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

     /*-
      * SPDX-License-Identifier: BSD-4-Clause
      *
      * 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: releng/12.0/sys/vm/swap_pager.c 340333 2018-11-10 20:36:48Z markj $");
    
    #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/pctrie.h>
    #include <sys/racct.h>
    #include <sys/resource.h>
    #include <sys/resourcevar.h>
    #include <sys/rwlock.h>
    #include <sys/sbuf.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>
   
   /*
    * MAX_PAGEOUT_CLUSTER must be a power of 2 between 1 and 64.
    * The 64-page limit is due to the radix code (kern/subr_blist.c).
    */
   #ifndef MAX_PAGEOUT_CLUSTER
   #define MAX_PAGEOUT_CLUSTER     32
   #endif
   
   #if !defined(SWB_NPAGES)
   #define SWB_NPAGES      MAX_PAGEOUT_CLUSTER
   #endif
   
   #define SWAP_META_PAGES         PCTRIE_COUNT
   
   /*
    * A swblk structure maps each page index within a
    * SWAP_META_PAGES-aligned and sized range to the address of an
    * on-disk swap block (or SWAPBLK_NONE). The collection of these
    * mappings for an entire vm object is implemented as a pc-trie.
    */
   struct swblk {
           vm_pindex_t     p;
           daddr_t         d[SWAP_META_PAGES];
   };
   
   static MALLOC_DEFINE(M_VMPGDATA, "vm_pgdata", "swap pager private data");
   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 struct sx swdev_syscall_lock;    /* serialize swap(on|off) */
   
   static u_long swap_reserved;
   static u_long swap_total;
   static int sysctl_page_shift(SYSCTL_HANDLER_ARGS);
   SYSCTL_PROC(_vm, OID_AUTO, swap_reserved, CTLTYPE_U64 | CTLFLAG_RD | CTLFLAG_MPSAFE,
       &swap_reserved, 0, sysctl_page_shift, "A", 
       "Amount of swap storage needed to back all allocated anonymous memory.");
   SYSCTL_PROC(_vm, OID_AUTO, swap_total, CTLTYPE_U64 | CTLFLAG_RD | CTLFLAG_MPSAFE,
       &swap_total, 0, sysctl_page_shift, "A", 
       "Total amount of available swap storage.");
   
   static int overcommit = 0;
   SYSCTL_INT(_vm, VM_OVERCOMMIT, overcommit, CTLFLAG_RW, &overcommit, 0,
       "Configure virtual memory overcommit behavior. See tuning(7) "
       "for details.");
   static unsigned long swzone;
   SYSCTL_ULONG(_vm, OID_AUTO, swzone, CTLFLAG_RD, &swzone, 0,
       "Actual size of swap metadata zone");
   static unsigned long swap_maxpages;
   SYSCTL_ULONG(_vm, OID_AUTO, swap_maxpages, CTLFLAG_RD, &swap_maxpages, 0,
       "Maximum amount of swap supported");
   
   /* bits from overcommit */
   #define SWAP_RESERVE_FORCE_ON           (1 << 0)
   #define SWAP_RESERVE_RLIMIT_ON          (1 << 1)
   #define SWAP_RESERVE_ALLOW_NONWIRED     (1 << 2)
   
   static int
   sysctl_page_shift(SYSCTL_HANDLER_ARGS)
   {
           uint64_t newval;
           u_long value = *(u_long *)arg1;
   
           newval = ((uint64_t)value) << PAGE_SHIFT;
           return (sysctl_handle_64(oidp, &newval, 0, req));
   }
   
   int
   swap_reserve(vm_ooffset_t incr)
   {
   
           return (swap_reserve_by_cred(incr, curthread->td_ucred));
   }
   
   int
   swap_reserve_by_cred(vm_ooffset_t incr, struct ucred *cred)
   {
           u_long r, s, prev, pincr;
           int res, error;
           static int curfail;
           static struct timeval lastfail;
           struct uidinfo *uip;
   
           uip = cred->cr_ruidinfo;
   
           KASSERT((incr & PAGE_MASK) == 0, ("%s: incr: %ju & PAGE_MASK", __func__,
               (uintmax_t)incr));
   
   #ifdef RACCT
           if (racct_enable) {
                   PROC_LOCK(curproc);
                   error = racct_add(curproc, RACCT_SWAP, incr);
                   PROC_UNLOCK(curproc);
                   if (error != 0)
                           return (0);
           }
   #endif
   
           pincr = atop(incr);
           res = 0;
           prev = atomic_fetchadd_long(&swap_reserved, pincr);
           r = prev + pincr;
           if (overcommit & SWAP_RESERVE_ALLOW_NONWIRED) {
                   s = vm_cnt.v_page_count - vm_cnt.v_free_reserved -
                       vm_wire_count();
           } else
                   s = 0;
           s += swap_total;
           if ((overcommit & SWAP_RESERVE_FORCE_ON) == 0 || r <= s ||
               (error = priv_check(curthread, PRIV_VM_SWAP_NOQUOTA)) == 0) {
                   res = 1;
           } else {
                   prev = atomic_fetchadd_long(&swap_reserved, -pincr);
                   if (prev < pincr)
                           panic("swap_reserved < incr on overcommit fail");
           }
           if (res) {
                   prev = atomic_fetchadd_long(&uip->ui_vmsize, pincr);
                   if ((overcommit & SWAP_RESERVE_RLIMIT_ON) != 0 &&
                       prev + pincr > lim_cur(curthread, RLIMIT_SWAP) &&
                       priv_check(curthread, PRIV_VM_SWAP_NORLIMIT)) {
                           res = 0;
                           prev = atomic_fetchadd_long(&uip->ui_vmsize, -pincr);
                           if (prev < pincr)
                                   panic("uip->ui_vmsize < incr on overcommit fail");
                   }
           }
           if (!res && ppsratecheck(&lastfail, &curfail, 1)) {
                   printf("uid %d, pid %d: swap reservation for %jd bytes failed\n",
                       uip->ui_uid, curproc->p_pid, incr);
           }
   
   #ifdef RACCT
           if (racct_enable && !res) {
                   PROC_LOCK(curproc);
                   racct_sub(curproc, RACCT_SWAP, incr);
                   PROC_UNLOCK(curproc);
           }
   #endif
   
           return (res);
   }
   
   void
   swap_reserve_force(vm_ooffset_t incr)
   {
           struct uidinfo *uip;
           u_long pincr;
   
           KASSERT((incr & PAGE_MASK) == 0, ("%s: incr: %ju & PAGE_MASK", __func__,
               (uintmax_t)incr));
   
           PROC_LOCK(curproc);
   #ifdef RACCT
           if (racct_enable)
                   racct_add_force(curproc, RACCT_SWAP, incr);
   #endif
           pincr = atop(incr);
           atomic_add_long(&swap_reserved, pincr);
           uip = curproc->p_ucred->cr_ruidinfo;
           atomic_add_long(&uip->ui_vmsize, pincr);
           PROC_UNLOCK(curproc);
   }
   
   void
   swap_release(vm_ooffset_t decr)
   {
           struct ucred *cred;
   
           PROC_LOCK(curproc);
           cred = curproc->p_ucred;
           swap_release_by_cred(decr, cred);
           PROC_UNLOCK(curproc);
   }
   
   void
   swap_release_by_cred(vm_ooffset_t decr, struct ucred *cred)
   {
           u_long prev, pdecr;
           struct uidinfo *uip;
   
           uip = cred->cr_ruidinfo;
   
           KASSERT((decr & PAGE_MASK) == 0, ("%s: decr: %ju & PAGE_MASK", __func__,
               (uintmax_t)decr));
   
           pdecr = atop(decr);
           prev = atomic_fetchadd_long(&swap_reserved, -pdecr);
           if (prev < pdecr)
                   panic("swap_reserved < decr");
   
           prev = atomic_fetchadd_long(&uip->ui_vmsize, -pdecr);
           if (prev < pdecr)
                   printf("negative vmsize for uid = %d\n", uip->ui_uid);
   #ifdef RACCT
           if (racct_enable)
                   racct_sub_cred(cred, RACCT_SWAP, decr);
   #endif
   }
   
