FreeBSD/Linux Kernel Cross Reference
sys/vm/swap_pager.c

     /*
      * Copyright (c) 1998 Matthew Dillon,
      * Copyright (c) 1994 John S. Dyson
      * Copyright (c) 1990 University of Utah.
      * Copyright (c) 1991, 1993
      *      The Regents of the University of California.  All rights reserved.
      *
      * This code is derived from software contributed to Berkeley by
      * the Systems Programming Group of the University of Utah Computer
     * Science Department.
     *
     * 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.
     *
     *                              New Swap System
     *                              Matthew Dillon
     *
     * Radix Bitmap 'blists'.
     *
     *      - The new swapper uses the new radix bitmap code.  This should scale
     *        to arbitrarily small or arbitrarily large swap spaces and an almost
     *        arbitrary degree of fragmentation.
     *
     * Features:
     *
     *      - on the fly reallocation of swap during putpages.  The new system
     *        does not try to keep previously allocated swap blocks for dirty
     *        pages.  
     *
     *      - on the fly deallocation of swap
     *
     *      - No more garbage collection required.  Unnecessarily allocated swap
     *        blocks only exist for dirty vm_page_t's now and these are already
     *        cycled (in a high-load system) by the pager.  We also do on-the-fly
     *        removal of invalidated swap blocks when a page is destroyed
     *        or renamed.
     *
     * from: Utah $Hdr: swap_pager.c 1.4 91/04/30$
     *
     *      @(#)swap_pager.c        8.9 (Berkeley) 3/21/94
     *
     * $FreeBSD$
     */
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/conf.h>
    #include <sys/kernel.h>
    #include <sys/proc.h>
    #include <sys/buf.h>
    #include <sys/vnode.h>
    #include <sys/malloc.h>
    #include <sys/vmmeter.h>
    #include <sys/sysctl.h>
    #include <sys/blist.h>
    #include <sys/lock.h>
    #include <sys/vmmeter.h>
    
    #ifndef MAX_PAGEOUT_CLUSTER
    #define MAX_PAGEOUT_CLUSTER 16
    #endif
    
    #define SWB_NPAGES      MAX_PAGEOUT_CLUSTER
    
    #include "opt_swap.h"
    #include <vm/vm.h>
    #include <vm/vm_object.h>
    #include <vm/vm_page.h>
    #include <vm/vm_pager.h>
    #include <vm/vm_pageout.h>
    #include <vm/swap_pager.h>
    #include <vm/vm_extern.h>
    #include <vm/vm_zone.h>
    
   #define SWM_FREE        0x02    /* free, period                 */
   #define SWM_POP         0x04    /* pop out                      */
   
   /*
    * vm_swap_size is in page-sized chunks now.  It was DEV_BSIZE'd chunks
    * in the old system.
    */
   
   extern int vm_swap_size;        /* number of free swap blocks, in pages */
   
   int swap_pager_full;            /* swap space exhaustion (task killing) */
   static int swap_pager_almost_full; /* swap space exhaustion (w/ hysteresis)*/
   static int nsw_rcount;          /* free read buffers                    */
   static int nsw_wcount_sync;     /* limit write buffers / synchronous    */
   static int nsw_wcount_async;    /* limit write buffers / asynchronous   */
   static int nsw_wcount_async_max;/* assigned maximum                     */
   static int nsw_cluster_max;     /* maximum VOP I/O allowed              */
   static int sw_alloc_interlock;  /* swap pager allocation interlock      */
   
   struct blist *swapblist;
   static struct swblock **swhash;
   static int swhash_mask;
   static int swap_async_max = 4;  /* maximum in-progress async I/O's      */
   
   extern struct vnode *swapdev_vp;        /* from vm_swap.c */
   
   SYSCTL_INT(_vm, OID_AUTO, swap_async_max,
           CTLFLAG_RW, &swap_async_max, 0, "Maximum running async swap ops");
   
   /*
    * "named" and "unnamed" anon region objects.  Try to reduce the overhead
    * of searching a named list by hashing it just a little.
    */
   
   #define NOBJLISTS               8
   
   #define NOBJLIST(handle)        \
           (&swap_pager_object_list[((int)(intptr_t)handle >> 4) & (NOBJLISTS-1)])
   
   static struct pagerlst  swap_pager_object_list[NOBJLISTS];
   struct pagerlst         swap_pager_un_object_list;
   vm_zone_t               swap_zone;
   
   /*
    * pagerops for OBJT_SWAP - "swap pager".  Some ops are also global procedure
    * calls hooked from other parts of the VM system and do not appear here.
    * (see vm/swap_pager.h).
    */
   
   static vm_object_t
                   swap_pager_alloc __P((void *handle, vm_ooffset_t size,
                                         vm_prot_t prot, vm_ooffset_t offset));
   static void     swap_pager_dealloc __P((vm_object_t object));
   static int      swap_pager_getpages __P((vm_object_t, vm_page_t *, int, int));
   static void     swap_pager_init __P((void));
   static void     swap_pager_unswapped __P((vm_page_t));
   static void     swap_pager_strategy __P((vm_object_t, struct buf *));
   
   struct pagerops swappagerops = {
           swap_pager_init,        /* early system initialization of pager */
           swap_pager_alloc,       /* allocate an OBJT_SWAP object         */
           swap_pager_dealloc,     /* deallocate an OBJT_SWAP object       */
           swap_pager_getpages,    /* pagein                               */
           swap_pager_putpages,    /* pageout                              */
           swap_pager_haspage,     /* get backing store status for page    */
           swap_pager_unswapped,   /* remove swap related to page          */
           swap_pager_strategy     /* pager strategy call                  */
   };
   
   /*
    * dmmax is in page-sized chunks with the new swap system.  It was
    * dev-bsized chunks in the old.  dmmax is always a power of 2.
    *
    * swap_*() routines are externally accessible.  swp_*() routines are
    * internal.
    */
   
   int dmmax;
   static int dmmax_mask;
   int nswap_lowat = 128;          /* in pages, swap_pager_almost_full warn */
   int nswap_hiwat = 512;          /* in pages, swap_pager_almost_full warn */
   
   static __inline void    swp_sizecheck __P((void));
   static void     swp_pager_sync_iodone __P((struct buf *bp));
   static void     swp_pager_async_iodone __P((struct buf *bp));
   
   /*
    * Swap bitmap functions
    */
   
   static __inline void    swp_pager_freeswapspace __P((daddr_t blk, int npages));
   static __inline daddr_t swp_pager_getswapspace __P((int npages));
   
   /*
    * Metadata functions
    */
   
   static void swp_pager_meta_build __P((vm_object_t, vm_pindex_t, daddr_t));
   static void swp_pager_meta_free __P((vm_object_t, vm_pindex_t, daddr_t));
   static void swp_pager_meta_free_all __P((vm_object_t));
   static daddr_t swp_pager_meta_ctl __P((vm_object_t, vm_pindex_t, int));
   
   /*
    * SWP_SIZECHECK() -    update swap_pager_full indication
    *      
    *      update the swap_pager_almost_full indication and warn when we are
    *      about to run out of swap space, using lowat/hiwat hysteresis.
    *
    *      Clear swap_pager_full ( task killing ) indication when lowat is met.
    *
    *      No restrictions on call
    *      This routine may not block.
    *      This routine must be called at splvm()
    */
   
   static __inline void
   swp_sizecheck()
   {
           if (vm_swap_size < nswap_lowat) {
                   if (swap_pager_almost_full == 0) {
                           printf("swap_pager: out of swap space\n");
                           swap_pager_almost_full = 1;
                   }
           } else {
                   swap_pager_full = 0;
                   if (vm_swap_size > nswap_hiwat)
                           swap_pager_almost_full = 0;
           }
   }
   
