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) 1982, 1986, 1989, 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
     *      @(#)vm_swap.c   8.5 (Berkeley) 2/17/94
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_mac.h"
    #include "opt_swap.h"
    #include "opt_vm.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/conf.h>
    #include <sys/kernel.h>
    #include <sys/priv.h>
    #include <sys/proc.h>
    #include <sys/bio.h>
    #include <sys/buf.h>
    #include <sys/disk.h>
    #include <sys/fcntl.h>
    #include <sys/mount.h>
    #include <sys/namei.h>
    #include <sys/vnode.h>
    #include <sys/malloc.h>
    #include <sys/sysctl.h>
    #include <sys/sysproto.h>
    #include <sys/blist.h>
    #include <sys/lock.h>
    #include <sys/sx.h>
    #include <sys/vmmeter.h>
    
    #include <security/mac/mac_framework.h>
    
    #include <vm/vm.h>
   #include <vm/pmap.h>
   #include <vm/vm_map.h>
   #include <vm/vm_kern.h>
   #include <vm/vm_object.h>
   #include <vm/vm_page.h>
   #include <vm/vm_pager.h>
   #include <vm/vm_pageout.h>
   #include <vm/vm_param.h>
   #include <vm/swap_pager.h>
   #include <vm/vm_extern.h>
   #include <vm/uma.h>
   
   #include <geom/geom.h>
   
   /*
    * SWB_NPAGES must be a power of 2.  It may be set to 1, 2, 4, 8, or 16
    * pages per allocation.  We recommend you stick with the default of 8.
    * The 16-page limit is due to the radix code (kern/subr_blist.c).
    */
   #ifndef MAX_PAGEOUT_CLUSTER
   #define MAX_PAGEOUT_CLUSTER 16
   #endif
   
   #if !defined(SWB_NPAGES)
   #define SWB_NPAGES      MAX_PAGEOUT_CLUSTER
   #endif
   
   /*
    * Piecemeal swap metadata structure.  Swap is stored in a radix tree.
    *
    * If SWB_NPAGES is 8 and sizeof(char *) == sizeof(daddr_t), our radix
    * is basically 8.  Assuming PAGE_SIZE == 4096, one tree level represents
    * 32K worth of data, two levels represent 256K, three levels represent
    * 2 MBytes.   This is acceptable.
    *
    * Overall memory utilization is about the same as the old swap structure.
    */
   #define SWCORRECT(n) (sizeof(void *) * (n) / sizeof(daddr_t))
   #define SWAP_META_PAGES         (SWB_NPAGES * 2)
   #define SWAP_META_MASK          (SWAP_META_PAGES - 1)
   
   struct swblock {
           struct swblock  *swb_hnext;
           vm_object_t     swb_object;
           vm_pindex_t     swb_index;
           int             swb_count;
           daddr_t         swb_pages[SWAP_META_PAGES];
   };
   
   static struct mtx sw_dev_mtx;
   static TAILQ_HEAD(, swdevt) swtailq = TAILQ_HEAD_INITIALIZER(swtailq);
   static struct swdevt *swdevhd;  /* Allocate from here next */
   static int nswapdev;            /* Number of swap devices */
   int swap_pager_avail;
   static int swdev_syscall_active = 0; /* serialize swap(on|off) */
   
   static void swapdev_strategy(struct buf *, struct swdevt *sw);
   
   #define SWM_FREE        0x02    /* free, period                 */
   #define SWM_POP         0x04    /* pop out                      */
   
   int swap_pager_full = 2;        /* swap space exhaustion (task killing) */
   static int swap_pager_almost_full = 1; /* 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 struct swblock **swhash;
   static int swhash_mask;
   static struct mtx swhash_mtx;
   
   static int swap_async_max = 4;  /* maximum in-progress async I/O's      */
   static struct sx sw_alloc_sx;
   
   
   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 mtx sw_alloc_mtx; /* protect list manipulation */ 
   static struct pagerlst  swap_pager_object_list[NOBJLISTS];
   static uma_zone_t       swap_zone;
   static struct vm_object swap_zone_obj;
   
   /*
    * 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(void *handle, vm_ooffset_t size,
                                         vm_prot_t prot, vm_ooffset_t offset);
   static void     swap_pager_dealloc(vm_object_t object);
   static int      swap_pager_getpages(vm_object_t, vm_page_t *, int, int);
   static void     swap_pager_putpages(vm_object_t, vm_page_t *, int, boolean_t, int *);
   static boolean_t
                   swap_pager_haspage(vm_object_t object, vm_pindex_t pindex, int *before, int *after);
   static void     swap_pager_init(void);
   static void     swap_pager_unswapped(vm_page_t);
   static void     swap_pager_swapoff(struct swdevt *sp);
   
   struct pagerops swappagerops = {
           .pgo_init =     swap_pager_init,        /* early system initialization of pager */
           .pgo_alloc =    swap_pager_alloc,       /* allocate an OBJT_SWAP object         */
           .pgo_dealloc =  swap_pager_dealloc,     /* deallocate an OBJT_SWAP object       */
           .pgo_getpages = swap_pager_getpages,    /* pagein                               */
           .pgo_putpages = swap_pager_putpages,    /* pageout                              */
           .pgo_haspage =  swap_pager_haspage,     /* get backing store status for page    */
           .pgo_pageunswapped = swap_pager_unswapped,      /* remove swap related to page          */
   };
   
   /*
    * 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.
    */
   static int dmmax;
   static int nswap_lowat = 128;   /* in pages, swap_pager_almost_full warn */
   static int nswap_hiwat = 512;   /* in pages, swap_pager_almost_full warn */
   
   SYSCTL_INT(_vm, OID_AUTO, dmmax,
           CTLFLAG_RD, &dmmax, 0, "Maximum size of a swap block");
   
   static void     swp_sizecheck(void);
   static void     swp_pager_async_iodone(struct buf *bp);
   static int      swapongeom(struct thread *, struct vnode *);
   static int      swaponvp(struct thread *, struct vnode *, u_long);
   static int      swapoff_one(struct swdevt *sp, struct thread *td);
   
   /*
    * Swap bitmap functions
    */
   static void     swp_pager_freeswapspace(daddr_t blk, int npages);
   static daddr_t  swp_pager_getswapspace(int npages);
   
