EINVAL);
                  bufmem_lowater = t;
          } else if (rnode->sysctl_data == &bufmem_hiwater) {
                  if (t - bufmem_lowater < 16)
                          return (EINVAL);
                  bufmem_hiwater = t;
          } else
                  return (EINVAL);
  
          sysctl_bufvm_common();
  
          return 0;
  }
  
  SYSCTL_SETUP(sysctl_kern_buf_setup, "sysctl kern.buf subtree setup")
  {
  
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT,
                         CTLTYPE_NODE, "kern", NULL,
                         NULL, 0, NULL, 0,
                         CTL_KERN, CTL_EOL);
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT,
                         CTLTYPE_NODE, "buf",
                         SYSCTL_DESCR("Kernel buffer cache information"),
                         sysctl_dobuf, 0, NULL, 0,
                         CTL_KERN, KERN_BUF, CTL_EOL);
  }
  
  SYSCTL_SETUP(sysctl_vm_buf_setup, "sysctl vm.buf* subtree setup")
  {
  
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT,
                         CTLTYPE_NODE, "vm", NULL,
                         NULL, 0, NULL, 0,
                         CTL_VM, CTL_EOL);
  
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
                         CTLTYPE_INT, "bufcache",
                         SYSCTL_DESCR("Percentage of physical memory to use for "
                                      "buffer cache"),
                         sysctl_bufcache_update, 0, &bufcache, 0,
                         CTL_VM, CTL_CREATE, CTL_EOL);
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT|CTLFLAG_READONLY,
                         CTLTYPE_QUAD, "bufmem",
                         SYSCTL_DESCR("Amount of kernel memory used by buffer "
                                      "cache"),
                         NULL, 0, &bufmem, 0,
                         CTL_VM, CTL_CREATE, CTL_EOL);
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
                         CTLTYPE_QUAD, "bufmem_lowater",
                         SYSCTL_DESCR("Minimum amount of kernel memory to "
                                      "reserve for buffer cache"),
                         sysctl_bufvm_update, 0, &bufmem_lowater, 0,
                         CTL_VM, CTL_CREATE, CTL_EOL);
          sysctl_createv(clog, 0, NULL, NULL,
                         CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
                         CTLTYPE_QUAD, "bufmem_hiwater",
                         SYSCTL_DESCR("Maximum amount of kernel memory to use "
                                      "for buffer cache"),
                         sysctl_bufvm_update, 0, &bufmem_hiwater, 0,
                         CTL_VM, CTL_CREATE, CTL_EOL);
  }
  
  #ifdef DEBUG
  /*
   * Print out statistics on the current allocation of the buffer pool.
   * Can be enabled to print out on every ``sync'' by setting "syncprt"
   * in vfs_syscalls.c using sysctl.
   */
  void
  vfs_bufstats(void)
  {
          int s, i, j, count;
          struct buf *bp;
          struct bqueue *dp;
          int counts[(MAXBSIZE / PAGE_SIZE) + 1];
          static const char *bname[BQUEUES] = { "LOCKED", "LRU", "AGE" };
  
          for (dp = bufqueues, i = 0; dp < &bufqueues[BQUEUES]; dp++, i++) {
                  count = 0;
                  for (j = 0; j <= MAXBSIZE/PAGE_SIZE; j++)
                          counts[j] = 0;
                  s = splbio();
                  TAILQ_FOREACH(bp, &dp->bq_queue, b_freelist) {
                          counts[bp->b_bufsize/PAGE_SIZE]++;
                          count++;
                  }
                  splx(s);
                  printf("%s: total-%d", bname[i], count);
                  for (j = 0; j <= MAXBSIZE/PAGE_SIZE; j++)
                          if (counts[j] != 0)
                                  printf(", %d-%d", j * PAGE_SIZE, counts[j]);
                  printf("\n");
          }
  }
  #endif /* DEBUG */
  