   /*
    * SWAP_PAGER_INIT() -  initialize the swap pager!
    *
    *      Expected to be started from system init.  NOTE:  This code is run 
    *      before much else so be careful what you depend on.  Most of the VM
    *      system has yet to be initialized at this point.
    */
   
   static void
   swap_pager_init()
   {
           /*
            * Initialize object lists
            */
           int i;
   
           for (i = 0; i < NOBJLISTS; ++i)
                   TAILQ_INIT(&swap_pager_object_list[i]);
           TAILQ_INIT(&swap_pager_un_object_list);
   
           /*
            * Device Stripe, in PAGE_SIZE'd blocks
            */
   
           dmmax = SWB_NPAGES * 2;
           dmmax_mask = ~(dmmax - 1);
   }
   
   /*
    * SWAP_PAGER_SWAP_INIT() - swap pager initialization from pageout process
    *
    *      Expected to be started from pageout process once, prior to entering
    *      its main loop.
    */
   
   void
   swap_pager_swap_init()
   {
           int n, n2;
   
           /*
            * Number of in-transit swap bp operations.  Don't
            * exhaust the pbufs completely.  Make sure we
            * initialize workable values (0 will work for hysteresis
            * but it isn't very efficient).
            *
            * The nsw_cluster_max is constrained by the bp->b_pages[]
            * array (MAXPHYS/PAGE_SIZE) and our locally defined
            * MAX_PAGEOUT_CLUSTER.   Also be aware that swap ops are
            * constrained by the swap device interleave stripe size.
            *
            * Currently we hardwire nsw_wcount_async to 4.  This limit is 
            * designed to prevent other I/O from having high latencies due to
            * our pageout I/O.  The value 4 works well for one or two active swap
            * devices but is probably a little low if you have more.  Even so,
            * a higher value would probably generate only a limited improvement
            * with three or four active swap devices since the system does not
            * typically have to pageout at extreme bandwidths.   We will want
            * at least 2 per swap devices, and 4 is a pretty good value if you
            * have one NFS swap device due to the command/ack latency over NFS.
            * So it all works out pretty well.
            */
   
           nsw_cluster_max = min((MAXPHYS/PAGE_SIZE), MAX_PAGEOUT_CLUSTER);
   
           nsw_rcount = (nswbuf + 1) / 2;
           nsw_wcount_sync = (nswbuf + 3) / 4;
           nsw_wcount_async = 4;
           nsw_wcount_async_max = nsw_wcount_async;
   
           /*
            * Initialize our zone.  Right now I'm just guessing on the number
            * we need based on the number of pages in the system.  Each swblock
            * can hold 16 pages, so this is probably overkill.  This reservation
            * is typically limited to around 32MB by default.  The initial
            * guess caps the swap space at 8 times the size of RAM.
            */
           n = cnt.v_page_count / 2;
           if (maxswzone && n > maxswzone / sizeof(struct swblock))
                   n = maxswzone / sizeof(struct swblock);
           n2 = n;
   
           do {
                   swap_zone = zinit(
                           "SWAPMETA", 
                           sizeof(struct swblock), 
                           n,
                           ZONE_INTERRUPT, 
                           1);
                   if (swap_zone != NULL)
                           break;
                   /*
                    * if the allocation failed, try a zone two thirds the
                    * size of the previous attempt.
                    */
                   n -= ((n + 2) / 3);
           } while (n > 0);
   
           if (swap_zone == NULL)
                   panic("swap_pager_swap_init: swap_zone == NULL");
           if (n2 != n)
                   printf("Swap zone entries reduced from %d to %d.\n", n2, n);
           n2 = n;
   
           /*
            * Initialize our meta-data hash table.  The swapper does not need to
            * be quite as efficient as the VM system, so we do not use an 
            * oversized hash table.
            *
            *      n:              size of hash table, must be power of 2
            *      swhash_mask:    hash table index mask
            */
   
           for (n = 1; n < n2 / 8; n *= 2)
                   ;
   
           swhash = malloc(sizeof(struct swblock *) * n, M_VMPGDATA, M_WAITOK);
           bzero(swhash, sizeof(struct swblock *) * n);
   
           swhash_mask = n - 1;
   }
   
   /*
    * SWAP_PAGER_ALLOC() - allocate a new OBJT_SWAP VM object and instantiate
    *                      its metadata structures.
    *
    *      This routine is called from the mmap and fork code to create a new
    *      OBJT_SWAP object.  We do this by creating an OBJT_DEFAULT object
    *      and then converting it with swp_pager_meta_build().
    *
    *      This routine may block in vm_object_allocate() and create a named
    *      object lookup race, so we must interlock.   We must also run at
    *      splvm() for the object lookup to handle races with interrupts, but
    *      we do not have to maintain splvm() in between the lookup and the
    *      add because (I believe) it is not possible to attempt to create
    *      a new swap object w/handle when a default object with that handle
    *      already exists.
    */
   
   static vm_object_t
   swap_pager_alloc(void *handle, vm_ooffset_t size, vm_prot_t prot,
                    vm_ooffset_t offset)
   {
           vm_object_t object;
   
           if (handle) {
                   /*
                    * Reference existing named region or allocate new one.  There
                    * should not be a race here against swp_pager_meta_build()
                    * as called from vm_page_remove() in regards to the lookup
                    * of the handle.
                    */
   
                   while (sw_alloc_interlock) {
                           sw_alloc_interlock = -1;
                           tsleep(&sw_alloc_interlock, PVM, "swpalc", 0);
                   }
                   sw_alloc_interlock = 1;
   
                   object = vm_pager_object_lookup(NOBJLIST(handle), handle);
   
                   if (object != NULL) {
                           vm_object_reference(object);
                   } else {
                           object = vm_object_allocate(OBJT_DEFAULT,
                                   OFF_TO_IDX(offset + PAGE_MASK + size));
                           object->handle = handle;
   
                           swp_pager_meta_build(object, 0, SWAPBLK_NONE);
                   }
   
                   if (sw_alloc_interlock < 0)
                           wakeup(&sw_alloc_interlock);
   
                   sw_alloc_interlock = 0;
           } else {
                   object = vm_object_allocate(OBJT_DEFAULT,
                           OFF_TO_IDX(offset + PAGE_MASK + size));
   
                   swp_pager_meta_build(object, 0, SWAPBLK_NONE);
           }
   
           return (object);
   }
   
   /*
    * SWAP_PAGER_DEALLOC() -       remove swap metadata from object
    *
    *      The swap backing for the object is destroyed.  The code is 
    *      designed such that we can reinstantiate it later, but this
    *      routine is typically called only when the entire object is
    *      about to be destroyed.
    *
    *      This routine may block, but no longer does. 
    *
    *      The object must be locked or unreferenceable.
    */
   
   static void
   swap_pager_dealloc(object)
           vm_object_t object;
   {
           int s;
   
           /*
            * Remove from list right away so lookups will fail if we block for
            * pageout completion.
            */
   
           if (object->handle == NULL) {
                   TAILQ_REMOVE(&swap_pager_un_object_list, object, pager_object_list);
           } else {
                   TAILQ_REMOVE(NOBJLIST(object->handle), object, pager_object_list);
           }
   
           vm_object_pip_wait(object, "swpdea");
   
           /*
            * Free all remaining metadata.  We only bother to free it from 
            * the swap meta data.  We do not attempt to free swapblk's still
            * associated with vm_page_t's for this object.  We do not care
            * if paging is still in progress on some objects.
            */
           s = splvm();
           swp_pager_meta_free_all(object);
           splx(s);
   }
   