   #define SWM_POP         0x01    /* pop out                      */
   
   static 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 int sysctl_swap_async_max(SYSCTL_HANDLER_ARGS);
   SYSCTL_PROC(_vm, OID_AUTO, swap_async_max, CTLTYPE_INT | CTLFLAG_RW |
       CTLFLAG_MPSAFE, NULL, 0, sysctl_swap_async_max, "I",
       "Maximum running async swap ops");
   static int sysctl_swap_fragmentation(SYSCTL_HANDLER_ARGS);
   SYSCTL_PROC(_vm, OID_AUTO, swap_fragmentation, CTLTYPE_STRING | CTLFLAG_RD |
       CTLFLAG_MPSAFE, NULL, 0, sysctl_swap_fragmentation, "A",
       "Swap Fragmentation Info");
   
   static struct sx sw_alloc_sx;
   
   /*
    * "named" and "unnamed" anon region objects.  Try to reduce the overhead
    * of searching a named list by hashing it just a little.
    */
   
   #define NOBJLISTS               8
   
   #define NOBJLIST(handle)        \
           (&swap_pager_object_list[((int)(intptr_t)handle >> 4) & (NOBJLISTS-1)])
   
   static struct pagerlst  swap_pager_object_list[NOBJLISTS];
   static uma_zone_t swblk_zone;
   static uma_zone_t swpctrie_zone;
   
   /*
    * pagerops for OBJT_SWAP - "swap pager".  Some ops are also global procedure
    * calls hooked from other parts of the VM system and do not appear here.
    * (see vm/swap_pager.h).
    */
   static vm_object_t
                   swap_pager_alloc(void *handle, vm_ooffset_t size,
                       vm_prot_t prot, vm_ooffset_t offset, struct ucred *);
   static void     swap_pager_dealloc(vm_object_t object);
   static int      swap_pager_getpages(vm_object_t, vm_page_t *, int, int *,
       int *);
   static int      swap_pager_getpages_async(vm_object_t, vm_page_t *, int, int *,
       int *, pgo_getpages_iodone_t, void *);
   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_getpages_async = swap_pager_getpages_async, /* pagein (async)              */
           .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          */
   };
   
   /*
    * swap_*() routines are externally accessible.  swp_*() routines are
    * internal.
    */
   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, &nsw_cluster_max, 0,
       "Maximum size of a swap block in pages");
   
   static void     swp_sizecheck(void);
   static void     swp_pager_async_iodone(struct buf *bp);
   static bool     swp_pager_swblk_empty(struct swblk *sb, int start, int limit);
   static int      swapongeom(struct vnode *);
   static int      swaponvp(struct thread *, struct vnode *, u_long);
   static int      swapoff_one(struct swdevt *sp, struct ucred *cred);
   
   /*
    * Swap bitmap functions
    */
   static void     swp_pager_freeswapspace(daddr_t blk, daddr_t npages);
   static daddr_t  swp_pager_getswapspace(int npages);
   
   /*
    * Metadata functions
    */
   static daddr_t swp_pager_meta_build(vm_object_t, vm_pindex_t, daddr_t);
   static void swp_pager_meta_free(vm_object_t, vm_pindex_t, vm_pindex_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);
   
   static void
   swp_pager_init_freerange(daddr_t *start, daddr_t *num)
   {
   
           *start = SWAPBLK_NONE;
           *num = 0;
   }
   
   static void
   swp_pager_update_freerange(daddr_t *start, daddr_t *num, daddr_t addr)
   {
   
           if (*start + *num == addr) {
                   (*num)++;
           } else {
                   swp_pager_freeswapspace(*start, *num);
                   *start = addr;
                   *num = 1;
           }
   }
   
   static void *
   swblk_trie_alloc(struct pctrie *ptree)
   {
   
           return (uma_zalloc(swpctrie_zone, M_NOWAIT | (curproc == pageproc ?
               M_USE_RESERVE : 0)));
   }
   
   static void
   swblk_trie_free(struct pctrie *ptree, void *node)
   {
   
           uma_zfree(swpctrie_zone, node);
   }
   
   PCTRIE_DEFINE(SWAP, swblk, p, swblk_trie_alloc, swblk_trie_free);
   
   /*
    * 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.
    */
   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;
           }
   }
   
   /*
    * 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_dev_mtx, "swapdev", NULL, MTX_DEF);
           sx_init(&sw_alloc_sx, "swspsx");
           sx_init(&swdev_syscall_lock, "swsysc");
   }
   
   /*
    * 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)
   {
           unsigned long 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, guessing on the number we need based
            * on the number of pages in the system.
            */
           n = vm_cnt.v_page_count / 2;
           if (maxswzone && n > maxswzone / sizeof(struct swblk))
                   n = maxswzone / sizeof(struct swblk);
           swpctrie_zone = uma_zcreate("swpctrie", pctrie_node_size(), NULL, NULL,
               pctrie_zone_init, NULL, UMA_ALIGN_PTR, UMA_ZONE_VM);
           if (swpctrie_zone == NULL)
                   panic("failed to create swap pctrie zone.");
           swblk_zone = uma_zcreate("swblk", sizeof(struct swblk), NULL, NULL,
               NULL, NULL, _Alignof(struct swblk) - 1, UMA_ZONE_VM);
           if (swblk_zone == NULL)
                   panic("failed to create swap blk zone.");
           n2 = n;
           do {
                   if (uma_zone_reserve_kva(swblk_zone, n))
                           break;
                   /*
                    * if the allocation failed, try a zone two thirds the
                    * size of the previous attempt.
                    */
                   n -= ((n + 2) / 3);
           } while (n > 0);
   
           /*
            * Often uma_zone_reserve_kva() cannot reserve exactly the
            * requested size.  Account for the difference when
            * calculating swap_maxpages.
            */
           n = uma_zone_get_max(swblk_zone);
   
           if (n < n2)
                   printf("Swap blk zone entries reduced from %lu to %lu.\n",
                       n2, n);
           swap_maxpages = n * SWAP_META_PAGES;
           swzone = n * sizeof(struct swblk);
           if (!uma_zone_reserve_kva(swpctrie_zone, n))
                   printf("Cannot reserve swap pctrie zone, "
                       "reduce kern.maxswzone.\n");
   }
   
   static vm_object_t
   swap_pager_alloc_init(void *handle, struct ucred *cred, vm_ooffset_t size,
       vm_ooffset_t offset)
   {
           vm_object_t object;
   
           if (cred != NULL) {
                   if (!swap_reserve_by_cred(size, cred))
                           return (NULL);
                   crhold(cred);
           }
   
           /*
            * The un_pager.swp.swp_blks trie is initialized by
            * vm_object_allocate() to ensure the correct order of
            * visibility to other threads.
            */
           object = vm_object_allocate(OBJT_SWAP, OFF_TO_IDX(offset +
               PAGE_MASK + size));
   
           object->handle = handle;
           if (cred != NULL) {
                   object->cred = cred;
                   object->charge = size;
           }
           return (object);
   }
   
   /*
    * 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.
    *
    *      This routine must ensure that no live duplicate is created for
    *      the named object request, which is protected against by
    *      holding the sw_alloc_sx lock in case handle != NULL.
    */
   static vm_object_t
   swap_pager_alloc(void *handle, vm_ooffset_t size, vm_prot_t prot,
       vm_ooffset_t offset, struct ucred *cred)
   {
           vm_object_t object;
   
           if (handle != NULL) {
                   /*
                    * 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 = swap_pager_alloc_init(handle, cred, size,
                               offset);
                           if (object != NULL) {
                                   TAILQ_INSERT_TAIL(NOBJLIST(object->handle),
                                       object, pager_object_list);
                           }
                   }
                   sx_xunlock(&sw_alloc_sx);
           } else {
                   object = swap_pager_alloc_init(handle, cred, size, offset);
           }
           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.
    *
    *      The object must be locked.
    */
   static void
   swap_pager_dealloc(vm_object_t object)
   {
   