   /*
    * Metadata functions
    */
   static struct swblock **swp_pager_hash(vm_object_t object, vm_pindex_t index);
   static void swp_pager_meta_build(vm_object_t, vm_pindex_t, daddr_t);
   static void swp_pager_meta_free(vm_object_t, vm_pindex_t, daddr_t);
   static void swp_pager_meta_free_all(vm_object_t);
   static daddr_t swp_pager_meta_ctl(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 void
   swp_sizecheck(void)
   {
   
           if (swap_pager_avail < 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 (swap_pager_avail > nswap_hiwat)
                           swap_pager_almost_full = 0;
           }
   }
   
   /*
    * SWP_PAGER_HASH() -   hash swap meta data
    *
    *      This is an 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 struct swblock **
   swp_pager_hash(vm_object_t object, vm_pindex_t index)
   {
           struct swblock **pswap;
           struct swblock *swap;
   
           index &= ~(vm_pindex_t)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);
   }
   
   /*
    * 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(void)
   {
           /*
            * Initialize object lists
            */
           int i;
   
           for (i = 0; i < NOBJLISTS; ++i)
                   TAILQ_INIT(&swap_pager_object_list[i]);
           mtx_init(&sw_alloc_mtx, "swap_pager list", NULL, MTX_DEF);
           mtx_init(&sw_dev_mtx, "swapdev", NULL, MTX_DEF);
   
           /*
            * Device Stripe, in PAGE_SIZE'd blocks
            */
           dmmax = SWB_NPAGES * 2;
   }
   
   /*
    * 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(void)
   {
           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);
   
           mtx_lock(&pbuf_mtx);
           nsw_rcount = (nswbuf + 1) / 2;
           nsw_wcount_sync = (nswbuf + 3) / 4;
           nsw_wcount_async = 4;
           nsw_wcount_async_max = nsw_wcount_async;
           mtx_unlock(&pbuf_mtx);
   
           /*
            * 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.
            */
           n = cnt.v_page_count / 2;
           if (maxswzone && n > maxswzone / sizeof(struct swblock))
                   n = maxswzone / sizeof(struct swblock);
           n2 = n;
           swap_zone = uma_zcreate("SWAPMETA", sizeof(struct swblock), NULL, NULL,
               NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE | UMA_ZONE_VM);
           if (swap_zone == NULL)
                   panic("failed to create swap_zone.");
           do {
                   if (uma_zone_set_obj(swap_zone, &swap_zone_obj, n))
                           break;
                   /*
                    * if the allocation failed, try a zone two thirds the
                    * size of the previous attempt.
                    */
                   n -= ((n + 2) / 3);
           } while (n > 0);
           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 | M_ZERO);
           swhash_mask = n - 1;
           mtx_init(&swhash_mtx, "swap_pager swhash", NULL, MTX_DEF);
   }
   
   /*
    * 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.
    *
    * MPSAFE
    */
   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;
           vm_pindex_t pindex;
   
           pindex = OFF_TO_IDX(offset + PAGE_MASK + size);
   
           if (handle) {
                   mtx_lock(&Giant);
                   /*
                    * 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.
                    */
                   sx_xlock(&sw_alloc_sx);
                   object = vm_pager_object_lookup(NOBJLIST(handle), handle);
   
                   if (object == NULL) {
                           object = vm_object_allocate(OBJT_DEFAULT, pindex);
                           object->handle = handle;
   
                           VM_OBJECT_LOCK(object);
                           swp_pager_meta_build(object, 0, SWAPBLK_NONE);
                           VM_OBJECT_UNLOCK(object);
                   }
                   sx_xunlock(&sw_alloc_sx);
                   mtx_unlock(&Giant);
           } else {
                   object = vm_object_allocate(OBJT_DEFAULT, pindex);
   
                   VM_OBJECT_LOCK(object);
                   swp_pager_meta_build(object, 0, SWAPBLK_NONE);
                   VM_OBJECT_UNLOCK(object);
           }
           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(vm_object_t object)
   {
   
           /*
            * Remove from list right away so lookups will fail if we block for
            * pageout completion.
            */
           if (object->handle != NULL) {
                   mtx_lock(&sw_alloc_mtx);
                   TAILQ_REMOVE(NOBJLIST(object->handle), object, pager_object_list);
                   mtx_unlock(&sw_alloc_mtx);
           }
   
           VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
           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.
            */
           swp_pager_meta_free_all(object);
   }
   
   /************************************************************************
    *                      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().
    *
    *      We allocate in round-robin fashion from the configured devices.
    */
   static daddr_t
   swp_pager_getswapspace(int npages)
   {
           daddr_t blk;
           struct swdevt *sp;
           int i;
   
           blk = SWAPBLK_NONE;
           mtx_lock(&sw_dev_mtx);
           sp = swdevhd;
           for (i = 0; i < nswapdev; i++) {
                   if (sp == NULL)
                           sp = TAILQ_FIRST(&swtailq);
                   if (!(sp->sw_flags & SW_CLOSING)) {
                           blk = blist_alloc(sp->sw_blist, npages);
                           if (blk != SWAPBLK_NONE) {
                                   blk += sp->sw_first;
                                   sp->sw_used += npages;
                                   swap_pager_avail -= npages;
                                   swp_sizecheck();
                                   swdevhd = TAILQ_NEXT(sp, sw_list);
                                   goto done;
                           }
                   }
                   sp = TAILQ_NEXT(sp, sw_list);
           }
           if (swap_pager_full != 2) {
                   printf("swap_pager_getswapspace(%d): failed\n", npages);
                   swap_pager_full = 2;
                   swap_pager_almost_full = 1;
           }
           swdevhd = NULL;
   done:
           mtx_unlock(&sw_dev_mtx);
           return (blk);
   }
   
   static int
   swp_pager_isondev(daddr_t blk, struct swdevt *sp)
   {
   
           return (blk >= sp->sw_first && blk < sp->sw_end);
   }
           
   static void
   swp_pager_strategy(struct buf *bp)
   {
           struct swdevt *sp;
   
           mtx_lock(&sw_dev_mtx);
           TAILQ_FOREACH(sp, &swtailq, sw_list) {
                   if (bp->b_blkno >= sp->sw_first && bp->b_blkno < sp->sw_end) {
                           mtx_unlock(&sw_dev_mtx);
                           sp->sw_strategy(bp, sp);
                           return;
                   }
           }
           panic("Swapdev not found");
   }
           