   /************************************************************************
    *                      SWAP PAGER BITMAP ROUTINES                      *
    ************************************************************************/
   
   /*
    * SWP_PAGER_GETSWAPSPACE() -   allocate raw swap space
    *
    *      Allocate swap for the requested number of pages.  The starting
    *      swap block number (a page index) is returned or SWAPBLK_NONE
    *      if the allocation failed.
    *
    *      Also has the side effect of advising that somebody made a mistake
    *      when they configured swap and didn't configure enough.
    *
    *      Must be called at splvm() to avoid races with bitmap frees from
    *      vm_page_remove() aka swap_pager_page_removed().
    *
    *      This routine may not block
    *      This routine must be called at splvm().
    */
   
   static __inline daddr_t
   swp_pager_getswapspace(npages)
           int npages;
   {
           daddr_t blk;
   
           if ((blk = blist_alloc(swapblist, npages)) == SWAPBLK_NONE) {
                   if (swap_pager_full != 2) {
                           printf("swap_pager_getswapspace: failed\n");
                           swap_pager_full = 2;
                           swap_pager_almost_full = 1;
                   }
           } else {
                   vm_swap_size -= npages;
                   swp_sizecheck();
           }
           return(blk);
   }
   
   /*
    * SWP_PAGER_FREESWAPSPACE() -  free raw swap space 
    *
    *      This routine returns the specified swap blocks back to the bitmap.
    *
    *      Note:  This routine may not block (it could in the old swap code),
    *      and through the use of the new blist routines it does not block.
    *
    *      We must be called at splvm() to avoid races with bitmap frees from
    *      vm_page_remove() aka swap_pager_page_removed().
    *
    *      This routine may not block
    *      This routine must be called at splvm().
    */
   
   static __inline void
   swp_pager_freeswapspace(blk, npages)
           daddr_t blk;
           int npages;
   {
           blist_free(swapblist, blk, npages);
           vm_swap_size += npages;
           swp_sizecheck();
   }
   
   /*
    * SWAP_PAGER_FREESPACE() -     frees swap blocks associated with a page
    *                              range within an object.
    *
    *      This is a globally accessible routine.
    *
    *      This routine removes swapblk assignments from swap metadata.
    *
    *      The external callers of this routine typically have already destroyed 
    *      or renamed vm_page_t's associated with this range in the object so 
    *      we should be ok.
    *
    *      This routine may be called at any spl.  We up our spl to splvm temporarily
    *      in order to perform the metadata removal.
    */
   
   void
   swap_pager_freespace(object, start, size)
           vm_object_t object;
           vm_pindex_t start;
           vm_size_t size;
   {
           int s = splvm();
           swp_pager_meta_free(object, start, size);
           splx(s);
   }
   
   /*
    * SWAP_PAGER_RESERVE() - reserve swap blocks in object
    *
    *      Assigns swap blocks to the specified range within the object.  The 
    *      swap blocks are not zerod.  Any previous swap assignment is destroyed.
    *
    *      Returns 0 on success, -1 on failure.
    */
   
   int
   swap_pager_reserve(vm_object_t object, vm_pindex_t start, vm_size_t size)
   {
           int s;
           int n = 0;
           daddr_t blk = SWAPBLK_NONE;
           vm_pindex_t beg = start;        /* save start index */
   
           s = splvm();
           while (size) {
                   if (n == 0) {
                           n = BLIST_MAX_ALLOC;
                           while ((blk = swp_pager_getswapspace(n)) == SWAPBLK_NONE) {
                                   n >>= 1;
                                   if (n == 0) {
                                           swp_pager_meta_free(object, beg, start - beg);
                                           splx(s);
                                           return(-1);
                                   }
                           }
                   }
                   swp_pager_meta_build(object, start, blk);
                   --size;
                   ++start;
                   ++blk;
                   --n;
           }
           swp_pager_meta_free(object, start, n);
           splx(s);
           return(0);
   }
   
   /*
    * SWAP_PAGER_COPY() -  copy blocks from source pager to destination pager
    *                      and destroy the source.
    *
    *      Copy any valid swapblks from the source to the destination.  In
    *      cases where both the source and destination have a valid swapblk,
    *      we keep the destination's.
    *
    *      This routine is allowed to block.  It may block allocating metadata
    *      indirectly through swp_pager_meta_build() or if paging is still in
    *      progress on the source. 
    *
    *      This routine can be called at any spl
    *
    *      XXX vm_page_collapse() kinda expects us not to block because we 
    *      supposedly do not need to allocate memory, but for the moment we
    *      *may* have to get a little memory from the zone allocator, but
    *      it is taken from the interrupt memory.  We should be ok. 
    *
    *      The source object contains no vm_page_t's (which is just as well)
    *
    *      The source object is of type OBJT_SWAP.
    *
    *      The source and destination objects must be locked or 
    *      inaccessible (XXX are they ?)
    */
   
   void
   swap_pager_copy(srcobject, dstobject, offset, destroysource)
           vm_object_t srcobject;
           vm_object_t dstobject;
           vm_pindex_t offset;
           int destroysource;
   {
           vm_pindex_t i;
           int s;
   
           s = splvm();
   
           /*
            * If destroysource is set, we remove the source object from the 
            * swap_pager internal queue now. 
            */
   
           if (destroysource) {
                   if (srcobject->handle == NULL) {
                           TAILQ_REMOVE(
                               &swap_pager_un_object_list, 
                               srcobject, 
                               pager_object_list
                           );
                   } else {
                           TAILQ_REMOVE(
                               NOBJLIST(srcobject->handle),
                               srcobject,
                               pager_object_list
                           );
                   }
           }
   
           /*
            * transfer source to destination.
            */
   
           for (i = 0; i < dstobject->size; ++i) {
                   daddr_t dstaddr;
   
                   /*
                    * Locate (without changing) the swapblk on the destination,
                    * unless it is invalid in which case free it silently, or
                    * if the destination is a resident page, in which case the
                    * source is thrown away.
                    */
   
                   dstaddr = swp_pager_meta_ctl(dstobject, i, 0);
   
                   if (dstaddr == SWAPBLK_NONE) {
                           /*
                            * Destination has no swapblk and is not resident,
                            * copy source.
                            */
                           daddr_t srcaddr;
   
                           srcaddr = swp_pager_meta_ctl(
                               srcobject, 
                               i + offset,
                               SWM_POP
                           );
   
                           if (srcaddr != SWAPBLK_NONE)
                                   swp_pager_meta_build(dstobject, i, srcaddr);
                   } else {
                           /*
                            * Destination has valid swapblk or it is represented
                            * by a resident page.  We destroy the sourceblock.
                            */
                           
                           swp_pager_meta_ctl(srcobject, i + offset, SWM_FREE);
                   }
           }
   
           /*
            * Free left over swap blocks in source.
            *
            * We have to revert the type to OBJT_DEFAULT so we do not accidently
            * double-remove the object from the swap queues.
            */
   
           if (destroysource) {
                   swp_pager_meta_free_all(srcobject);
                   /*
                    * Reverting the type is not necessary, the caller is going
                    * to destroy srcobject directly, but I'm doing it here
                    * for consistency since we've removed the object from its
                    * queues.
                    */
                   srcobject->type = OBJT_DEFAULT;
           }
           splx(s);
   }
   