           VM_OBJECT_ASSERT_WLOCKED(object);
           KASSERT((object->flags & OBJ_DEAD) != 0, ("dealloc of reachable obj"));
   
           /*
            * Remove from list right away so lookups will fail if we block for
            * pageout completion.
            */
           if (object->handle != NULL) {
                   VM_OBJECT_WUNLOCK(object);
                   sx_xlock(&sw_alloc_sx);
                   TAILQ_REMOVE(NOBJLIST(object->handle), object,
                       pager_object_list);
                   sx_xunlock(&sw_alloc_sx);
                   VM_OBJECT_WLOCK(object);
           }
   
           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);
           object->handle = NULL;
           object->type = OBJT_DEAD;
   }
   
   /************************************************************************
    *                      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.
    *
    *      This routine may not sleep.
    *
    *      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);
                           if ((sp->sw_flags & SW_UNMAPPED) != 0 &&
                               unmapped_buf_allowed) {
                                   bp->b_data = unmapped_buf;
                                   bp->b_offset = 0;
                           } else {
                                   pmap_qenter((vm_offset_t)bp->b_data,
                                       &bp->b_pages[0], bp->b_bcount / PAGE_SIZE);
                           }
                           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.
    *
    *      This routine may not sleep.
    */
   static void
   swp_pager_freeswapspace(daddr_t blk, daddr_t npages)
   {
           struct swdevt *sp;
   
           if (npages == 0)
                   return;
           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");
   }
   
   /*
    * SYSCTL_SWAP_FRAGMENTATION() -        produce raw swap space stats
    */
   static int
   sysctl_swap_fragmentation(SYSCTL_HANDLER_ARGS)
   {
           struct sbuf sbuf;
           struct swdevt *sp;
           const char *devname;
           int error;
   
           error = sysctl_wire_old_buffer(req, 0);
           if (error != 0)
                   return (error);
           sbuf_new_for_sysctl(&sbuf, NULL, 128, req);
           mtx_lock(&sw_dev_mtx);
           TAILQ_FOREACH(sp, &swtailq, sw_list) {
                   if (vn_isdisk(sp->sw_vp, NULL))
                           devname = devtoname(sp->sw_vp->v_rdev);
                   else
                           devname = "[file]";
                   sbuf_printf(&sbuf, "\nFree space on device %s:\n", devname);
                   blist_stats(sp->sw_blist, &sbuf);
           }
           mtx_unlock(&sw_dev_mtx);
           error = sbuf_finish(&sbuf);
           sbuf_delete(&sbuf);
           return (error);
   }
   
   /*
    * 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.
    *
    *      The object must be locked.
    */
   void
   swap_pager_freespace(vm_object_t object, vm_pindex_t start, vm_size_t size)
   {
   
           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 zeroed.  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 */
           daddr_t addr, n_free, s_free;
   
           swp_pager_init_freerange(&s_free, &n_free);
           VM_OBJECT_WLOCK(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_WUNLOCK(object);
                                           return (-1);
                                   }
                           }
                   }
                   addr = swp_pager_meta_build(object, start, blk);
                   if (addr != SWAPBLK_NONE)
                           swp_pager_update_freerange(&s_free, &n_free, addr);
                   --size;
                   ++start;
                   ++blk;
                   --n;
           }
           swp_pager_freeswapspace(s_free, n_free);
           swp_pager_meta_free(object, start, n);
           VM_OBJECT_WUNLOCK(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 sleep.  It may sleep allocating metadata
    *      indirectly through swp_pager_meta_build() or if paging is still in
    *      progress on the source.
    *
    *      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.
    *      Both object locks may temporarily be released.
    */
   void
   swap_pager_copy(vm_object_t srcobject, vm_object_t dstobject,
       vm_pindex_t offset, int destroysource)
   {
           vm_pindex_t i;
           daddr_t dstaddr, n_free, s_free, srcaddr;
   
           VM_OBJECT_ASSERT_WLOCKED(srcobject);
           VM_OBJECT_ASSERT_WLOCKED(dstobject);
   
           /*
            * If destroysource is set, we remove the source object from the
            * swap_pager internal queue now.
            */
           if (destroysource && srcobject->handle != NULL) {
                   vm_object_pip_add(srcobject, 1);
                   VM_OBJECT_WUNLOCK(srcobject);
                   vm_object_pip_add(dstobject, 1);
                   VM_OBJECT_WUNLOCK(dstobject);
                   sx_xlock(&sw_alloc_sx);
                   TAILQ_REMOVE(NOBJLIST(srcobject->handle), srcobject,
                       pager_object_list);
                   sx_xunlock(&sw_alloc_sx);
                   VM_OBJECT_WLOCK(dstobject);
                   vm_object_pip_wakeup(dstobject);
                   VM_OBJECT_WLOCK(srcobject);
                   vm_object_pip_wakeup(srcobject);
           }
   
           /*
            * Transfer source to destination.
            */
           swp_pager_init_freerange(&s_free, &n_free);
           for (i = 0; i < dstobject->size; ++i) {
                   srcaddr = swp_pager_meta_ctl(srcobject, i + offset, SWM_POP);
                   if (srcaddr == SWAPBLK_NONE)
                           continue;
                   dstaddr = swp_pager_meta_ctl(dstobject, i, 0);
                   if (dstaddr != SWAPBLK_NONE) {
                           /*
                            * Destination has valid swapblk or it is represented
                            * by a resident page.  We destroy the source block.
                            */
                           swp_pager_update_freerange(&s_free, &n_free, srcaddr);
                           continue;
                   }
   
                   /*
                    * Destination has no swapblk and is not resident,
                    * copy source.
                    *
                    * swp_pager_meta_build() can sleep.
                    */
                   vm_object_pip_add(srcobject, 1);
                   VM_OBJECT_WUNLOCK(srcobject);
                   vm_object_pip_add(dstobject, 1);
                   dstaddr = swp_pager_meta_build(dstobject, i, srcaddr);
                   KASSERT(dstaddr == SWAPBLK_NONE,
                       ("Unexpected destination swapblk"));
                  vm_object_pip_wakeup(dstobject);
                  VM_OBJECT_WLOCK(srcobject);
                  vm_object_pip_wakeup(srcobject);
          }
          swp_pager_freeswapspace(s_free, n_free);
  
          /*
           * Free left over swap blocks in source.
           *
           * We have to revert the type to OBJT_DEFAULT so we do not accidentally
           * 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.
   */
  static boolean_t
  swap_pager_haspage(vm_object_t object, vm_pindex_t pindex, int *before,
      int *after)
  {
          daddr_t blk, blk0;
          int i;
  
          VM_OBJECT_ASSERT_LOCKED(object);
  
          /*
           * 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) {
                  for (i = 1; i < SWB_NPAGES; i++) {
                          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) {
                  for (i = 1; i < SWB_NPAGES; i++) {
                          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 sleep.
   *
   *      The object containing the page must be locked.
   */
  static void
  swap_pager_unswapped(vm_page_t m)
  {
          daddr_t srcaddr;
  
          srcaddr = swp_pager_meta_ctl(m->object, m->pindex, SWM_POP);
          if (srcaddr != SWAPBLK_NONE)
                  swp_pager_freeswapspace(srcaddr, 1);
  }
  
  /*
   * swap_pager_getpages() - bring pages in from swap
   *
   *      Attempt to page in the pages in array "ma" of length "count".  The
   *      caller may optionally specify that additional pages preceding and
   *      succeeding the specified range be paged in.  The number of such pages
   *      is returned in the "rbehind" and "rahead" parameters, and they will
   *      be in the inactive queue upon return.
   *
   *      The pages in "ma" must be busied and will remain busied upon return.
   */
  static int
  swap_pager_getpages(vm_object_t object, vm_page_t *ma, int count, int *rbehind,
      int *rahead)
  {
          struct buf *bp;
          vm_page_t bm, mpred, msucc, p;
          vm_pindex_t pindex;
          daddr_t blk;
          int i, maxahead, maxbehind, reqcount;
  
          reqcount = count;
  