   
   /*
    * 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 void
   swp_pager_freeswapspace(daddr_t blk, int npages)
   {
           struct swdevt *sp;
   
           mtx_lock(&sw_dev_mtx);
           TAILQ_FOREACH(sp, &swtailq, sw_list) {
                   if (blk >= sp->sw_first && blk < sp->sw_end) {
                           sp->sw_used -= npages;
                           /*
                            * If we are attempting to stop swapping on
                            * this device, we don't want to mark any
                            * blocks free lest they be reused.  
                            */
                           if ((sp->sw_flags & SW_CLOSING) == 0) {
                                   blist_free(sp->sw_blist, blk - sp->sw_first,
                                       npages);
                                   swap_pager_avail += npages;
                                   swp_sizecheck();
                           }
                           mtx_unlock(&sw_dev_mtx);
                           return;
                   }
           }
           panic("Swapdev not found");
   }
   
   /*
    * 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(vm_object_t object, vm_pindex_t start, vm_size_t size)
   {
   
           VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
           swp_pager_meta_free(object, start, size);
   }
   
   /*
    * 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 n = 0;
           daddr_t blk = SWAPBLK_NONE;
           vm_pindex_t beg = start;        /* save start index */
   
           VM_OBJECT_LOCK(object);
           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);
                                           VM_OBJECT_UNLOCK(object);
                                           return (-1);
                                   }
                           }
                   }
                   swp_pager_meta_build(object, start, blk);
                   --size;
                   ++start;
                   ++blk;
                   --n;
           }
           swp_pager_meta_free(object, start, n);
           VM_OBJECT_UNLOCK(object);
           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(vm_object_t srcobject, vm_object_t dstobject,
       vm_pindex_t offset, int destroysource)
   {
           vm_pindex_t i;
   
           VM_OBJECT_LOCK_ASSERT(srcobject, MA_OWNED);
           VM_OBJECT_LOCK_ASSERT(dstobject, MA_OWNED);
   
           /*
            * If destroysource is set, we remove the source object from the 
            * swap_pager internal queue now. 
            */
           if (destroysource) {
                   if (srcobject->handle != NULL) {
                           mtx_lock(&sw_alloc_mtx);
                           TAILQ_REMOVE(
                               NOBJLIST(srcobject->handle),
                               srcobject,
                               pager_object_list
                           );
                           mtx_unlock(&sw_alloc_mtx);
                   }
           }
   
           /*
            * 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() can sleep.
                                    */
                                   vm_object_pip_add(srcobject, 1);
                                   VM_OBJECT_UNLOCK(srcobject);
                                   vm_object_pip_add(dstobject, 1);
                                   swp_pager_meta_build(dstobject, i, srcaddr);
                                   vm_object_pip_wakeup(dstobject);
                                   VM_OBJECT_LOCK(srcobject);
                                   vm_object_pip_wakeup(srcobject);
                           }
                   } 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;
           }
   }
   
   /*
    * 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.
    */
   static boolean_t
   swap_pager_haspage(vm_object_t object, vm_pindex_t pindex, int *before, int *after)
   {
           daddr_t blk0;
   
           VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
           /*
            * do we have good backing store at the requested index ?
            */
           blk0 = swp_pager_meta_ctl(object, pindex, 0);
   
           if (blk0 == SWAPBLK_NONE) {
                   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);
           }
           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(vm_page_t m)
   {
   
           VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED);
           swp_pager_meta_ctl(m->object, m->pindex, SWM_FREE);
   }
   
   /*
    * 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(vm_object_t object, vm_page_t *m, int count, int reqpage)
   {
           struct buf *bp;
           vm_page_t mreq;
           int i;
           int j;
           daddr_t blk;
   
           mreq = m[reqpage];
   
           KASSERT(mreq->object == object,
               ("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 but we know it to
            * not span devices.   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.
            */
           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;
           }
           ++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;
           }
   
           /*
            * free pages outside our collection range.   Note: we never free
            * mreq, it must remain busy throughout.
            */
           if (0 < i || j < count) {
                   int k;
   
                   vm_page_lock_queues();
                   for (k = 0; k < i; ++k)
                           vm_page_free(m[k]);
                   for (k = j; k < count; ++k)
                           vm_page_free(m[k]);
                   vm_page_unlock_queues();
           }
   
           /*
            * 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);
  
          /*
           * Getpbuf() can sleep.
           */
          VM_OBJECT_UNLOCK(object);
          /*
           * Get a swap buffer header to perform the IO
           */
          bp = getpbuf(&nsw_rcount);
          bp->b_flags |= B_PAGING;
  
          /*
           * map our page(s) into kva for input
           */
          pmap_qenter((vm_offset_t)bp->b_data, m + i, j - i);
  
          bp->b_iocmd = BIO_READ;
          bp->b_iodone = swp_pager_async_iodone;
          bp->b_rcred = crhold(thread0.td_ucred);
          bp->b_wcred = crhold(thread0.td_ucred);
          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;
  
          VM_OBJECT_LOCK(object);
          {
                  int k;
  
                  for (k = i; k < j; ++k) {
                          bp->b_pages[k - i] = m[k];
                          m[k]->oflags |= VPO_SWAPINPROG;
                  }
          }
          bp->b_npages = j - i;
  
          PCPU_INC(cnt.v_swapin);
          PCPU_ADD(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(object, bp->b_npages);
          VM_OBJECT_UNLOCK(object);
  
          /*
           * 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 swapdev_strategy
           */
          BUF_KERNPROC(bp);
          swp_pager_strategy(bp);
  
          /*
           * wait for the page we want to complete.  VPO_SWAPINPROG is always
           * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
           * is set in the meta-data.
           */
          VM_OBJECT_LOCK(object);
          while ((mreq->oflags & VPO_SWAPINPROG) != 0) {
                  mreq->oflags |= VPO_WANTED;
                  vm_page_lock_queues();
                  vm_page_flag_set(mreq, PG_REFERENCED);
                  vm_page_unlock_queues();
                  PCPU_INC(cnt.v_intrans);
                  if (msleep(mreq, VM_OBJECT_MTX(object), PSWP, "swread", hz*20)) {
                          printf(
  "swap_pager: indefinite wait buffer: bufobj: %p, blkno: %jd, size: %ld\n",
                              bp->b_bufobj, (intmax_t)bp->b_blkno, bp->b_bcount);
                  }
          }
  
          /*
           * mreq is left busied 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(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);
          VM_OBJECT_UNLOCK(object);
  