   /*
    * SWAP_PAGER_HASPAGE() -       determine if we have good backing store for
    *                              the requested page.
    *
    *      We determine whether good backing store exists for the requested
    *      page and return TRUE if it does, FALSE if it doesn't.
    *
    *      If TRUE, we also try to determine how much valid, contiguous backing
    *      store exists before and after the requested page within a reasonable
    *      distance.  We do not try to restrict it to the swap device stripe
    *      (that is handled in getpages/putpages).  It probably isn't worth
    *      doing here.
    */
   
   boolean_t
   swap_pager_haspage(object, pindex, before, after)
           vm_object_t object;
           vm_pindex_t pindex;
           int *before;
           int *after;
   {
           daddr_t blk0;
           int s;
   
           /*
            * do we have good backing store at the requested index ?
            */
   
           s = splvm();
           blk0 = swp_pager_meta_ctl(object, pindex, 0);
   
           if (blk0 == SWAPBLK_NONE) {
                   splx(s);
                   if (before)
                           *before = 0;
                   if (after)
                           *after = 0;
                   return (FALSE);
           }
   
           /*
            * find backwards-looking contiguous good backing store
            */
   
           if (before != NULL) {
                   int i;
   
                   for (i = 1; i < (SWB_NPAGES/2); ++i) {
                           daddr_t blk;
   
                           if (i > pindex)
                                   break;
                           blk = swp_pager_meta_ctl(object, pindex - i, 0);
                           if (blk != blk0 - i)
                                   break;
                   }
                   *before = (i - 1);
           }
   
           /*
            * find forward-looking contiguous good backing store
            */
   
           if (after != NULL) {
                   int i;
   
                   for (i = 1; i < (SWB_NPAGES/2); ++i) {
                           daddr_t blk;
   
                           blk = swp_pager_meta_ctl(object, pindex + i, 0);
                           if (blk != blk0 + i)
                                   break;
                   }
                   *after = (i - 1);
           }
           splx(s);
           return (TRUE);
   }
   
   /*
    * SWAP_PAGER_PAGE_UNSWAPPED() - remove swap backing store related to page
    *
    *      This removes any associated swap backing store, whether valid or
    *      not, from the page.  
    *
    *      This routine is typically called when a page is made dirty, at
    *      which point any associated swap can be freed.  MADV_FREE also
    *      calls us in a special-case situation
    *
    *      NOTE!!!  If the page is clean and the swap was valid, the caller
    *      should make the page dirty before calling this routine.  This routine
    *      does NOT change the m->dirty status of the page.  Also: MADV_FREE
    *      depends on it.
    *
    *      This routine may not block
    *      This routine must be called at splvm()
    */
   
   static void
   swap_pager_unswapped(m)
           vm_page_t m;
   {
           swp_pager_meta_ctl(m->object, m->pindex, SWM_FREE);
   }
   
   /*
    * SWAP_PAGER_STRATEGY() - read, write, free blocks
    *
    *      This implements the vm_pager_strategy() interface to swap and allows
    *      other parts of the system to directly access swap as backing store
    *      through vm_objects of type OBJT_SWAP.  This is intended to be a 
    *      cacheless interface ( i.e. caching occurs at higher levels ).
    *      Therefore we do not maintain any resident pages.  All I/O goes
    *      directly to and from the swap device.
    *      
    *      Note that b_blkno is scaled for PAGE_SIZE
    *
    *      We currently attempt to run I/O synchronously or asynchronously as
    *      the caller requests.  This isn't perfect because we loose error
    *      sequencing when we run multiple ops in parallel to satisfy a request.
    *      But this is swap, so we let it all hang out.
    */
   
   static void     
   swap_pager_strategy(vm_object_t object, struct buf *bp)
   {
           vm_pindex_t start;
           int count;
           int s;
           char *data;
           struct buf *nbp = NULL;
   
           if (bp->b_bcount & PAGE_MASK) {
                   bp->b_error = EINVAL;
                   bp->b_flags |= B_ERROR | B_INVAL;
                   biodone(bp);
                   printf("swap_pager_strategy: bp %p b_vp %p blk %d size %d, not page bounded\n", bp, bp->b_vp, (int)bp->b_pblkno, (int)bp->b_bcount);
                   return;
           }
   
           /*
            * Clear error indication, initialize page index, count, data pointer.
            */
   
           bp->b_error = 0;
           bp->b_flags &= ~B_ERROR;
           bp->b_resid = bp->b_bcount;
   
           start = bp->b_pblkno;
           count = howmany(bp->b_bcount, PAGE_SIZE);
           data = bp->b_data;
   
           s = splvm();
   
           /*
            * Deal with B_FREEBUF
            */
   
           if (bp->b_flags & B_FREEBUF) {
                   /*
                    * FREE PAGE(s) - destroy underlying swap that is no longer
                    *                needed.
                    */
                   swp_pager_meta_free(object, start, count);
                   splx(s);
                   bp->b_resid = 0;
                   biodone(bp);
                   return;
           }
   
           /*
            * Execute read or write
            */
   
           while (count > 0) {
                   daddr_t blk;
   
                   /*
                    * Obtain block.  If block not found and writing, allocate a
                    * new block and build it into the object.
                    */
   
                   blk = swp_pager_meta_ctl(object, start, 0);
                   if ((blk == SWAPBLK_NONE) && (bp->b_flags & B_READ) == 0) {
                           blk = swp_pager_getswapspace(1);
                           if (blk == SWAPBLK_NONE) {
                                   bp->b_error = ENOMEM;
                                   bp->b_flags |= B_ERROR;
                                   break;
                           }
                           swp_pager_meta_build(object, start, blk);
                   }
                           
                   /*
                    * Do we have to flush our current collection?  Yes if:
                    *
                    *      - no swap block at this index
                    *      - swap block is not contiguous
                    *      - we cross a physical disk boundry in the
                    *        stripe.
                    */
   
                   if (
                       nbp && (nbp->b_blkno + btoc(nbp->b_bcount) != blk ||
                        ((nbp->b_blkno ^ blk) & dmmax_mask)
                       )
                   ) {
                           splx(s);
                           if (bp->b_flags & B_READ) {
                                   ++cnt.v_swapin;
                                   cnt.v_swappgsin += btoc(nbp->b_bcount);
                           } else {
                                   ++cnt.v_swapout;
                                   cnt.v_swappgsout += btoc(nbp->b_bcount);
                                   nbp->b_dirtyend = nbp->b_bcount;
                           }
                           flushchainbuf(nbp);
                           s = splvm();
                           nbp = NULL;
                   }
   
                   /*
                    * Add new swapblk to nbp, instantiating nbp if necessary.
                    * Zero-fill reads are able to take a shortcut.
                    */
   
                   if (blk == SWAPBLK_NONE) {
                           /*
                            * We can only get here if we are reading.  Since
                            * we are at splvm() we can safely modify b_resid,
                            * even if chain ops are in progress.
                            */
                           bzero(data, PAGE_SIZE);
                           bp->b_resid -= PAGE_SIZE;
                   } else {
                           if (nbp == NULL) {
                                   nbp = getchainbuf(bp, swapdev_vp, (bp->b_flags & B_READ) | B_ASYNC);
                                   nbp->b_blkno = blk;
                                   nbp->b_bcount = 0;
                                   nbp->b_data = data;
                           }
                           nbp->b_bcount += PAGE_SIZE;
                   }
                   --count;
                   ++start;
                   data += PAGE_SIZE;
           }
   
           /*
            *  Flush out last buffer
            */
   
           splx(s);
   
           if (nbp) {
                   if ((bp->b_flags & B_ASYNC) == 0)
                           nbp->b_flags &= ~B_ASYNC;
                   if (nbp->b_flags & B_READ) {
                           ++cnt.v_swapin;
                           cnt.v_swappgsin += btoc(nbp->b_bcount);
                   } else {
                           ++cnt.v_swapout;
                           cnt.v_swappgsout += btoc(nbp->b_bcount);
                           nbp->b_dirtyend = nbp->b_bcount;
                   }
                   flushchainbuf(nbp);
                   /* nbp = NULL; */
           }
   