          /*
           * Determine the final number of read-behind pages and
           * allocate them BEFORE releasing the object lock.  Otherwise,
           * there can be a problematic race with vm_object_split().
           * Specifically, vm_object_split() might first transfer pages
           * that precede ma[0] in the current object to a new object,
           * and then this function incorrectly recreates those pages as
           * read-behind pages in the current object.
           */
          if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, &maxahead))
                  return (VM_PAGER_FAIL);
  
          /*
           * Clip the readahead and readbehind ranges to exclude resident pages.
           */
          if (rahead != NULL) {
                  KASSERT(reqcount - 1 <= maxahead,
                      ("page count %d extends beyond swap block", reqcount));
                  *rahead = imin(*rahead, maxahead - (reqcount - 1));
                  pindex = ma[reqcount - 1]->pindex;
                  msucc = TAILQ_NEXT(ma[reqcount - 1], listq);
                  if (msucc != NULL && msucc->pindex - pindex - 1 < *rahead)
                          *rahead = msucc->pindex - pindex - 1;
          }
          if (rbehind != NULL) {
                  *rbehind = imin(*rbehind, maxbehind);
                  pindex = ma[0]->pindex;
                  mpred = TAILQ_PREV(ma[0], pglist, listq);
                  if (mpred != NULL && pindex - mpred->pindex - 1 < *rbehind)
                          *rbehind = pindex - mpred->pindex - 1;
          }
  
          bm = ma[0];
          for (i = 0; i < count; i++)
                  ma[i]->oflags |= VPO_SWAPINPROG;
  
          /*
           * Allocate readahead and readbehind pages.
           */
          if (rbehind != NULL) {
                  for (i = 1; i <= *rbehind; i++) {
                          p = vm_page_alloc(object, ma[0]->pindex - i,
                              VM_ALLOC_NORMAL);
                          if (p == NULL)
                                  break;
                          p->oflags |= VPO_SWAPINPROG;
                          bm = p;
                  }
                  *rbehind = i - 1;
          }
          if (rahead != NULL) {
                  for (i = 0; i < *rahead; i++) {
                          p = vm_page_alloc(object,
                              ma[reqcount - 1]->pindex + i + 1, VM_ALLOC_NORMAL);
                          if (p == NULL)
                                  break;
                          p->oflags |= VPO_SWAPINPROG;
                  }
                  *rahead = i;
          }
          if (rbehind != NULL)
                  count += *rbehind;
          if (rahead != NULL)
                  count += *rahead;
  
          vm_object_pip_add(object, count);
  
          pindex = bm->pindex;
          blk = swp_pager_meta_ctl(object, pindex, 0);
          KASSERT(blk != SWAPBLK_NONE,
              ("no swap blocking containing %p(%jx)", object, (uintmax_t)pindex));
  
          VM_OBJECT_WUNLOCK(object);
          bp = getpbuf(&nsw_rcount);
          /* Pages cannot leave the object while busy. */
          for (i = 0, p = bm; i < count; i++, p = TAILQ_NEXT(p, listq)) {
                  MPASS(p->pindex == bm->pindex + i);
                  bp->b_pages[i] = p;
          }
  
          bp->b_flags |= B_PAGING;
          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;
          bp->b_bcount = PAGE_SIZE * count;
          bp->b_bufsize = PAGE_SIZE * count;
          bp->b_npages = count;
          bp->b_pgbefore = rbehind != NULL ? *rbehind : 0;
          bp->b_pgafter = rahead != NULL ? *rahead : 0;
  
          VM_CNT_INC(v_swapin);
          VM_CNT_ADD(v_swappgsin, count);
  
          /*
           * 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 ma[] 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 pages we want to complete.  VPO_SWAPINPROG is always
           * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
           * is set in the metadata for each page in the request.
           */
          VM_OBJECT_WLOCK(object);
          while ((ma[0]->oflags & VPO_SWAPINPROG) != 0) {
                  ma[0]->oflags |= VPO_SWAPSLEEP;
                  VM_CNT_INC(v_intrans);
                  if (VM_OBJECT_SLEEP(object, &object->paging_in_progress, 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);
                  }
          }
  
          /*
           * If we had an unrecoverable read error pages will not be valid.
           */
          for (i = 0; i < reqcount; i++)
                  if (ma[i]->valid != VM_PAGE_BITS_ALL)
                          return (VM_PAGER_ERROR);
  
          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_getpages_async():
   *
   *      Right now this is emulation of asynchronous operation on top of
   *      swap_pager_getpages().
   */
  static int
  swap_pager_getpages_async(vm_object_t object, vm_page_t *ma, int count,
      int *rbehind, int *rahead, pgo_getpages_iodone_t iodone, void *arg)
  {
          int r, error;
  
          r = swap_pager_getpages(object, ma, count, rbehind, rahead);
          VM_OBJECT_WUNLOCK(object);
          switch (r) {
          case VM_PAGER_OK:
                  error = 0;
                  break;
          case VM_PAGER_ERROR:
                  error = EIO;
                  break;
          case VM_PAGER_FAIL:
                  error = EINVAL;
                  break;
          default:
                  panic("unhandled swap_pager_getpages() error %d", r);
          }
          (iodone)(arg, ma, count, error);
          VM_OBJECT_WLOCK(object);
  
          return (r);
  }
  
  /*
   *      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.
   */
  static void
  swap_pager_putpages(vm_object_t object, vm_page_t *ma, int count,
      int flags, int *rtvals)
  {
          int i, n;
          boolean_t sync;
          daddr_t addr, n_free, s_free;
  
          swp_pager_init_freerange(&s_free, &n_free);
          if (count && ma[0]->object != object) {
                  panic("swap_pager_putpages: object mismatch %p/%p",
                      object,
                      ma[0]->object
                  );
          }
  
          /*
           * Step 1
           *
           * Turn object into OBJT_SWAP
           * check for bogus sysops
           * force sync if not pageout process
           */
          if (object->type != OBJT_SWAP) {
                  addr = swp_pager_meta_build(object, 0, SWAPBLK_NONE);
                  KASSERT(addr == SWAPBLK_NONE,
                      ("unexpected object swap block"));
          }
          VM_OBJECT_WUNLOCK(object);
  
          n = 0;
          if (curproc != pageproc)
                  sync = TRUE;
          else
                  sync = (flags & VM_PAGER_PUT_SYNC) != 0;
  
          /*
           * Step 2
           *
           * 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;
  
                  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_WLOCK(object);
                  for (j = 0; j < n; ++j) {
                          vm_page_t mreq = ma[i+j];
  
                          addr = swp_pager_meta_build(mreq->object, mreq->pindex,
                              blk + j);
                          if (addr != SWAPBLK_NONE)
                                  swp_pager_update_freerange(&s_free, &n_free,
                                      addr);
                          MPASS(mreq->dirty == VM_PAGE_BITS_ALL);
                          mreq->oflags |= VPO_SWAPINPROG;
                          bp->b_pages[j] = mreq;
                  }
                  VM_OBJECT_WUNLOCK(object);
                  bp->b_npages = n;
                  /*
                   * Must set dirty range for NFS to work.
                   */
                  bp->b_dirtyoff = 0;
                  bp->b_dirtyend = bp->b_bcount;
  
                  VM_CNT_INC(v_swapout);
                  VM_CNT_ADD(v_swappgsout, bp->b_npages);
  
                  /*
                   * We unconditionally set rtvals[] to VM_PAGER_PEND so that we
                   * can call the async completion routine at the end of a
                   * synchronous I/O operation.  Otherwise, our caller would
                   * perform duplicate unbusy and wakeup operations on the page
                   * and object, respectively.
                   */
                  for (j = 0; j < n; j++)
                          rtvals[i + j] = VM_PAGER_PEND;
  
                  /*
                   * 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);
                          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.
                   */
                  bwait(bp, PVM, "swwrt");
  