          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.
           */
          mtx_lock(&pbuf_mtx);
          if (swap_async_max != nsw_wcount_async_max) {
                  int n;
  
                  /*
                   * 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.
                   */
                  n -= nsw_wcount_async_max;
                  if (nsw_wcount_async + n >= 0) {
                          nsw_wcount_async += n;
                          nsw_wcount_async_max += n;
                          wakeup(&nsw_wcount_async);
                  }
          }
          mtx_unlock(&pbuf_mtx);
  
          /*
           * 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 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);
  
                  /*
                   * 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;
                          continue;
                  }
  
                  /*
                   * All I/O parameters have been satisfied, build the I/O
                   * request and assign the swap space.
                   */
                  if (sync == TRUE) {
                          bp = getpbuf(&nsw_wcount_sync);
                  } else {
                          bp = getpbuf(&nsw_wcount_async);
                          bp->b_flags = B_ASYNC;
                  }
                  bp->b_flags |= B_PAGING;
                  bp->b_iocmd = BIO_WRITE;
  
                  pmap_qenter((vm_offset_t)bp->b_data, &m[i], n);
  
                  bp->b_rcred = crhold(thread0.td_ucred);
                  bp->b_wcred = crhold(thread0.td_ucred);
                  bp->b_bcount = PAGE_SIZE * n;
                  bp->b_bufsize = PAGE_SIZE * n;
                  bp->b_blkno = blk;
  
                  VM_OBJECT_LOCK(object);
                  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;
  
                          mreq->oflags |= VPO_SWAPINPROG;
                          bp->b_pages[j] = mreq;
                  }
                  VM_OBJECT_UNLOCK(object);
                  bp->b_npages = n;
                  /*
                   * Must set dirty range for NFS to work.
                   */
                  bp->b_dirtyoff = 0;
                  bp->b_dirtyend = bp->b_bcount;
  
                  PCPU_INC(cnt.v_swapout);
                  PCPU_ADD(cnt.v_swappgsout, bp->b_npages);
  
                  /*
                   * asynchronous
                   *
                   * NOTE: b_blkno is destroyed by the call to swapdev_strategy
                   */
                  if (sync == FALSE) {
                          bp->b_iodone = swp_pager_async_iodone;
                          BUF_KERNPROC(bp);
                          swp_pager_strategy(bp);
  
                          for (j = 0; j < n; ++j)
                                  rtvals[i+j] = VM_PAGER_PEND;
                          /* restart outter loop */
                          continue;
                  }
  
                  /*
                   * synchronous
                   *
                   * NOTE: b_blkno is destroyed by the call to swapdev_strategy
                   */
                  bp->b_iodone = bdone;
                  swp_pager_strategy(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.
                   */
                  bwait(bp, PVM, "swwrt");
                  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);
          }
          VM_OBJECT_LOCK(object);
  }
  
  /*
   *      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(struct buf *bp)
  {
          int i;
          vm_object_t object = NULL;
  
          /*
           * report error
           */
          if (bp->b_ioflags & BIO_ERROR) {
                  printf(
                      "swap_pager: I/O error - %s failed; blkno %ld,"
                          "size %ld, error %d\n",
                      ((bp->b_iocmd == BIO_READ) ? "pagein" : "pageout"),
                      (long)bp->b_blkno, 
                      (long)bp->b_bcount,
                      bp->b_error
                  );
          }
  
          /*
           * remove the mapping for kernel virtual
           */
          pmap_qremove((vm_offset_t)bp->b_data, bp->b_npages);
  
          if (bp->b_npages) {
                  object = bp->b_pages[0]->object;
                  VM_OBJECT_LOCK(object);
          }
          vm_page_lock_queues();
          /*
           * 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];
  
                  m->oflags &= ~VPO_SWAPINPROG;
  
                  if (bp->b_ioflags & BIO_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_iocmd == BIO_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 VPO_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.
                                   */
                                  m->valid = 0;
                                  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_iocmd == BIO_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.
                           *
                           * 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);
  
                          /*
                           * We have to wake specifically requested pages
                           * up too because we cleared VPO_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_unlock_queues();
  
          /*
           * adjust pip.  NOTE: the original parent may still have its own
           * pip refs on the object.
           */
          if (object != NULL) {
                  vm_object_pip_wakeupn(object, bp->b_npages);
                  VM_OBJECT_UNLOCK(object);
          }
  
          /* 
           * swapdev_strategy() manually sets b_vp and b_bufobj before calling 
           * bstrategy(). Set them back to NULL now we're done with it, or we'll
           * trigger a KASSERT in relpbuf().
           */
          if (bp->b_vp) {
                      bp->b_vp = NULL;
                      bp->b_bufobj = NULL;
          }
          /*
           * release the physical I/O buffer
           */
          relpbuf(
              bp, 
              ((bp->b_iocmd == BIO_READ) ? &nsw_rcount : 
                  ((bp->b_flags & B_ASYNC) ? 
                      &nsw_wcount_async : 
                      &nsw_wcount_sync
                  )
              )
          );
  }
  
  /*
   *      swap_pager_isswapped:
   *
   *      Return 1 if at least one page in the given object is paged
   *      out to the given swap device.
   *
   *      This routine may not block.
   */
  int
  swap_pager_isswapped(vm_object_t object, struct swdevt *sp)
  {
          daddr_t index = 0;
          int bcount;
          int i;
  
          VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
          if (object->type != OBJT_SWAP)
                  return (0);
  
          mtx_lock(&swhash_mtx);
          for (bcount = 0; bcount < object->un_pager.swp.swp_bcount; bcount++) {
                  struct swblock *swap;
  
                  if ((swap = *swp_pager_hash(object, index)) != NULL) {
                          for (i = 0; i < SWAP_META_PAGES; ++i) {
                                  if (swp_pager_isondev(swap->swb_pages[i], sp)) {
                                          mtx_unlock(&swhash_mtx);
                                          return (1);
                                  }
                          }
                  }
                  index += SWAP_META_PAGES;
                  if (index > 0x20000000)
                          panic("swap_pager_isswapped: failed to locate all swap meta blocks");
          }
          mtx_unlock(&swhash_mtx);
          return (0);
  }
  