           /*
            * Wait for completion.
            */
   
           if (bp->b_flags & B_ASYNC) {
                   autochaindone(bp);
           } else {
                   waitchainbuf(bp, 0, 1);
           }
   }
   
   /*
    * SWAP_PAGER_GETPAGES() - bring pages in from swap
    *
    *      Attempt to retrieve (m, count) pages from backing store, but make
    *      sure we retrieve at least m[reqpage].  We try to load in as large
    *      a chunk surrounding m[reqpage] as is contiguous in swap and which
    *      belongs to the same object.
    *
   *      The code is designed for asynchronous operation and 
   *      immediate-notification of 'reqpage' but tends not to be
   *      used that way.  Please do not optimize-out this algorithmic
   *      feature, I intend to improve on it in the future.
   *
   *      The parent has a single vm_object_pip_add() reference prior to
   *      calling us and we should return with the same.
   *
   *      The parent has BUSY'd the pages.  We should return with 'm'
   *      left busy, but the others adjusted.
   */
  
  static int
  swap_pager_getpages(object, m, count, reqpage)
          vm_object_t object;
          vm_page_t *m;
          int count, reqpage;
  {
          struct buf *bp;
          vm_page_t mreq;
          int s;
          int i;
          int j;
          daddr_t blk;
          vm_offset_t kva;
          vm_pindex_t lastpindex;
  
          mreq = m[reqpage];
  
          if (mreq->object != object) {
                  panic("swap_pager_getpages: object mismatch %p/%p", 
                      object, 
                      mreq->object
                  );
          }
          /*
           * Calculate range to retrieve.  The pages have already been assigned
           * their swapblks.  We require a *contiguous* range that falls entirely
           * within a single device stripe.   If we do not supply it, bad things
           * happen.  Note that blk, iblk & jblk can be SWAPBLK_NONE, but the 
           * loops are set up such that the case(s) are handled implicitly.
           *
           * The swp_*() calls must be made at splvm().  vm_page_free() does
           * not need to be, but it will go a little faster if it is.
           */
  
          s = splvm();
          blk = swp_pager_meta_ctl(mreq->object, mreq->pindex, 0);
  
          for (i = reqpage - 1; i >= 0; --i) {
                  daddr_t iblk;
  
                  iblk = swp_pager_meta_ctl(m[i]->object, m[i]->pindex, 0);
                  if (blk != iblk + (reqpage - i))
                          break;
                  if ((blk ^ iblk) & dmmax_mask)
                          break;
          }
          ++i;
  
          for (j = reqpage + 1; j < count; ++j) {
                  daddr_t jblk;
  
                  jblk = swp_pager_meta_ctl(m[j]->object, m[j]->pindex, 0);
                  if (blk != jblk - (j - reqpage))
                          break;
                  if ((blk ^ jblk) & dmmax_mask)
                          break;
          }
  
          /*
           * free pages outside our collection range.   Note: we never free
           * mreq, it must remain busy throughout.
           */
  
          {
                  int k;
  
                  for (k = 0; k < i; ++k)
                          vm_page_free(m[k]);
                  for (k = j; k < count; ++k)
                          vm_page_free(m[k]);
          }
          splx(s);
  
  
          /*
           * Return VM_PAGER_FAIL if we have nothing to do.  Return mreq 
           * still busy, but the others unbusied.
           */
  
          if (blk == SWAPBLK_NONE)
                  return(VM_PAGER_FAIL);
  
          /*
           * Get a swap buffer header to perform the IO
           */
  
          bp = getpbuf(&nsw_rcount);
          kva = (vm_offset_t) bp->b_data;
  
          /*
           * map our page(s) into kva for input
           *
           * NOTE: B_PAGING is set by pbgetvp()
           */
  
          pmap_qenter(kva, m + i, j - i);
  
          bp->b_flags = B_READ | B_CALL;
          bp->b_iodone = swp_pager_async_iodone;
          bp->b_rcred = bp->b_wcred = proc0.p_ucred;
          bp->b_data = (caddr_t) kva;
          crhold(bp->b_rcred);
          crhold(bp->b_wcred);
          bp->b_blkno = blk - (reqpage - i);
          bp->b_bcount = PAGE_SIZE * (j - i);
          bp->b_bufsize = PAGE_SIZE * (j - i);
          bp->b_pager.pg_reqpage = reqpage - i;
  
          {
                  int k;
  
                  for (k = i; k < j; ++k) {
                          bp->b_pages[k - i] = m[k];
                          vm_page_flag_set(m[k], PG_SWAPINPROG);
                  }
          }
          bp->b_npages = j - i;
  
          pbgetvp(swapdev_vp, bp);
  
          cnt.v_swapin++;
          cnt.v_swappgsin += bp->b_npages;
  
          /*
           * We still hold the lock on mreq, and our automatic completion routine
           * does not remove it.
           */
  
          vm_object_pip_add(mreq->object, bp->b_npages);
          lastpindex = m[j-1]->pindex;
  
          /*
           * perform the I/O.  NOTE!!!  bp cannot be considered valid after
           * this point because we automatically release it on completion.
           * Instead, we look at the one page we are interested in which we
           * still hold a lock on even through the I/O completion.
           *
           * The other pages in our m[] array are also released on completion,
           * so we cannot assume they are valid anymore either.
           *
           * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
           */
  
          BUF_KERNPROC(bp);
          VOP_STRATEGY(bp->b_vp, bp);
  
          /*
           * wait for the page we want to complete.  PG_SWAPINPROG is always
           * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
           * is set in the meta-data.
           */
  
          s = splvm();
  
          while ((mreq->flags & PG_SWAPINPROG) != 0) {
                  vm_page_flag_set(mreq, PG_WANTED | PG_REFERENCED);
                  cnt.v_intrans++;
                  if (tsleep(mreq, PSWP, "swread", hz*20)) {
                          printf(
                              "swap_pager: indefinite wait buffer: device:"
                                  " %s, blkno: %ld, size: %ld\n",
                              devtoname(bp->b_dev), (long)bp->b_blkno,
                              bp->b_bcount
                          );
                  }
          }
  
          splx(s);
  
          /*
           * mreq is left bussied after completion, but all the other pages
           * are freed.  If we had an unrecoverable read error the page will
           * not be valid.
           */
  
          if (mreq->valid != VM_PAGE_BITS_ALL) {
                  return(VM_PAGER_ERROR);
          } else {
                  return(VM_PAGER_OK);
          }
  
          /*
           * A final note: in a low swap situation, we cannot deallocate swap
           * and mark a page dirty here because the caller is likely to mark
           * the page clean when we return, causing the page to possibly revert 
           * to all-zero's later.
           */
  }
  
  /*
   *      swap_pager_putpages: 
   *
   *      Assign swap (if necessary) and initiate I/O on the specified pages.
   *
   *      We support both OBJT_DEFAULT and OBJT_SWAP objects.  DEFAULT objects
   *      are automatically converted to SWAP objects.
   *
   *      In a low memory situation we may block in VOP_STRATEGY(), but the new 
   *      vm_page reservation system coupled with properly written VFS devices 
   *      should ensure that no low-memory deadlock occurs.  This is an area
   *      which needs work.
   *
   *      The parent has N vm_object_pip_add() references prior to
   *      calling us and will remove references for rtvals[] that are
   *      not set to VM_PAGER_PEND.  We need to remove the rest on I/O
   *      completion.
   *
   *      The parent has soft-busy'd the pages it passes us and will unbusy
   *      those whos rtvals[] entry is not set to VM_PAGER_PEND on return.
   *      We need to unbusy the rest on I/O completion.
   */
  
  void
  swap_pager_putpages(object, m, count, sync, rtvals)
          vm_object_t object;
          vm_page_t *m;
          int count;
          boolean_t sync;
          int *rtvals;
  {
          int i;
          int n = 0;
  
          if (count && m[0]->object != object) {
                  panic("swap_pager_getpages: object mismatch %p/%p", 
                      object, 
                      m[0]->object
                  );
          }
          /*
           * Step 1
           *
           * Turn object into OBJT_SWAP
           * check for bogus sysops
           * force sync if not pageout process
           */
  
          if (object->type != OBJT_SWAP)
                  swp_pager_meta_build(object, 0, SWAPBLK_NONE);
  
          if (curproc != pageproc)
                  sync = TRUE;
  