                  /*
                   * 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_WLOCK(object);
          swp_pager_freeswapspace(s_free, n_free);
  }
  
  /*
   *      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.
   *
   *      This routine may not sleep.
   */
  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
           */
          if (buf_mapped(bp))
                  pmap_qremove((vm_offset_t)bp->b_data, bp->b_npages);
          else
                  bp->b_data = bp->b_kvabase;
  
          if (bp->b_npages) {
                  object = bp->b_pages[0]->object;
                  VM_OBJECT_WLOCK(object);
          }
  
          /*
           * 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 (m->oflags & VPO_SWAPSLEEP) {
                          m->oflags &= ~VPO_SWAPSLEEP;
                          wakeup(&object->paging_in_progress);
                  }
  
                  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) {
                                  /*
                                   * 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;
                          } else {
                                  /*
                                   * If a write error occurs, reactivate page
                                   * so it doesn't clog the inactive list,
                                   * then finish the I/O.
                                   */
                                  MPASS(m->dirty == VM_PAGE_BITS_ALL);
                                  vm_page_lock(m);
                                  vm_page_activate(m);
                                  vm_page_unlock(m);
                                  vm_page_sunbusy(m);
                          }
                  } else if (bp->b_iocmd == BIO_READ) {
                          /*
                           * 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.
                           */
                          KASSERT(!pmap_page_is_mapped(m),
                              ("swp_pager_async_iodone: page %p is mapped", m));
                          KASSERT(m->dirty == 0,
                              ("swp_pager_async_iodone: page %p is dirty", m));
  
                          m->valid = VM_PAGE_BITS_ALL;
                          if (i < bp->b_pgbefore ||
                              i >= bp->b_npages - bp->b_pgafter)
                                  vm_page_readahead_finish(m);
                  } else {
                          /*
                           * For write success, clear the dirty
                           * status, then finish the I/O ( which decrements the
                           * busy count and possibly wakes waiter's up ).
                           * A page is only written to swap after a period of
                           * inactivity.  Therefore, we do not expect it to be
                           * reused.
                           */
                          KASSERT(!pmap_page_is_write_mapped(m),
                              ("swp_pager_async_iodone: page %p is not write"
                              " protected", m));
                          vm_page_undirty(m);
                          vm_page_lock(m);
                          vm_page_deactivate_noreuse(m);
                          vm_page_unlock(m);
                          vm_page_sunbusy(m);
                  }
          }
  
          /*
           * 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_WUNLOCK(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
                  )
              )
          );
  }
  
  int
  swap_pager_nswapdev(void)
  {
  
          return (nswapdev);
  }
  
  /*
   * SWP_PAGER_FORCE_PAGEIN() - force a swap block to be paged in
   *
   *      This routine dissociates the page at the given index within an object
   *      from its backing store, paging it in if it does not reside in memory.
   *      If the page is paged in, it is marked dirty and placed in the laundry
   *      queue.  The page is marked dirty because it no longer has backing
   *      store.  It is placed in the laundry queue because it has not been
   *      accessed recently.  Otherwise, it would already reside in memory.
   *
   *      We also attempt to swap in all other pages in the swap block.
   *      However, 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);
          if (m->valid == VM_PAGE_BITS_ALL) {
                  vm_object_pip_wakeup(object);
                  vm_page_dirty(m);
  #ifdef INVARIANTS
                  vm_page_lock(m);
                  if (m->wire_count == 0 && m->queue == PQ_NONE)
                          panic("page %p is neither wired nor queued", m);
                  vm_page_unlock(m);
  #endif
                  vm_page_xunbusy(m);
                  vm_pager_page_unswapped(m);
                  return;
          }
  
          if (swap_pager_getpages(object, &m, 1, NULL, NULL) != VM_PAGER_OK)
                  panic("swap_pager_force_pagein: read from swap failed");/*XXX*/
          vm_object_pip_wakeup(object);
          vm_page_dirty(m);
          vm_page_lock(m);
          vm_page_launder(m);
          vm_page_unlock(m);
          vm_page_xunbusy(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 swblk *sb;
          vm_object_t object;
          vm_pindex_t pi;
          int i, retries;
  
          sx_assert(&swdev_syscall_lock, SA_XLOCKED);
  
          retries = 0;
  full_rescan:
          mtx_lock(&vm_object_list_mtx);
          TAILQ_FOREACH(object, &vm_object_list, object_list) {
                  if (object->type != OBJT_SWAP)
                          continue;
                  mtx_unlock(&vm_object_list_mtx);
                  /* Depends on type-stability. */
                  VM_OBJECT_WLOCK(object);
  
                  /*
                   * Dead objects are eventually terminated on their own.
                   */
                  if ((object->flags & OBJ_DEAD) != 0)
                          goto next_obj;
  
                  /*
                   * Sync with fences placed after pctrie
                   * initialization.  We must not access pctrie below
                   * unless we checked that our object is swap and not
                   * dead.
                   */
                  atomic_thread_fence_acq();
                  if (object->type != OBJT_SWAP)
                          goto next_obj;
  
                  for (pi = 0; (sb = SWAP_PCTRIE_LOOKUP_GE(
                      &object->un_pager.swp.swp_blks, pi)) != NULL; ) {
                          pi = sb->p + SWAP_META_PAGES;
                          for (i = 0; i < SWAP_META_PAGES; i++) {
                                  if (sb->d[i] == SWAPBLK_NONE)
                                          continue;
                                  if (swp_pager_isondev(sb->d[i], sp))
                                          swp_pager_force_pagein(object,
                                              sb->p + i);
                          }
                  }
  next_obj:
                  VM_OBJECT_WUNLOCK(object);
                  mtx_lock(&vm_object_list_mtx);
          }
          mtx_unlock(&vm_object_list_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;
          }
          EVENTHANDLER_INVOKE(swapoff, sp);
  }
  
  /************************************************************************
   *                              SWAP META DATA                          *
   ************************************************************************
   *
   *      These routines manipulate the swap metadata stored in the
   *      OBJT_SWAP object.
   *
   *      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_SWBLK_EMPTY() - is a range of blocks free?
   */
  static bool
  swp_pager_swblk_empty(struct swblk *sb, int start, int limit)
  {
          int i;
  
          MPASS(0 <= start && start <= limit && limit <= SWAP_META_PAGES);
          for (i = start; i < limit; i++) {
                  if (sb->d[i] != SWAPBLK_NONE)
                          return (false);
          }
          return (true);
  }
     
  /*
   * 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 returned.
   */
  static daddr_t
  swp_pager_meta_build(vm_object_t object, vm_pindex_t pindex, daddr_t swapblk)
  {
          static volatile int swblk_zone_exhausted, swpctrie_zone_exhausted;
          struct swblk *sb, *sb1;
          vm_pindex_t modpi, rdpi;
          daddr_t prev_swapblk;
          int error, i;
  
          VM_OBJECT_ASSERT_WLOCKED(object);
  
          /*
           * Convert default object to swap object if necessary
           */
          if (object->type != OBJT_SWAP) {
                  pctrie_init(&object->un_pager.swp.swp_blks);
  