  /*
   * SWP_PAGER_FORCE_PAGEIN() - force a swap block to be paged in
   *
   *      This routine dissociates the page at the given index within a
   *      swap block from its backing store, paging it in if necessary.
   *      If the page is paged in, it is placed in the inactive queue,
   *      since it had its backing store ripped out from under it.
   *      We also attempt to swap in all other pages in the swap block,
   *      we only guarantee that the one at the specified index is
   *      paged in.
   *
   *      XXX - The code to page the whole block in doesn't work, so we
   *            revert to the one-by-one behavior for now.  Sigh.
   */
  static inline void
  swp_pager_force_pagein(vm_object_t object, vm_pindex_t pindex)
  {
          vm_page_t m;
  
          vm_object_pip_add(object, 1);
          m = vm_page_grab(object, pindex, VM_ALLOC_NORMAL|VM_ALLOC_RETRY);
          if (m->valid == VM_PAGE_BITS_ALL) {
                  vm_object_pip_subtract(object, 1);
                  vm_page_lock_queues();
                  vm_page_activate(m);
                  vm_page_dirty(m);
                  vm_page_unlock_queues();
                  vm_page_wakeup(m);
                  vm_pager_page_unswapped(m);
                  return;
          }
  
          if (swap_pager_getpages(object, &m, 1, 0) != VM_PAGER_OK)
                  panic("swap_pager_force_pagein: read from swap failed");/*XXX*/
          vm_object_pip_subtract(object, 1);
          vm_page_lock_queues();
          vm_page_dirty(m);
          vm_page_dontneed(m);
          vm_page_unlock_queues();
          vm_page_wakeup(m);
          vm_pager_page_unswapped(m);
  }
  
  /*
   *      swap_pager_swapoff:
   *
   *      Page in all of the pages that have been paged out to the
   *      given device.  The corresponding blocks in the bitmap must be
   *      marked as allocated and the device must be flagged SW_CLOSING.
   *      There may be no processes swapped out to the device.
   *
   *      This routine may block.
   */
  static void
  swap_pager_swapoff(struct swdevt *sp)
  {
          struct swblock *swap;
          int i, j, retries;
  
          GIANT_REQUIRED;
  
          retries = 0;
  full_rescan:
          mtx_lock(&swhash_mtx);
          for (i = 0; i <= swhash_mask; i++) { /* '<=' is correct here */
  restart:
                  for (swap = swhash[i]; swap != NULL; swap = swap->swb_hnext) {
                          vm_object_t object = swap->swb_object;
                          vm_pindex_t pindex = swap->swb_index;
                          for (j = 0; j < SWAP_META_PAGES; ++j) {
                                  if (swp_pager_isondev(swap->swb_pages[j], sp)) {
                                          /* avoid deadlock */
                                          if (!VM_OBJECT_TRYLOCK(object)) {
                                                  break;
                                          } else {
                                                  mtx_unlock(&swhash_mtx);
                                                  swp_pager_force_pagein(object,
                                                      pindex + j);
                                                  VM_OBJECT_UNLOCK(object);
                                                  mtx_lock(&swhash_mtx);
                                                  goto restart;
                                          }
                                  }
                          }
                  }
          }
          mtx_unlock(&swhash_mtx);
          if (sp->sw_used) {
                  /*
                   * Objects may be locked or paging to the device being
                   * removed, so we will miss their pages and need to
                   * make another pass.  We have marked this device as
                   * SW_CLOSING, so the activity should finish soon.
                   */
                  retries++;
                  if (retries > 100) {
                          panic("swapoff: failed to locate %d swap blocks",
                              sp->sw_used);
                  }
                  pause("swpoff", hz / 20);
                  goto full_rescan;
          }
  }
  
  /************************************************************************
   *                              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_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 pindex, daddr_t swapblk)
  {
          struct swblock *swap;
          struct swblock **pswap;
          int idx;
  
          VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
          /*
           * 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) {
                          mtx_lock(&sw_alloc_mtx);
                          TAILQ_INSERT_TAIL(
                              NOBJLIST(object->handle),
                              object, 
                              pager_object_list
                          );
                          mtx_unlock(&sw_alloc_mtx);
                  }
          }
          
          /*
           * 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:
          mtx_lock(&swhash_mtx);
          pswap = swp_pager_hash(object, pindex);
  
          if ((swap = *pswap) == NULL) {
                  int i;
  
                  if (swapblk == SWAPBLK_NONE)
                          goto done;
  
                  swap = *pswap = uma_zalloc(swap_zone, M_NOWAIT);
                  if (swap == NULL) {
                          mtx_unlock(&swhash_mtx);
                          VM_OBJECT_UNLOCK(object);
                          if (uma_zone_exhausted(swap_zone))
                                  printf("swap zone exhausted, increase kern.maxswzone\n");
                          VM_WAIT;
                          VM_OBJECT_LOCK(object);
                          goto retry;
                  }
  
                  swap->swb_hnext = NULL;
                  swap->swb_object = object;
                  swap->swb_index = pindex & ~(vm_pindex_t)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
           */
          idx = pindex & SWAP_META_MASK;
  
          if (swap->swb_pages[idx] != SWAPBLK_NONE) {
                  swp_pager_freeswapspace(swap->swb_pages[idx], 1);
                  --swap->swb_count;
          }
  
          /*
           * Enter block into metadata
           */
          swap->swb_pages[idx] = swapblk;
          if (swapblk != SWAPBLK_NONE)
                  ++swap->swb_count;
  done:
          mtx_unlock(&swhash_mtx);
  }
  
  /*
   * 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)
  {
  
          VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
          if (object->type != OBJT_SWAP)
                  return;
  
          while (count > 0) {
                  struct swblock **pswap;
                  struct swblock *swap;
  
                  mtx_lock(&swhash_mtx);
                  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;
                                          uma_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;
                  }
                  mtx_unlock(&swhash_mtx);
          }
  }
  
  /*
   * 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;
  
          VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
          if (object->type != OBJT_SWAP)
                  return;
  
          while (object->un_pager.swp.swp_bcount) {
                  struct swblock **pswap;
                  struct swblock *swap;
  
                  mtx_lock(&swhash_mtx);
                  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;
                          uma_zfree(swap_zone, swap);
                          --object->un_pager.swp.swp_bcount;
                  }
                  mtx_unlock(&swhash_mtx);
                  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 pindex, int flags)
  {
          struct swblock **pswap;
          struct swblock *swap;
          daddr_t r1;
          int idx;
  
          VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
          /*
           * 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;
          mtx_lock(&swhash_mtx);
          pswap = swp_pager_hash(object, pindex);
  
          if ((swap = *pswap) != NULL) {
                  idx = pindex & SWAP_META_MASK;
                  r1 = swap->swb_pages[idx];
  