          /*
           * Step 2
           *
           * Update nsw parameters from swap_async_max sysctl values.  
           * Do not let the sysop crash the machine with bogus numbers.
           */
  
          if (swap_async_max != nsw_wcount_async_max) {
                  int n;
                  int s;
  
                  /*
                   * limit range
                   */
                  if ((n = swap_async_max) > nswbuf / 2)
                          n = nswbuf / 2;
                  if (n < 1)
                          n = 1;
                  swap_async_max = n;
  
                  /*
                   * Adjust difference ( if possible ).  If the current async
                   * count is too low, we may not be able to make the adjustment
                   * at this time.
                   */
                  s = splvm();
                  n -= nsw_wcount_async_max;
                  if (nsw_wcount_async + n >= 0) {
                          nsw_wcount_async += n;
                          nsw_wcount_async_max += n;
                          wakeup(&nsw_wcount_async);
                  }
                  splx(s);
          }
  
          /*
           * Step 3
           *
           * Assign swap blocks and issue I/O.  We reallocate swap on the fly.
           * The page is left dirty until the pageout operation completes
           * successfully.
           */
  
          for (i = 0; i < count; i += n) {
                  int s;
                  int j;
                  struct buf *bp;
                  daddr_t blk;
  
                  /*
                   * Maximum I/O size is limited by a number of factors.
                   */
  
                  n = min(BLIST_MAX_ALLOC, count - i);
                  n = min(n, nsw_cluster_max);
  
                  s = splvm();
  
                  /*
                   * Get biggest block of swap we can.  If we fail, fall
                   * back and try to allocate a smaller block.  Don't go
                   * overboard trying to allocate space if it would overly
                   * fragment swap.
                   */
                  while (
                      (blk = swp_pager_getswapspace(n)) == SWAPBLK_NONE &&
                      n > 4
                  ) {
                          n >>= 1;
                  }
                  if (blk == SWAPBLK_NONE) {
                          for (j = 0; j < n; ++j)
                                  rtvals[i+j] = VM_PAGER_FAIL;
                          splx(s);
                          continue;
                  }
  
                  /*
                   * The I/O we are constructing cannot cross a physical
                   * disk boundry in the swap stripe.  Note: we are still
                   * at splvm().
                   */
                  if ((blk ^ (blk + n)) & dmmax_mask) {
                          j = ((blk + dmmax) & dmmax_mask) - blk;
                          swp_pager_freeswapspace(blk + j, n - j);
                          n = j;
                  }
  
                  /*
                   * All I/O parameters have been satisfied, build the I/O
                   * request and assign the swap space.
                   *
                   * NOTE: B_PAGING is set by pbgetvp()
                   */
  
                  if (sync == TRUE) {
                          bp = getpbuf(&nsw_wcount_sync);
                          bp->b_flags = B_CALL;
                  } else {
                          bp = getpbuf(&nsw_wcount_async);
                          bp->b_flags = B_CALL | B_ASYNC;
                  }
                  bp->b_spc = NULL;       /* not used, but NULL-out anyway */
  
                  pmap_qenter((vm_offset_t)bp->b_data, &m[i], n);
  
                  bp->b_rcred = bp->b_wcred = proc0.p_ucred;
                  bp->b_bcount = PAGE_SIZE * n;
                  bp->b_bufsize = PAGE_SIZE * n;
                  bp->b_blkno = blk;
  
                  crhold(bp->b_rcred);
                  crhold(bp->b_wcred);
  
                  pbgetvp(swapdev_vp, bp);
  
                  for (j = 0; j < n; ++j) {
                          vm_page_t mreq = m[i+j];
  
                          swp_pager_meta_build(
                              mreq->object, 
                              mreq->pindex,
                              blk + j
                          );
                          vm_page_dirty(mreq);
                          rtvals[i+j] = VM_PAGER_OK;
  
                          vm_page_flag_set(mreq, PG_SWAPINPROG);
                          bp->b_pages[j] = mreq;
                  }
                  bp->b_npages = n;
                  /*
                   * Must set dirty range for NFS to work.
                   */
                  bp->b_dirtyoff = 0;
                  bp->b_dirtyend = bp->b_bcount;
  
                  cnt.v_swapout++;
                  cnt.v_swappgsout += bp->b_npages;
                  swapdev_vp->v_numoutput++;
  
                  splx(s);
  
                  /*
                   * asynchronous
                   *
                   * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
                   */
  
                  if (sync == FALSE) {
                          bp->b_iodone = swp_pager_async_iodone;
                          BUF_KERNPROC(bp);
                          VOP_STRATEGY(bp->b_vp, bp);
  
                          for (j = 0; j < n; ++j)
                                  rtvals[i+j] = VM_PAGER_PEND;
                          continue;
                  }
  
                  /*
                   * synchronous
                   *
                   * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
                   */
  
                  bp->b_iodone = swp_pager_sync_iodone;
                  VOP_STRATEGY(bp->b_vp, bp);
  
                  /*
                   * Wait for the sync I/O to complete, then update rtvals.
                   * We just set the rtvals[] to VM_PAGER_PEND so we can call
                   * our async completion routine at the end, thus avoiding a
                   * double-free.
                   */
                  s = splbio();
  
                  while ((bp->b_flags & B_DONE) == 0) {
                          tsleep(bp, PVM, "swwrt", 0);
                  }
  
                  for (j = 0; j < n; ++j)
                          rtvals[i+j] = VM_PAGER_PEND;
  
                  /*
                   * Now that we are through with the bp, we can call the
                   * normal async completion, which frees everything up.
                   */
  
                  swp_pager_async_iodone(bp);
  
                  splx(s);
          }
  }
  
  /*
   *      swap_pager_sync_iodone:
   *
   *      Completion routine for synchronous reads and writes from/to swap.
   *      We just mark the bp is complete and wake up anyone waiting on it.
   *
   *      This routine may not block.  This routine is called at splbio() or better.
   */
  
  static void
  swp_pager_sync_iodone(bp)
          struct buf *bp;
  {
          bp->b_flags |= B_DONE;
          bp->b_flags &= ~B_ASYNC;
          wakeup(bp);
  }
  
  /*
   *      swp_pager_async_iodone:
   *
   *      Completion routine for asynchronous reads and writes from/to swap.
   *      Also called manually by synchronous code to finish up a bp.
   *
   *      For READ operations, the pages are PG_BUSY'd.  For WRITE operations, 
   *      the pages are vm_page_t->busy'd.  For READ operations, we PG_BUSY 
   *      unbusy all pages except the 'main' request page.  For WRITE 
   *      operations, we vm_page_t->busy'd unbusy all pages ( we can do this 
   *      because we marked them all VM_PAGER_PEND on return from putpages ).
   *
   *      This routine may not block.
   *      This routine is called at splbio() or better
   *
   *      We up ourselves to splvm() as required for various vm_page related
   *      calls.
   */
  
  static void
  swp_pager_async_iodone(bp)
          register struct buf *bp;
  {
          int s;
          int i;
          vm_object_t object = NULL;
  
          bp->b_flags |= B_DONE;
  