  /* ------------------------------ */
  
  static POOL_INIT(bufiopool, sizeof(struct buf), 0, 0, 0, "biopl", NULL);
  
  static struct buf *
  getiobuf1(int prflags)
  {
          struct buf *bp;
          int s;
  
          s = splbio();
          bp = pool_get(&bufiopool, prflags);
          splx(s);
          if (bp != NULL) {
                  BUF_INIT(bp);
          }
          return bp;
  }
  
  struct buf *
  getiobuf(void)
  {
  
          return getiobuf1(PR_WAITOK);
  }
  
  struct buf *
  getiobuf_nowait(void)
  {
  
          return getiobuf1(PR_NOWAIT);
  }
  
  void
  putiobuf(struct buf *bp)
  {
          int s;
  
          s = splbio();
          pool_put(&bufiopool, bp);
          splx(s);
  }
  
  /*
   * nestiobuf_iodone: b_iodone callback for nested buffers.
   */
  
  void
  nestiobuf_iodone(struct buf *bp)
  {
          struct buf *mbp = bp->b_private;
          int error;
          int donebytes;
  
          KASSERT(bp->b_bcount <= bp->b_bufsize);
          KASSERT(mbp != bp);
  
          error = 0;
          if ((bp->b_flags & B_ERROR) != 0) {
                  error = EIO;
                  /* check if an error code was returned */
                  if (bp->b_error)
                          error = bp->b_error;
          } else if ((bp->b_bcount < bp->b_bufsize) || (bp->b_resid > 0)) {
                  /*
                   * Not all got transfered, raise an error. We have no way to
                   * propagate these conditions to mbp.
                   */
                  error = EIO;
          }
  
          donebytes = bp->b_bufsize;
  
          putiobuf(bp);
          nestiobuf_done(mbp, donebytes, error);
  }
  
  /*
   * nestiobuf_setup: setup a "nested" buffer.
   *
   * => 'mbp' is a "master" buffer which is being divided into sub pieces.
   * => 'bp' should be a buffer allocated by getiobuf or getiobuf_nowait.
   * => 'offset' is a byte offset in the master buffer.
   * => 'size' is a size in bytes of this nested buffer.
   */
  
  void
  nestiobuf_setup(struct buf *mbp, struct buf *bp, int offset, size_t size)
  {
          const int b_read = mbp->b_flags & B_READ;
          struct vnode *vp = mbp->b_vp;
  
          KASSERT(mbp->b_bcount >= offset + size);
          bp->b_vp = vp;
          bp->b_flags = B_BUSY | B_CALL | B_ASYNC | b_read;
          bp->b_iodone = nestiobuf_iodone;
          bp->b_data = mbp->b_data + offset;
          bp->b_resid = bp->b_bcount = size;
          bp->b_bufsize = bp->b_bcount;
          bp->b_private = mbp;
          BIO_COPYPRIO(bp, mbp);
          if (!b_read && vp != NULL) {
                  int s;
  
                  s = splbio();
                  V_INCR_NUMOUTPUT(vp);
                  splx(s);
          }
  }
  
  /*
   * nestiobuf_done: propagate completion to the master buffer.
   *
   * => 'donebytes' specifies how many bytes in the 'mbp' is completed.
   * => 'error' is an errno(2) that 'donebytes' has been completed with.
   */
  
  void
  nestiobuf_done(struct buf *mbp, int donebytes, int error)
  {
          int s;
  
          if (donebytes == 0) {
                  return;
          }
          s = splbio();
          KASSERT(mbp->b_resid >= donebytes);
          if (error) {
                  mbp->b_flags |= B_ERROR;
                  mbp->b_error = error;
          }
          mbp->b_resid -= donebytes;
          if (mbp->b_resid == 0) {
                  if ((mbp->b_flags & B_ERROR) != 0) {
                          mbp->b_resid = mbp->b_bcount; /* be conservative */
                  }
                  biodone(mbp);
          }
          splx(s);
  }

Cache object: a61f2457adc08acbf27de4264afa826b