                  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[idx] = SWAPBLK_NONE;
                                  if (--swap->swb_count == 0) {
                                          *pswap = swap->swb_hnext;
                                          uma_zfree(swap_zone, swap);
                                          --object->un_pager.swp.swp_bcount;
                                  }
                          } 
                  }
          }
          mtx_unlock(&swhash_mtx);
          return (r1);
  }
  
  /*
   * System call swapon(name) enables swapping on device name,
   * which must be in the swdevsw.  Return EBUSY
   * if already swapping on this device.
   */
  #ifndef _SYS_SYSPROTO_H_
  struct swapon_args {
          char *name;
  };
  #endif
  
  /* 
   * MPSAFE
   */
  /* ARGSUSED */
  int
  swapon(struct thread *td, struct swapon_args *uap)
  {
          struct vattr attr;
          struct vnode *vp;
          struct nameidata nd;
          int error;
  
          error = priv_check(td, PRIV_SWAPON);
          if (error)
                  return (error);
  
          mtx_lock(&Giant);
          while (swdev_syscall_active)
              tsleep(&swdev_syscall_active, PUSER - 1, "swpon", 0);
          swdev_syscall_active = 1;
  
          /*
           * Swap metadata may not fit in the KVM if we have physical
           * memory of >1GB.
           */
          if (swap_zone == NULL) {
                  error = ENOMEM;
                  goto done;
          }
  
          NDINIT(&nd, LOOKUP, ISOPEN | FOLLOW | AUDITVNODE1, UIO_USERSPACE,
              uap->name, td);
          error = namei(&nd);
          if (error)
                  goto done;
  
          NDFREE(&nd, NDF_ONLY_PNBUF);
          vp = nd.ni_vp;
  
          if (vn_isdisk(vp, &error)) {
                  error = swapongeom(td, vp);
          } else if (vp->v_type == VREG &&
              (vp->v_mount->mnt_vfc->vfc_flags & VFCF_NETWORK) != 0 &&
              (error = VOP_GETATTR(vp, &attr, td->td_ucred, td)) == 0) {
                  /*
                   * Allow direct swapping to NFS regular files in the same
                   * way that nfs_mountroot() sets up diskless swapping.
                   */
                  error = swaponvp(td, vp, attr.va_size / DEV_BSIZE);
          }
  
          if (error)
                  vrele(vp);
  done:
          swdev_syscall_active = 0;
          wakeup_one(&swdev_syscall_active);
          mtx_unlock(&Giant);
          return (error);
  }
  
  static void
  swaponsomething(struct vnode *vp, void *id, u_long nblks, sw_strategy_t *strategy, sw_close_t *close, dev_t dev)
  {
          struct swdevt *sp, *tsp;
          swblk_t dvbase;
          u_long mblocks;
  
          /*
           * If we go beyond this, we get overflows in the radix
           * tree bitmap code.
           */
          mblocks = 0x40000000 / BLIST_META_RADIX;
          if (nblks > mblocks) {
                  printf("WARNING: reducing size to maximum of %lu blocks per swap unit\n",
                          mblocks);
                  nblks = mblocks;
          }
          /*
           * nblks is in DEV_BSIZE'd chunks, convert to PAGE_SIZE'd chunks.
           * First chop nblks off to page-align it, then convert.
           * 
           * sw->sw_nblks is in page-sized chunks now too.
           */
          nblks &= ~(ctodb(1) - 1);
          nblks = dbtoc(nblks);
  
          sp = malloc(sizeof *sp, M_VMPGDATA, M_WAITOK | M_ZERO);
          sp->sw_vp = vp;
          sp->sw_id = id;
          sp->sw_dev = dev;
          sp->sw_flags = 0;
          sp->sw_nblks = nblks;
          sp->sw_used = 0;
          sp->sw_strategy = strategy;
          sp->sw_close = close;
  
          sp->sw_blist = blist_create(nblks);
          /*
           * Do not free the first two block in order to avoid overwriting
           * any bsd label at the front of the partition
           */
          blist_free(sp->sw_blist, 2, nblks - 2);
  
          dvbase = 0;
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(tsp, &swtailq, sw_list) {
                  if (tsp->sw_end >= dvbase) {
                          /*
                           * We put one uncovered page between the devices
                           * in order to definitively prevent any cross-device
                           * I/O requests
                           */
                          dvbase = tsp->sw_end + 1;
                  }
          }
          sp->sw_first = dvbase;
          sp->sw_end = dvbase + nblks;
          TAILQ_INSERT_TAIL(&swtailq, sp, sw_list);
          nswapdev++;
          swap_pager_avail += nblks;
          swp_sizecheck();
          mtx_unlock(&sw_dev_mtx);
  }
  
  /*
   * SYSCALL: swapoff(devname)
   *
   * Disable swapping on the given device.
   *
   * XXX: Badly designed system call: it should use a device index
   * rather than filename as specification.  We keep sw_vp around
   * only to make this work.
   */
  #ifndef _SYS_SYSPROTO_H_
  struct swapoff_args {
          char *name;
  };
  #endif
  
  /*
   * MPSAFE
   */
  /* ARGSUSED */
  int
  swapoff(struct thread *td, struct swapoff_args *uap)
  {
          struct vnode *vp;
          struct nameidata nd;
          struct swdevt *sp;
          int error;
  
          error = priv_check(td, PRIV_SWAPOFF);
          if (error)
                  return (error);
  
          mtx_lock(&Giant);
          while (swdev_syscall_active)
              tsleep(&swdev_syscall_active, PUSER - 1, "swpoff", 0);
          swdev_syscall_active = 1;
  
          NDINIT(&nd, LOOKUP, FOLLOW | AUDITVNODE1, UIO_USERSPACE, uap->name,
              td);
          error = namei(&nd);
          if (error)
                  goto done;
          NDFREE(&nd, NDF_ONLY_PNBUF);
          vp = nd.ni_vp;
  