          /*
           * report error
           */
  
          if (bp->b_flags & B_ERROR) {
                  printf(
                      "swap_pager: I/O error - %s failed; blkno %ld,"
                          "size %ld, error %d\n",
                      ((bp->b_flags & B_READ) ? "pagein" : "pageout"),
                      (long)bp->b_blkno, 
                      (long)bp->b_bcount,
                      bp->b_error
                  );
          }
  
          /*
           * set object, raise to splvm().
           */
  
          if (bp->b_npages)
                  object = bp->b_pages[0]->object;
          s = splvm();
  
          /*
           * remove the mapping for kernel virtual
           */
  
          pmap_qremove((vm_offset_t)bp->b_data, bp->b_npages);
  
          /*
           * cleanup pages.  If an error occurs writing to swap, we are in
           * very serious trouble.  If it happens to be a disk error, though,
           * we may be able to recover by reassigning the swap later on.  So
           * in this case we remove the m->swapblk assignment for the page 
           * but do not free it in the rlist.  The errornous block(s) are thus
           * never reallocated as swap.  Redirty the page and continue.
           */
  
          for (i = 0; i < bp->b_npages; ++i) {
                  vm_page_t m = bp->b_pages[i];
  
                  vm_page_flag_clear(m, PG_SWAPINPROG);
  
                  if (bp->b_flags & B_ERROR) {
                          /*
                           * If an error occurs I'd love to throw the swapblk
                           * away without freeing it back to swapspace, so it
                           * can never be used again.  But I can't from an 
                           * interrupt.
                           */
  
                          if (bp->b_flags & B_READ) {
                                  /*
                                   * When reading, reqpage needs to stay
                                   * locked for the parent, but all other
                                   * pages can be freed.  We still want to
                                   * wakeup the parent waiting on the page,
                                   * though.  ( also: pg_reqpage can be -1 and 
                                   * not match anything ).
                                   *
                                   * We have to wake specifically requested pages
                                   * up too because we cleared PG_SWAPINPROG and
                                   * someone may be waiting for that.
                                   *
                                   * NOTE: for reads, m->dirty will probably
                                   * be overridden by the original caller of
                                   * getpages so don't play cute tricks here.
                                   *
                                   * XXX IT IS NOT LEGAL TO FREE THE PAGE HERE
                                   * AS THIS MESSES WITH object->memq, and it is
                                   * not legal to mess with object->memq from an
                                   * interrupt.
                                   */
  
                                  m->valid = 0;
                                  vm_page_flag_clear(m, PG_ZERO);
  
                                  if (i != bp->b_pager.pg_reqpage)
                                          vm_page_free(m);
                                  else
                                          vm_page_flash(m);
                                  /*
                                   * If i == bp->b_pager.pg_reqpage, do not wake 
                                   * the page up.  The caller needs to.
                                   */
                          } else {
                                  /*
                                   * If a write error occurs, reactivate page
                                   * so it doesn't clog the inactive list,
                                   * then finish the I/O.
                                   */
                                  vm_page_dirty(m);
                                  vm_page_activate(m);
                                  vm_page_io_finish(m);
                          }
                  } else if (bp->b_flags & B_READ) {
                          /*
                           * For read success, clear dirty bits.  Nobody should
                           * have this page mapped but don't take any chances,
                           * make sure the pmap modify bits are also cleared.
                           *
                           * NOTE: for reads, m->dirty will probably be 
                           * overridden by the original caller of getpages so
                           * we cannot set them in order to free the underlying
                           * swap in a low-swap situation.  I don't think we'd
                           * want to do that anyway, but it was an optimization
                           * that existed in the old swapper for a time before
                           * it got ripped out due to precisely this problem.
                           *
                           * clear PG_ZERO in page.
                           *
                           * If not the requested page then deactivate it.
                           *
                           * Note that the requested page, reqpage, is left
                           * busied, but we still have to wake it up.  The
                           * other pages are released (unbusied) by 
                           * vm_page_wakeup().  We do not set reqpage's
                           * valid bits here, it is up to the caller.
                           */
  
                          pmap_clear_modify(m);
                          m->valid = VM_PAGE_BITS_ALL;
                          vm_page_undirty(m);
                          vm_page_flag_clear(m, PG_ZERO);
  
                          /*
                           * We have to wake specifically requested pages
                           * up too because we cleared PG_SWAPINPROG and
                           * could be waiting for it in getpages.  However,
                           * be sure to not unbusy getpages specifically
                           * requested page - getpages expects it to be 
                           * left busy.
                           */
                          if (i != bp->b_pager.pg_reqpage) {
                                  vm_page_deactivate(m);
                                  vm_page_wakeup(m);
                          } else {
                                  vm_page_flash(m);
                          }
                  } else {
                          /*
                           * For write success, clear the modify and dirty 
                           * status, then finish the I/O ( which decrements the 
                           * busy count and possibly wakes waiter's up ).
                           */
                          pmap_clear_modify(m);
                          vm_page_undirty(m);
                          vm_page_io_finish(m);
                          if (!vm_page_count_severe() || !vm_page_try_to_cache(m))
                                  vm_page_protect(m, VM_PROT_READ);
                  }
          }
  
          /*
           * adjust pip.  NOTE: the original parent may still have its own
           * pip refs on the object.
           */
  
          if (object)
                  vm_object_pip_wakeupn(object, bp->b_npages);
  
          /*
           * release the physical I/O buffer
           */
  
          relpbuf(
              bp, 
              ((bp->b_flags & B_READ) ? &nsw_rcount : 
                  ((bp->b_flags & B_ASYNC) ? 
                      &nsw_wcount_async : 
                      &nsw_wcount_sync
                  )
              )
          );
          splx(s);
  }
  
  /************************************************************************
   *                              SWAP META DATA                          *
   ************************************************************************
   *
   *      These routines manipulate the swap metadata stored in the 
   *      OBJT_SWAP object.  All swp_*() routines must be called at
   *      splvm() because swap can be freed up by the low level vm_page
   *      code which might be called from interrupts beyond what splbio() covers.
   *
   *      Swap metadata is implemented with a global hash and not directly
   *      linked into the object.  Instead the object simply contains
   *      appropriate tracking counters.
   */
  
  /*
   * SWP_PAGER_HASH() -   hash swap meta data
   *
   *      This is an inline helper function which hashes the swapblk given
   *      the object and page index.  It returns a pointer to a pointer
   *      to the object, or a pointer to a NULL pointer if it could not
   *      find a swapblk.
   *
   *      This routine must be called at splvm().
   */
  
  static __inline struct swblock **
  swp_pager_hash(vm_object_t object, vm_pindex_t index)
  {
          struct swblock **pswap;
          struct swblock *swap;
  
          index &= ~SWAP_META_MASK;
          pswap = &swhash[(index ^ (int)(intptr_t)object) & swhash_mask];
  
          while ((swap = *pswap) != NULL) {
                  if (swap->swb_object == object &&
                      swap->swb_index == index
                  ) {
                          break;
                  }
                  pswap = &swap->swb_hnext;
          }
          return(pswap);
  }
  
  /*
   * SWP_PAGER_META_BUILD() -     add swap block to swap meta data for object
   *
   *      We first convert the object to a swap object if it is a default
   *      object.
   *
   *      The specified swapblk is added to the object's swap metadata.  If
   *      the swapblk is not valid, it is freed instead.  Any previously
   *      assigned swapblk is freed.
   *
   *      This routine must be called at splvm(), except when used to convert
   *      an OBJT_DEFAULT object into an OBJT_SWAP object.
  