                  /*
                   * Ensure that swap_pager_swapoff()'s iteration over
                   * object_list does not see a garbage pctrie.
                   */
                  atomic_thread_fence_rel();
  
                  object->type = OBJT_SWAP;
                  KASSERT(object->handle == NULL, ("default pager with handle"));
          }
  
          rdpi = rounddown(pindex, SWAP_META_PAGES);
          sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks, rdpi);
          if (sb == NULL) {
                  if (swapblk == SWAPBLK_NONE)
                          return (SWAPBLK_NONE);
                  for (;;) {
                          sb = uma_zalloc(swblk_zone, M_NOWAIT | (curproc ==
                              pageproc ? M_USE_RESERVE : 0));
                          if (sb != NULL) {
                                  sb->p = rdpi;
                                  for (i = 0; i < SWAP_META_PAGES; i++)
                                          sb->d[i] = SWAPBLK_NONE;
                                  if (atomic_cmpset_int(&swblk_zone_exhausted,
                                      1, 0))
                                          printf("swblk zone ok\n");
                                  break;
                          }
                          VM_OBJECT_WUNLOCK(object);
                          if (uma_zone_exhausted(swblk_zone)) {
                                  if (atomic_cmpset_int(&swblk_zone_exhausted,
                                      0, 1))
                                          printf("swap blk zone exhausted, "
                                              "increase kern.maxswzone\n");
                                  vm_pageout_oom(VM_OOM_SWAPZ);
                                  pause("swzonxb", 10);
                          } else
                                  uma_zwait(swblk_zone);
                          VM_OBJECT_WLOCK(object);
                          sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
                              rdpi);
                          if (sb != NULL)
                                  /*
                                   * Somebody swapped out a nearby page,
                                   * allocating swblk at the rdpi index,
                                   * while we dropped the object lock.
                                   */
                                  goto allocated;
                  }
                  for (;;) {
                          error = SWAP_PCTRIE_INSERT(
                              &object->un_pager.swp.swp_blks, sb);
                          if (error == 0) {
                                  if (atomic_cmpset_int(&swpctrie_zone_exhausted,
                                      1, 0))
                                          printf("swpctrie zone ok\n");
                                  break;
                          }
                          VM_OBJECT_WUNLOCK(object);
                          if (uma_zone_exhausted(swpctrie_zone)) {
                                  if (atomic_cmpset_int(&swpctrie_zone_exhausted,
                                      0, 1))
                                          printf("swap pctrie zone exhausted, "
                                              "increase kern.maxswzone\n");
                                  vm_pageout_oom(VM_OOM_SWAPZ);
                                  pause("swzonxp", 10);
                          } else
                                  uma_zwait(swpctrie_zone);
                          VM_OBJECT_WLOCK(object);
                          sb1 = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
                              rdpi);
                          if (sb1 != NULL) {
                                  uma_zfree(swblk_zone, sb);
                                  sb = sb1;
                                  goto allocated;
                          }
                  }
          }
  allocated:
          MPASS(sb->p == rdpi);
  
          modpi = pindex % SWAP_META_PAGES;
          /* Return prior contents of metadata. */
          prev_swapblk = sb->d[modpi];
          /* Enter block into metadata. */
          sb->d[modpi] = swapblk;
  
          /*
           * Free the swblk if we end up with the empty page run.
           */
          if (swapblk == SWAPBLK_NONE &&
              swp_pager_swblk_empty(sb, 0, SWAP_META_PAGES)) {
                  SWAP_PCTRIE_REMOVE(&object->un_pager.swp.swp_blks, rdpi);
                  uma_zfree(swblk_zone, sb);
          }
          return (prev_swapblk);
  }
  
  /*
   * 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.
   */
  static void
  swp_pager_meta_free(vm_object_t object, vm_pindex_t pindex, vm_pindex_t count)
  {
          struct swblk *sb;
          daddr_t n_free, s_free;
          vm_pindex_t last;
          int i, limit, start;
  
          VM_OBJECT_ASSERT_WLOCKED(object);
          if (object->type != OBJT_SWAP || count == 0)
                  return;
  
          swp_pager_init_freerange(&s_free, &n_free);
          last = pindex + count;
          for (;;) {
                  sb = SWAP_PCTRIE_LOOKUP_GE(&object->un_pager.swp.swp_blks,
                      rounddown(pindex, SWAP_META_PAGES));
                  if (sb == NULL || sb->p >= last)
                          break;
                  start = pindex > sb->p ? pindex - sb->p : 0;
                  limit = last - sb->p < SWAP_META_PAGES ? last - sb->p :
                      SWAP_META_PAGES;
                  for (i = start; i < limit; i++) {
                          if (sb->d[i] == SWAPBLK_NONE)
                                  continue;
                          swp_pager_update_freerange(&s_free, &n_free, sb->d[i]);
                          sb->d[i] = SWAPBLK_NONE;
                  }
                  pindex = sb->p + SWAP_META_PAGES;
                  if (swp_pager_swblk_empty(sb, 0, start) &&
                      swp_pager_swblk_empty(sb, limit, SWAP_META_PAGES)) {
                          SWAP_PCTRIE_REMOVE(&object->un_pager.swp.swp_blks,
                              sb->p);
                          uma_zfree(swblk_zone, sb);
                  }
          }
          swp_pager_freeswapspace(s_free, n_free);
  }
  
  /*
   * SWP_PAGER_META_FREE_ALL() - destroy all swap metadata associated with object
   *
   *      This routine locates and destroys all swap metadata associated with
   *      an object.
   */
  static void
  swp_pager_meta_free_all(vm_object_t object)
  {
          struct swblk *sb;
          daddr_t n_free, s_free;
          vm_pindex_t pindex;
          int i;
  
          VM_OBJECT_ASSERT_WLOCKED(object);
          if (object->type != OBJT_SWAP)
                  return;
  
          swp_pager_init_freerange(&s_free, &n_free);
          for (pindex = 0; (sb = SWAP_PCTRIE_LOOKUP_GE(
              &object->un_pager.swp.swp_blks, pindex)) != NULL;) {
                  pindex = sb->p + SWAP_META_PAGES;
                  for (i = 0; i < SWAP_META_PAGES; i++) {
                          if (sb->d[i] == SWAPBLK_NONE)
                                  continue;
                          swp_pager_update_freerange(&s_free, &n_free, sb->d[i]);
                  }
                  SWAP_PCTRIE_REMOVE(&object->un_pager.swp.swp_blks, sb->p);
                  uma_zfree(swblk_zone, sb);
          }
          swp_pager_freeswapspace(s_free, n_free);
  }
  
  /*
   * SWP_PAGER_METACTL() -  misc control of swap meta data.
   *
   *      This routine is capable of looking up, or removing swapblk
   *      assignments in the swap meta data.  It returns the swapblk being
   *      looked-up, popped, or SWAPBLK_NONE if the block was invalid.
   *
   *      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.
   *
   *      SWM_POP         remove from meta data but do not free it
   */
  static daddr_t
  swp_pager_meta_ctl(vm_object_t object, vm_pindex_t pindex, int flags)
  {
          struct swblk *sb;
          daddr_t r1;
  
          if ((flags & SWM_POP) != 0)
                  VM_OBJECT_ASSERT_WLOCKED(object);
          else
                  VM_OBJECT_ASSERT_LOCKED(object);
  
          /*
           * The meta data only exists if the object is OBJT_SWAP
           * and even then might not be allocated yet.
           */
          if (object->type != OBJT_SWAP)
                  return (SWAPBLK_NONE);
  
          sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
              rounddown(pindex, SWAP_META_PAGES));
          if (sb == NULL)
                  return (SWAPBLK_NONE);
          r1 = sb->d[pindex % SWAP_META_PAGES];
          if (r1 == SWAPBLK_NONE)
                  return (SWAPBLK_NONE);
          if ((flags & SWM_POP) != 0) {
                  sb->d[pindex % SWAP_META_PAGES] = SWAPBLK_NONE;
                  if (swp_pager_swblk_empty(sb, 0, SWAP_META_PAGES)) {
                          SWAP_PCTRIE_REMOVE(&object->un_pager.swp.swp_blks,
                              rounddown(pindex, SWAP_META_PAGES));
                          uma_zfree(swblk_zone, sb);
                  }
          }
          return (r1);
  }
  
  /*
   * Returns the least page index which is greater than or equal to the
   * parameter pindex and for which there is a swap block allocated.
   * Returns object's size if the object's type is not swap or if there
   * are no allocated swap blocks for the object after the requested
   * pindex.
   */
  vm_pindex_t
  swap_pager_find_least(vm_object_t object, vm_pindex_t pindex)
  {
          struct swblk *sb;
          int i;
  