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  if (sp->sw_vp == vp)
                          break;
          }
          mtx_unlock(&sw_dev_mtx);
          if (sp == NULL) {
                  error = EINVAL;
                  goto done;
          }
          error = swapoff_one(sp, td);
  done:
          swdev_syscall_active = 0;
          wakeup_one(&swdev_syscall_active);
          mtx_unlock(&Giant);
          return (error);
  }
  
  static int
  swapoff_one(struct swdevt *sp, struct thread *td)
  {
          u_long nblks, dvbase;
  #ifdef MAC
          int error;
  #endif
  
          mtx_assert(&Giant, MA_OWNED);
  #ifdef MAC
          (void) vn_lock(sp->sw_vp, LK_EXCLUSIVE | LK_RETRY, td);
          error = mac_check_system_swapoff(td->td_ucred, sp->sw_vp);
          (void) VOP_UNLOCK(sp->sw_vp, 0, td);
          if (error != 0)
                  return (error);
  #endif
          nblks = sp->sw_nblks;
  
          /*
           * We can turn off this swap device safely only if the
           * available virtual memory in the system will fit the amount
           * of data we will have to page back in, plus an epsilon so
           * the system doesn't become critically low on swap space.
           */
          if (cnt.v_free_count + cnt.v_cache_count + swap_pager_avail <
              nblks + nswap_lowat) {
                  return (ENOMEM);
          }
  
          /*
           * Prevent further allocations on this device.
           */
          mtx_lock(&sw_dev_mtx);
          sp->sw_flags |= SW_CLOSING;
          for (dvbase = 0; dvbase < sp->sw_end; dvbase += dmmax) {
                  swap_pager_avail -= blist_fill(sp->sw_blist,
                       dvbase, dmmax);
          }
          mtx_unlock(&sw_dev_mtx);
  
          /*
           * Page in the contents of the device and close it.
           */
          swap_pager_swapoff(sp);
  
          sp->sw_close(td, sp);
          sp->sw_id = NULL;
          mtx_lock(&sw_dev_mtx);
          TAILQ_REMOVE(&swtailq, sp, sw_list);
          nswapdev--;
          if (nswapdev == 0) {
                  swap_pager_full = 2;
                  swap_pager_almost_full = 1;
          }
          if (swdevhd == sp)
                  swdevhd = NULL;
          mtx_unlock(&sw_dev_mtx);
          blist_destroy(sp->sw_blist);
          free(sp, M_VMPGDATA);
          return (0);
  }
  
  void
  swapoff_all(void)
  {
          struct swdevt *sp, *spt;
          const char *devname;
          int error;
   
          mtx_lock(&Giant);
          while (swdev_syscall_active)
                  tsleep(&swdev_syscall_active, PUSER - 1, "swpoff", 0);
          swdev_syscall_active = 1;
   
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH_SAFE(sp, &swtailq, sw_list, spt) {
                  mtx_unlock(&sw_dev_mtx);
                  if (vn_isdisk(sp->sw_vp, NULL))
                          devname = sp->sw_vp->v_rdev->si_name;
                  else
                          devname = "[file]";
                  error = swapoff_one(sp, &thread0);
                  if (error != 0) {
                          printf("Cannot remove swap device %s (error=%d), "
                              "skipping.\n", devname, error);
                  } else if (bootverbose) {
                          printf("Swap device %s removed.\n", devname);
                  }
                  mtx_lock(&sw_dev_mtx);
          }
          mtx_unlock(&sw_dev_mtx);
   
          swdev_syscall_active = 0;
          wakeup_one(&swdev_syscall_active);
          mtx_unlock(&Giant);
  }
  
  void
  swap_pager_status(int *total, int *used)
  {
          struct swdevt *sp;
  
          *total = 0;
          *used = 0;
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  *total += sp->sw_nblks;
                  *used += sp->sw_used;
          }
          mtx_unlock(&sw_dev_mtx);
  }
  
  static int
  sysctl_vm_swap_info(SYSCTL_HANDLER_ARGS)
  {
          int     *name = (int *)arg1;
          int     error, n;
          struct xswdev xs;
          struct swdevt *sp;
  
          if (arg2 != 1) /* name length */
                  return (EINVAL);
  
          n = 0;
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  if (n == *name) {
                          mtx_unlock(&sw_dev_mtx);
                          xs.xsw_version = XSWDEV_VERSION;
                          xs.xsw_dev = sp->sw_dev;
                          xs.xsw_flags = sp->sw_flags;
                          xs.xsw_nblks = sp->sw_nblks;
                          xs.xsw_used = sp->sw_used;
  
                          error = SYSCTL_OUT(req, &xs, sizeof(xs));
                          return (error);
                  }
                  n++;
          }
          mtx_unlock(&sw_dev_mtx);
          return (ENOENT);
  }
  
  SYSCTL_INT(_vm, OID_AUTO, nswapdev, CTLFLAG_RD, &nswapdev, 0,
      "Number of swap devices");
  SYSCTL_NODE(_vm, OID_AUTO, swap_info, CTLFLAG_RD, sysctl_vm_swap_info,
      "Swap statistics by device");
  
  /*
   * vmspace_swap_count() - count the approximate swap useage in pages for a
   *                        vmspace.
   *
   *      The map must be locked.
   *
   *      Swap useage is determined by taking the proportional swap used by
   *      VM objects backing the VM map.  To make up for fractional losses,
   *      if the VM object has any swap use at all the associated map entries
   *      count for at least 1 swap page.
   */
  int
  vmspace_swap_count(struct vmspace *vmspace)
  {
          vm_map_t map = &vmspace->vm_map;
          vm_map_entry_t cur;
          int count = 0;
  
          for (cur = map->header.next; cur != &map->header; cur = cur->next) {
                  vm_object_t object;
  
                  if ((cur->eflags & MAP_ENTRY_IS_SUB_MAP) == 0 &&
                      (object = cur->object.vm_object) != NULL) {
                          VM_OBJECT_LOCK(object);
                          if (object->type == OBJT_SWAP &&
                              object->un_pager.swp.swp_bcount != 0) {
                                  int n = (cur->end - cur->start) / PAGE_SIZE;
  
                                  count += object->un_pager.swp.swp_bcount *
                                      SWAP_META_PAGES * n / object->size + 1;
                          }
                          VM_OBJECT_UNLOCK(object);
                  }
          }
          return (count);
  }
  
  /*
   * GEOM backend
   *
   * Swapping onto disk devices.
   *
   */
  
  static g_orphan_t swapgeom_orphan;
  
  static struct g_class g_swap_class = {
          .name = "SWAP",
          .version = G_VERSION,
          .orphan = swapgeom_orphan,
  };
  