   */
  
  static void
  swp_pager_meta_build(
          vm_object_t object, 
          vm_pindex_t index,
          daddr_t swapblk
  ) {
          struct swblock *swap;
          struct swblock **pswap;
  
          /*
           * Convert default object to swap object if necessary
           */
  
          if (object->type != OBJT_SWAP) {
                  object->type = OBJT_SWAP;
                  object->un_pager.swp.swp_bcount = 0;
  
                  if (object->handle != NULL) {
                          TAILQ_INSERT_TAIL(
                              NOBJLIST(object->handle),
                              object, 
                              pager_object_list
                          );
                  } else {
                          TAILQ_INSERT_TAIL(
                              &swap_pager_un_object_list,
                              object, 
                              pager_object_list
                          );
                  }
          }
          
          /*
           * Locate hash entry.  If not found create, but if we aren't adding
           * anything just return.  If we run out of space in the map we wait
           * and, since the hash table may have changed, retry.
           */
  
  retry:
          pswap = swp_pager_hash(object, index);
  
          if ((swap = *pswap) == NULL) {
                  int i;
  
                  if (swapblk == SWAPBLK_NONE)
                          return;
  
                  swap = *pswap = zalloc(swap_zone);
                  if (swap == NULL) {
                          if (swap_zone->zpagecount >= swap_zone->zpagemax)
                                  printf("swap zone exhausted, increase kern.maxswzone\n");
                          VM_WAIT;
                          goto retry;
                  }
                  swap->swb_hnext = NULL;
                  swap->swb_object = object;
                  swap->swb_index = index & ~SWAP_META_MASK;
                  swap->swb_count = 0;
  
                  ++object->un_pager.swp.swp_bcount;
  
                  for (i = 0; i < SWAP_META_PAGES; ++i)
                          swap->swb_pages[i] = SWAPBLK_NONE;
          }
  
          /*
           * Delete prior contents of metadata
           */
  
          index &= SWAP_META_MASK;
  
          if (swap->swb_pages[index] != SWAPBLK_NONE) {
                  swp_pager_freeswapspace(swap->swb_pages[index], 1);
                  --swap->swb_count;
          }
  
          /*
           * Enter block into metadata
           */
  
          swap->swb_pages[index] = swapblk;
          if (swapblk != SWAPBLK_NONE)
                  ++swap->swb_count;
  }
  
  /*
   * SWP_PAGER_META_FREE() - free a range of blocks in the object's swap metadata
   *
   *      The requested range of blocks is freed, with any associated swap 
   *      returned to the swap bitmap.
   *
   *      This routine will free swap metadata structures as they are cleaned 
   *      out.  This routine does *NOT* operate on swap metadata associated
   *      with resident pages.
   *
   *      This routine must be called at splvm()
   */
  
  static void
  swp_pager_meta_free(vm_object_t object, vm_pindex_t index, daddr_t count)
  {
          if (object->type != OBJT_SWAP)
                  return;
  
          while (count > 0) {
                  struct swblock **pswap;
                  struct swblock *swap;
  
                  pswap = swp_pager_hash(object, index);
  
                  if ((swap = *pswap) != NULL) {
                          daddr_t v = swap->swb_pages[index & SWAP_META_MASK];
  
                          if (v != SWAPBLK_NONE) {
                                  swp_pager_freeswapspace(v, 1);
                                  swap->swb_pages[index & SWAP_META_MASK] =
                                          SWAPBLK_NONE;
                                  if (--swap->swb_count == 0) {
                                          *pswap = swap->swb_hnext;
                                          zfree(swap_zone, swap);
                                          --object->un_pager.swp.swp_bcount;
                                  }
                          }
                          --count;
                          ++index;
                  } else {
                          int n = SWAP_META_PAGES - (index & SWAP_META_MASK);
                          count -= n;
                          index += n;
                  }
          }
  }
  
  /*
   * SWP_PAGER_META_FREE_ALL() - destroy all swap metadata associated with object
   *
   *      This routine locates and destroys all swap metadata associated with
   *      an object.
   *
   *      This routine must be called at splvm()
   */
  
  static void
  swp_pager_meta_free_all(vm_object_t object)
  {
          daddr_t index = 0;
  
          if (object->type != OBJT_SWAP)
                  return;
  
          while (object->un_pager.swp.swp_bcount) {
                  struct swblock **pswap;
                  struct swblock *swap;
  
                  pswap = swp_pager_hash(object, index);
                  if ((swap = *pswap) != NULL) {
                          int i;
  
                          for (i = 0; i < SWAP_META_PAGES; ++i) {
                                  daddr_t v = swap->swb_pages[i];
                                  if (v != SWAPBLK_NONE) {
                                          --swap->swb_count;
                                          swp_pager_freeswapspace(v, 1);
                                  }
                          }
                          if (swap->swb_count != 0)
                                  panic("swap_pager_meta_free_all: swb_count != 0");
                          *pswap = swap->swb_hnext;
                          zfree(swap_zone, swap);
                          --object->un_pager.swp.swp_bcount;
                  }
                  index += SWAP_META_PAGES;
                  if (index > 0x20000000)
                          panic("swp_pager_meta_free_all: failed to locate all swap meta blocks");
          }
  }
  
  /*
   * SWP_PAGER_METACTL() -  misc control of swap and vm_page_t meta data.
   *
   *      This routine is capable of looking up, popping, or freeing
   *      swapblk assignments in the swap meta data or in the vm_page_t.
   *      The routine typically returns the swapblk being looked-up, or popped,
   *      or SWAPBLK_NONE if the block was freed, or SWAPBLK_NONE if the block
   *      was invalid.  This routine will automatically free any invalid 
   *      meta-data swapblks.
   *
   *      It is not possible to store invalid swapblks in the swap meta data
   *      (other then a literal 'SWAPBLK_NONE'), so we don't bother checking.
   *
   *      When acting on a busy resident page and paging is in progress, we 
   *      have to wait until paging is complete but otherwise can act on the 
   *      busy page.
   *
   *      This routine must be called at splvm().
   *
   *      SWM_FREE        remove and free swap block from metadata
   *      SWM_POP         remove from meta data but do not free.. pop it out
   */
  
  static daddr_t
  swp_pager_meta_ctl(
          vm_object_t object,
          vm_pindex_t index,
          int flags
  ) {
          struct swblock **pswap;
          struct swblock *swap;
          daddr_t r1;
  
          /*
           * The meta data only exists of the object is OBJT_SWAP 
           * and even then might not be allocated yet.
           */
  
          if (object->type != OBJT_SWAP)
                  return(SWAPBLK_NONE);
  
          r1 = SWAPBLK_NONE;
          pswap = swp_pager_hash(object, index);
  
          if ((swap = *pswap) != NULL) {
                  index &= SWAP_META_MASK;
                  r1 = swap->swb_pages[index];
  
                  if (r1 != SWAPBLK_NONE) {
                          if (flags & SWM_FREE) {
                                  swp_pager_freeswapspace(r1, 1);
                                  r1 = SWAPBLK_NONE;
                          }
                          if (flags & (SWM_FREE|SWM_POP)) {
                                  swap->swb_pages[index] = SWAPBLK_NONE;
                                  if (--swap->swb_count == 0) {
                                          *pswap = swap->swb_hnext;
                                          zfree(swap_zone, swap);
                                          --object->un_pager.swp.swp_bcount;
                                  }
                          } 
                  }
          }
          return(r1);
  }
  

Cache object: a5981e65825a2977bff67912cf60722e