          VM_OBJECT_ASSERT_LOCKED(object);
          if (object->type != OBJT_SWAP)
                  return (object->size);
  
          sb = SWAP_PCTRIE_LOOKUP_GE(&object->un_pager.swp.swp_blks,
              rounddown(pindex, SWAP_META_PAGES));
          if (sb == NULL)
                  return (object->size);
          if (sb->p < pindex) {
                  for (i = pindex % SWAP_META_PAGES; i < SWAP_META_PAGES; i++) {
                          if (sb->d[i] != SWAPBLK_NONE)
                                  return (sb->p + i);
                  }
                  sb = SWAP_PCTRIE_LOOKUP_GE(&object->un_pager.swp.swp_blks,
                      roundup(pindex, SWAP_META_PAGES));
                  if (sb == NULL)
                          return (object->size);
          }
          for (i = 0; i < SWAP_META_PAGES; i++) {
                  if (sb->d[i] != SWAPBLK_NONE)
                          return (sb->p + i);
          }
  
          /*
           * We get here if a swblk is present in the trie but it
           * doesn't map any blocks.
           */
          MPASS(0);
          return (object->size);
  }
  
  /*
   * 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
  sys_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);
  
          sx_xlock(&swdev_syscall_lock);
  
          /*
           * Swap metadata may not fit in the KVM if we have physical
           * memory of >1GB.
           */
          if (swblk_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(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)) == 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:
          sx_xunlock(&swdev_syscall_lock);
          return (error);
  }
  
  /*
   * Check that the total amount of swap currently configured does not
   * exceed half the theoretical maximum.  If it does, print a warning
   * message.
   */
  static void
  swapon_check_swzone(void)
  {
          unsigned long maxpages, npages;
  
          npages = swap_total;
          /* absolute maximum we can handle assuming 100% efficiency */
          maxpages = uma_zone_get_max(swblk_zone) * SWAP_META_PAGES;
  
          /* recommend using no more than half that amount */
          if (npages > maxpages / 2) {
                  printf("warning: total configured swap (%lu pages) "
                      "exceeds maximum recommended amount (%lu pages).\n",
                      npages, maxpages / 2);
                  printf("warning: increase kern.maxswzone "
                      "or reduce amount of swap.\n");
          }
  }
  
  static void
  swaponsomething(struct vnode *vp, void *id, u_long nblks,
      sw_strategy_t *strategy, sw_close_t *close, dev_t dev, int flags)
  {
          struct swdevt *sp, *tsp;
          swblk_t dvbase;
          u_long 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);
  
          /*
           * 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 swap size to maximum of %luMB per unit\n",
                      mblocks / 1024 / 1024 * PAGE_SIZE);
                  nblks = mblocks;
          }
  
          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_flags = flags;
  
          sp->sw_blist = blist_create(nblks, M_WAITOK);
          /*
           * 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 - 2;
          swap_total += nblks;
          swapon_check_swzone();
          swp_sizecheck();
          mtx_unlock(&sw_dev_mtx);
          EVENTHANDLER_INVOKE(swapon, sp);
  }
  
  /*
   * 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
  sys_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);
  
          sx_xlock(&swdev_syscall_lock);
  
          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->td_ucred);
  done:
          sx_xunlock(&swdev_syscall_lock);
          return (error);
  }
  
  static int
  swapoff_one(struct swdevt *sp, struct ucred *cred)
  {
          u_long nblks;
  #ifdef MAC
          int error;
  #endif
  
          sx_assert(&swdev_syscall_lock, SA_XLOCKED);
  #ifdef MAC
          (void) vn_lock(sp->sw_vp, LK_EXCLUSIVE | LK_RETRY);
          error = mac_system_check_swapoff(cred, sp->sw_vp);
          (void) VOP_UNLOCK(sp->sw_vp, 0);
          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 (vm_free_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;
          swap_pager_avail -= blist_fill(sp->sw_blist, 0, nblks);
          swap_total -= nblks;
          mtx_unlock(&sw_dev_mtx);
  
          /*
           * Page in the contents of the device and close it.
           */
          swap_pager_swapoff(sp);
  
          sp->sw_close(curthread, sp);
          mtx_lock(&sw_dev_mtx);
          sp->sw_id = NULL;
          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;
  
          sx_xlock(&swdev_syscall_lock);
  
          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 = devtoname(sp->sw_vp->v_rdev);
                  else
                          devname = "[file]";
                  error = swapoff_one(sp, thread0.td_ucred);
                  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);
  
          sx_xunlock(&swdev_syscall_lock);
  }
  
  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);
  }
  
  int
  swap_dev_info(int name, struct xswdev *xs, char *devname, size_t len)
  {
          struct swdevt *sp;
          const char *tmp_devname;
          int error, n;
  
          n = 0;
          error = ENOENT;
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  if (n != name) {
                          n++;
                          continue;
                  }
                  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;
                  if (devname != NULL) {
                          if (vn_isdisk(sp->sw_vp, NULL))
                                  tmp_devname = devtoname(sp->sw_vp->v_rdev);
                          else
                                  tmp_devname = "[file]";
                          strncpy(devname, tmp_devname, len);
                  }
                  error = 0;
                  break;
          }
          mtx_unlock(&sw_dev_mtx);
          return (error);
  }
  
  #if defined(COMPAT_FREEBSD11)
  #define XSWDEV_VERSION_11       1
  struct xswdev11 {
          u_int   xsw_version;
          uint32_t xsw_dev;
          int     xsw_flags;
          int     xsw_nblks;
          int     xsw_used;
  };
  #endif
  
  static int
  sysctl_vm_swap_info(SYSCTL_HANDLER_ARGS)
  {
          struct xswdev xs;
  #if defined(COMPAT_FREEBSD11)
          struct xswdev11 xs11;
  #endif
          int error;
  
          if (arg2 != 1)                  /* name length */
                  return (EINVAL);
          error = swap_dev_info(*(int *)arg1, &xs, NULL, 0);
          if (error != 0)
                  return (error);
  #if defined(COMPAT_FREEBSD11)
          if (req->oldlen == sizeof(xs11)) {
                  xs11.xsw_version = XSWDEV_VERSION_11;
                  xs11.xsw_dev = xs.xsw_dev; /* truncation */
                  xs11.xsw_flags = xs.xsw_flags;
                  xs11.xsw_nblks = xs.xsw_nblks;
                  xs11.xsw_used = xs.xsw_used;
                  error = SYSCTL_OUT(req, &xs11, sizeof(xs11));
          } else
  #endif
                  error = SYSCTL_OUT(req, &xs, sizeof(xs));
          return (error);
  }
  
  SYSCTL_INT(_vm, OID_AUTO, nswapdev, CTLFLAG_RD, &nswapdev, 0,
      "Number of swap devices");
  SYSCTL_NODE(_vm, OID_AUTO, swap_info, CTLFLAG_RD | CTLFLAG_MPSAFE,
      sysctl_vm_swap_info,
      "Swap statistics by device");
  
  /*
   * Count the approximate swap usage in pages for a vmspace.  The
   * shadowed or not yet copied on write swap blocks are not accounted.
   * The map must be locked.
   */
  long
  vmspace_swap_count(struct vmspace *vmspace)
  {
          vm_map_t map;
          vm_map_entry_t cur;
          vm_object_t object;
          struct swblk *sb;
          vm_pindex_t e, pi;
          long count;
          int i;
  
          map = &vmspace->vm_map;
          count = 0;
  
          for (cur = map->header.next; cur != &map->header; cur = cur->next) {
                  if ((cur->eflags & MAP_ENTRY_IS_SUB_MAP) != 0)
                          continue;
                  object = cur->object.vm_object;
                  if (object == NULL || object->type != OBJT_SWAP)
                          continue;
                  VM_OBJECT_RLOCK(object);
                  if (object->type != OBJT_SWAP)
                          goto unlock;
                  pi = OFF_TO_IDX(cur->offset);
                  e = pi + OFF_TO_IDX(cur->end - cur->start);
                  for (;; pi = sb->p + SWAP_META_PAGES) {
                          sb = SWAP_PCTRIE_LOOKUP_GE(
                              &object->un_pager.swp.swp_blks, pi);
                          if (sb == NULL || sb->p >= e)
                                  break;
                          for (i = 0; i < SWAP_META_PAGES; i++) {
                                  if (sb->p + i < e &&
                                      sb->d[i] != SWAPBLK_NONE)
                                          count++;
                          }
                  }
  unlock:
                  VM_OBJECT_RUNLOCK(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_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);
  }
  