  DECLARE_GEOM_CLASS(g_swap_class, g_class);
  
  
  static void
  swapgeom_done(struct bio *bp2)
  {
          struct buf *bp;
  
          bp = bp2->bio_caller2;
          bp->b_ioflags = bp2->bio_flags;
          if (bp2->bio_error)
                  bp->b_ioflags |= BIO_ERROR;
          bp->b_resid = bp->b_bcount - bp2->bio_completed;
          bp->b_error = bp2->bio_error;
          bufdone(bp);
          g_destroy_bio(bp2);
  }
  
  static void
  swapgeom_strategy(struct buf *bp, struct swdevt *sp)
  {
          struct bio *bio;
          struct g_consumer *cp;
  
          cp = sp->sw_id;
          if (cp == NULL) {
                  bp->b_error = ENXIO;
                  bp->b_ioflags |= BIO_ERROR;
                  bufdone(bp);
                  return;
          }
          bio = g_alloc_bio();
  #if 0
          /*
           * XXX: We shouldn't really sleep here when we run out of buffers
           * XXX: but the alternative is worse right now.
           */
          if (bio == NULL) {
                  bp->b_error = ENOMEM;
                  bp->b_ioflags |= BIO_ERROR;
                  bufdone(bp);
                  return;
          }
  #endif
          bio->bio_caller2 = bp;
          bio->bio_cmd = bp->b_iocmd;
          bio->bio_data = bp->b_data;
          bio->bio_offset = (bp->b_blkno - sp->sw_first) * PAGE_SIZE;
          bio->bio_length = bp->b_bcount;
          bio->bio_done = swapgeom_done;
          g_io_request(bio, cp);
          return;
  }
  
  static void
  swapgeom_orphan(struct g_consumer *cp)
  {
          struct swdevt *sp;
  
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list)
                  if (sp->sw_id == cp)
                          sp->sw_id = NULL;
          mtx_unlock(&sw_dev_mtx);
  }
  
  static void
  swapgeom_close_ev(void *arg, int flags)
  {
          struct g_consumer *cp;
  
          cp = arg;
          g_access(cp, -1, -1, 0);
          g_detach(cp);
          g_destroy_consumer(cp);
  }
  
  static void
  swapgeom_close(struct thread *td, struct swdevt *sw)
  {
  
          /* XXX: direct call when Giant untangled */
          g_waitfor_event(swapgeom_close_ev, sw->sw_id, M_WAITOK, NULL);
  }
  
  
  struct swh0h0 {
          struct cdev *dev;
          struct vnode *vp;
          int     error;
  };
  
  static void
  swapongeom_ev(void *arg, int flags)
  {
          struct swh0h0 *swh;
          struct g_provider *pp;
          struct g_consumer *cp;
          static struct g_geom *gp;
          struct swdevt *sp;
          u_long nblks;
          int error;
  
          swh = arg;
          swh->error = 0;
          pp = g_dev_getprovider(swh->dev);
          if (pp == NULL) {
                  swh->error = ENODEV;
                  return;
          }
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  cp = sp->sw_id;
                  if (cp != NULL && cp->provider == pp) {
                          mtx_unlock(&sw_dev_mtx);
                          swh->error = EBUSY;
                          return;
                  }
          }
          mtx_unlock(&sw_dev_mtx);
          if (gp == NULL)
                  gp = g_new_geomf(&g_swap_class, "swap", NULL);
          cp = g_new_consumer(gp);
          g_attach(cp, pp);
          /*
           * XXX: Everytime you think you can improve the margin for
           * footshooting, somebody depends on the ability to do so:
           * savecore(8) wants to write to our swapdev so we cannot
           * set an exclusive count :-(
           */
          error = g_access(cp, 1, 1, 0);
          if (error) {
                  g_detach(cp);
                  g_destroy_consumer(cp);
                  swh->error = error;
                  return;
          }
          nblks = pp->mediasize / DEV_BSIZE;
          swaponsomething(swh->vp, cp, nblks, swapgeom_strategy,
              swapgeom_close, dev2udev(swh->dev));
          swh->error = 0;
          return;
  }
  
  static int
  swapongeom(struct thread *td, struct vnode *vp)
  {
          int error;
          struct swh0h0 swh;
  
          vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
  
          swh.dev = vp->v_rdev;
          swh.vp = vp;
          swh.error = 0;
          /* XXX: direct call when Giant untangled */
          error = g_waitfor_event(swapongeom_ev, &swh, M_WAITOK, NULL);
          if (!error)
                  error = swh.error;
          VOP_UNLOCK(vp, 0, td);
          return (error);
  }
  
  /*
   * VNODE backend
   *
   * This is used mainly for network filesystem (read: probably only tested
   * with NFS) swapfiles.
   *
   */
  
  static void
  swapdev_strategy(struct buf *bp, struct swdevt *sp)
  {
          struct vnode *vp2;
  
          bp->b_blkno = ctodb(bp->b_blkno - sp->sw_first);
  
          vp2 = sp->sw_id;
          vhold(vp2);
          if (bp->b_iocmd == BIO_WRITE) {
                  if (bp->b_bufobj)
                          bufobj_wdrop(bp->b_bufobj);
                  bufobj_wref(&vp2->v_bufobj);
          }
          if (bp->b_bufobj != &vp2->v_bufobj)
                  bp->b_bufobj = &vp2->v_bufobj;
          bp->b_vp = vp2;
          bp->b_iooffset = dbtob(bp->b_blkno);
          bstrategy(bp);
          return;
  }
  
  static void
  swapdev_close(struct thread *td, struct swdevt *sp)
  {
  
          VOP_CLOSE(sp->sw_vp, FREAD | FWRITE, td->td_ucred, td);
          vrele(sp->sw_vp);
  }
  
  
  static int
  swaponvp(struct thread *td, struct vnode *vp, u_long nblks)
  {
          struct swdevt *sp;
          int error;
  
          if (nblks == 0)
                  return (ENXIO);
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  if (sp->sw_id == vp) {
                          mtx_unlock(&sw_dev_mtx);
                          return (EBUSY);
                  }
          }
          mtx_unlock(&sw_dev_mtx);
      
          (void) vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
  #ifdef MAC
          error = mac_check_system_swapon(td->td_ucred, vp);
          if (error == 0)
  #endif
                  error = VOP_OPEN(vp, FREAD | FWRITE, td->td_ucred, td, NULL);
          (void) VOP_UNLOCK(vp, 0, td);
          if (error)
                  return (error);
  
          swaponsomething(vp, vp, nblks, swapdev_strategy, swapdev_close,
              NODEV);
          return (0);
  }

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