  /*
   * Add a reference to the g_consumer for an inflight transaction.
   */
  static void
  swapgeom_acquire(struct g_consumer *cp)
  {
  
          mtx_assert(&sw_dev_mtx, MA_OWNED);
          cp->index++;
  }
  
  /*
   * Remove a reference from the g_consumer.  Post a close event if all
   * references go away, since the function might be called from the
   * biodone context.
   */
  static void
  swapgeom_release(struct g_consumer *cp, struct swdevt *sp)
  {
  
          mtx_assert(&sw_dev_mtx, MA_OWNED);
          cp->index--;
          if (cp->index == 0) {
                  if (g_post_event(swapgeom_close_ev, cp, M_NOWAIT, NULL) == 0)
                          sp->sw_id = NULL;
          }
  }
  
  static void
  swapgeom_done(struct bio *bp2)
  {
          struct swdevt *sp;
          struct buf *bp;
          struct g_consumer *cp;
  
          bp = bp2->bio_caller2;
          cp = bp2->bio_from;
          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);
          sp = bp2->bio_caller1;
          mtx_lock(&sw_dev_mtx);
          swapgeom_release(cp, sp);
          mtx_unlock(&sw_dev_mtx);
          g_destroy_bio(bp2);
  }
  
  static void
  swapgeom_strategy(struct buf *bp, struct swdevt *sp)
  {
          struct bio *bio;
          struct g_consumer *cp;
  
          mtx_lock(&sw_dev_mtx);
          cp = sp->sw_id;
          if (cp == NULL) {
                  mtx_unlock(&sw_dev_mtx);
                  bp->b_error = ENXIO;
                  bp->b_ioflags |= BIO_ERROR;
                  bufdone(bp);
                  return;
          }
          swapgeom_acquire(cp);
          mtx_unlock(&sw_dev_mtx);
          if (bp->b_iocmd == BIO_WRITE)
                  bio = g_new_bio();
          else
                  bio = g_alloc_bio();
          if (bio == NULL) {
                  mtx_lock(&sw_dev_mtx);
                  swapgeom_release(cp, sp);
                  mtx_unlock(&sw_dev_mtx);
                  bp->b_error = ENOMEM;
                  bp->b_ioflags |= BIO_ERROR;
                  bufdone(bp);
                  return;
          }
  
          bio->bio_caller1 = sp;
          bio->bio_caller2 = bp;
          bio->bio_cmd = bp->b_iocmd;
          bio->bio_offset = (bp->b_blkno - sp->sw_first) * PAGE_SIZE;
          bio->bio_length = bp->b_bcount;
          bio->bio_done = swapgeom_done;
          if (!buf_mapped(bp)) {
                  bio->bio_ma = bp->b_pages;
                  bio->bio_data = unmapped_buf;
                  bio->bio_ma_offset = (vm_offset_t)bp->b_offset & PAGE_MASK;
                  bio->bio_ma_n = bp->b_npages;
                  bio->bio_flags |= BIO_UNMAPPED;
          } else {
                  bio->bio_data = bp->b_data;
                  bio->bio_ma = NULL;
          }
          g_io_request(bio, cp);
          return;
  }
  
  static void
  swapgeom_orphan(struct g_consumer *cp)
  {
          struct swdevt *sp;
          int destroy;
  
          mtx_lock(&sw_dev_mtx);
          TAILQ_FOREACH(sp, &swtailq, sw_list) {
                  if (sp->sw_id == cp) {
                          sp->sw_flags |= SW_CLOSING;
                          break;
                  }
          }
          /*
           * Drop reference we were created with. Do directly since we're in a
           * special context where we don't have to queue the call to
           * swapgeom_close_ev().
           */
          cp->index--;
          destroy = ((sp != NULL) && (cp->index == 0));
          if (destroy)
                  sp->sw_id = NULL;
          mtx_unlock(&sw_dev_mtx);
          if (destroy)
                  swapgeom_close_ev(cp, 0);
  }
  
  static void
  swapgeom_close(struct thread *td, struct swdevt *sw)
  {
          struct g_consumer *cp;
  
          mtx_lock(&sw_dev_mtx);
          cp = sw->sw_id;
          sw->sw_id = NULL;
          mtx_unlock(&sw_dev_mtx);
  
          /*
           * swapgeom_close() may be called from the biodone context,
           * where we cannot perform topology changes.  Delegate the
           * work to the events thread.
           */
          if (cp != NULL)
                  g_waitfor_event(swapgeom_close_ev, cp, M_WAITOK, NULL);
  }
  
  static int
  swapongeom_locked(struct cdev *dev, struct vnode *vp)
  {
          struct g_provider *pp;
          struct g_consumer *cp;
          static struct g_geom *gp;
          struct swdevt *sp;
          u_long nblks;
          int error;
  
          pp = g_dev_getprovider(dev);
          if (pp == NULL)
                  return (ENODEV);
          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);
                          return (EBUSY);
                  }
          }
          mtx_unlock(&sw_dev_mtx);
          if (gp == NULL)
                  gp = g_new_geomf(&g_swap_class, "swap");
          cp = g_new_consumer(gp);
          cp->index = 1;  /* Number of active I/Os, plus one for being active. */
          cp->flags |=  G_CF_DIRECT_SEND | G_CF_DIRECT_RECEIVE;
          g_attach(cp, pp);
          /*
           * XXX: Every time 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 != 0) {
                  g_detach(cp);
                  g_destroy_consumer(cp);
                  return (error);
          }
          nblks = pp->mediasize / DEV_BSIZE;
          swaponsomething(vp, cp, nblks, swapgeom_strategy,
              swapgeom_close, dev2udev(dev),
              (pp->flags & G_PF_ACCEPT_UNMAPPED) != 0 ? SW_UNMAPPED : 0);
          return (0);
  }
  
  static int
  swapongeom(struct vnode *vp)
  {
          int error;
  
          vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
          if (vp->v_type != VCHR || (vp->v_iflag & VI_DOOMED) != 0) {
                  error = ENOENT;
          } else {
                  g_topology_lock();
                  error = swapongeom_locked(vp->v_rdev, vp);
                  g_topology_unlock();
          }
          VOP_UNLOCK(vp, 0);
          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);
  #ifdef MAC
          error = mac_system_check_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);
          if (error)
                  return (error);
  
          swaponsomething(vp, vp, nblks, swapdev_strategy, swapdev_close,
              NODEV, 0);
          return (0);
  }
  
  static int
  sysctl_swap_async_max(SYSCTL_HANDLER_ARGS)
  {
          int error, new, n;
  
          new = nsw_wcount_async_max;
          error = sysctl_handle_int(oidp, &new, 0, req);
          if (error != 0 || req->newptr == NULL)
                  return (error);
  
          if (new > nswbuf / 2 || new < 1)
                  return (EINVAL);
  
          mtx_lock(&pbuf_mtx);
          while (nsw_wcount_async_max != new) {
                  /*
                   * Adjust difference.  If the current async count is too low,
                   * we will need to sqeeze our update slowly in.  Sleep with a
                   * higher priority than getpbuf() to finish faster.
                   */
                  n = new - nsw_wcount_async_max;
                  if (nsw_wcount_async + n >= 0) {
                          nsw_wcount_async += n;
                          nsw_wcount_async_max += n;
                          wakeup(&nsw_wcount_async);
                  } else {
                          nsw_wcount_async_max -= nsw_wcount_async;
                          nsw_wcount_async = 0;
                          msleep(&nsw_wcount_async, &pbuf_mtx, PSWP,
                              "swpsysctl", 0);
                  }
          }
          mtx_unlock(&pbuf_mtx);
  
          return (0);
  }

Cache object: da5de3b2af2d2b63e6fda0dbe1616f40