Merge ^/head r358263 through r358268.

[FreeBSD/FreeBSD.git] / 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("$FreeBSD$");

#include "opt_vm.h"

#include <sys/param.h>
#include <sys/bio.h>
#include <sys/blist.h>
#include <sys/buf.h>
#include <sys/conf.h>
#include <sys/disk.h>
#include <sys/disklabel.h>
#include <sys/eventhandler.h>
#include <sys/fcntl.h>
#include <sys/lock.h>
#include <sys/kernel.h>
#include <sys/mount.h>
#include <sys/namei.h>
#include <sys/malloc.h>
#include <sys/pctrie.h>
#include <sys/priv.h>
#include <sys/proc.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/systm.h>
#include <sys/sx.h>
#include <sys/vmmeter.h>
#include <sys/vnode.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);

static SYSCTL_NODE(_vm_stats, OID_AUTO, swap, CTLFLAG_RD, 0, "VM swap stats");

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");

static counter_u64_t swap_free_deferred;
SYSCTL_COUNTER_U64(_vm_stats_swap, OID_AUTO, free_deferred,
    CTLFLAG_RD, &swap_free_deferred,
    "Number of pages that deferred freeing swap space");

static counter_u64_t swap_free_completed;
SYSCTL_COUNTER_U64(_vm_stats_swap, OID_AUTO, free_completed,
    CTLFLAG_RD, &swap_free_completed,
    "Number of deferred frees completed");

/* 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
}

static int swap_pager_full = 2; /* swap space exhaustion (task killing) */
static int swap_pager_almost_full = 1; /* swap space exhaustion (w/hysteresis)*/
static struct mtx swbuf_mtx;    /* to sync nsw_wcount_async */
static int nsw_wcount_async;    /* limit async write buffers */
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 swwbuf_zone;
static uma_zone_t swrbuf_zone;
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);
static void     swap_pager_update_writecount(vm_object_t object,
    vm_offset_t start, vm_offset_t end);
static void     swap_pager_release_writecount(vm_object_t object,
    vm_offset_t start, vm_offset_t end);

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          */
        .pgo_update_writecount = swap_pager_update_writecount,
        .pgo_release_writecount = swap_pager_release_writecount,
};

/*
 * 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 void     swp_pager_free_empty_swblk(vm_object_t, struct swblk *sb);
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_transfer(vm_object_t src, vm_object_t dst,
    vm_pindex_t pindex, vm_pindex_t count);
static void swp_pager_meta_free_all(vm_object_t);
static daddr_t swp_pager_meta_lookup(vm_object_t, vm_pindex_t);

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");
}

static void
swap_pager_counters(void)
{

        swap_free_deferred = counter_u64_alloc(M_WAITOK);
        swap_free_completed = counter_u64_alloc(M_WAITOK);
}
SYSINIT(swap_counters, SI_SUB_CPU, SI_ORDER_ANY, swap_pager_counters, NULL);

/*
 * 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, which has MAXPHYS / PAGE_SIZE entries, and our locally
         * defined MAX_PAGEOUT_CLUSTER.   Also be aware that swap ops are
         * constrained by the swap device interleave stripe size.
         *
         * Currently we hardwire nsw_wcount_async to 4.  This limit is
         * designed to prevent other I/O from having high latencies due to
         * our pageout I/O.  The value 4 works well for one or two active swap
         * devices but is probably a little low if you have more.  Even so,
         * a higher value would probably generate only a limited improvement
         * with three or four active swap devices since the system does not
         * typically have to pageout at extreme bandwidths.   We will want
         * at least 2 per swap devices, and 4 is a pretty good value if you
         * have one NFS swap device due to the command/ack latency over NFS.
         * So it all works out pretty well.
         */
        nsw_cluster_max = min(MAXPHYS / PAGE_SIZE, MAX_PAGEOUT_CLUSTER);

        nsw_wcount_async = 4;
        nsw_wcount_async_max = nsw_wcount_async;
        mtx_init(&swbuf_mtx, "async swbuf mutex", NULL, MTX_DEF);

        swwbuf_zone = pbuf_zsecond_create("swwbuf", nswbuf / 4);
        swrbuf_zone = pbuf_zsecond_create("swrbuf", nswbuf / 2);

        /*
         * Initialize our zone, taking the user's requested size or
         * estimating the number we need based on the number of pages
         * in the system.
         */
        n = maxswzone != 0 ? maxswzone / sizeof(struct swblk) :
            vm_cnt.v_page_count / 2;
        swpctrie_zone = uma_zcreate("swpctrie", pctrie_node_size(), NULL, NULL,
            pctrie_zone_init, NULL, UMA_ALIGN_PTR, 0);
        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, 0);
        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 changed from %lu to %lu.\n",
                    n2, n);
        /* absolute maximum we can handle assuming 100% efficiency */
        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->un_pager.swp.writemappings = 0;
        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->flags & OBJ_ANON) == 0 && 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 up to 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 *io_npages)
{
        daddr_t blk;
        struct swdevt *sp;
        int mpages, npages;

        KASSERT(*io_npages >= 1,
            ("%s: npages not positive", __func__));
        blk = SWAPBLK_NONE;
        mpages = *io_npages;
        npages = imin(BLIST_MAX_ALLOC, mpages);
        mtx_lock(&sw_dev_mtx);
        sp = swdevhd;
        while (!TAILQ_EMPTY(&swtailq)) {
                if (sp == NULL)
                        sp = TAILQ_FIRST(&swtailq);
                if ((sp->sw_flags & SW_CLOSING) == 0)
                        blk = blist_alloc(sp->sw_blist, &npages, mpages);
                if (blk != SWAPBLK_NONE)
                        break;
                sp = TAILQ_NEXT(sp, sw_list);
                if (swdevhd == sp) {
                        if (npages == 1)
                                break;
                        mpages = npages - 1;
                        npages >>= 1;
                }
        }
        if (blk != SWAPBLK_NONE) {
                *io_npages = npages;
                blk += sp->sw_first;
                sp->sw_used += npages;
                swap_pager_avail -= npages;
                swp_sizecheck();
                swdevhd = TAILQ_NEXT(sp, sw_list);
        } else {
                if (swap_pager_full != 2) {
                        printf("swp_pager_getswapspace(%d): failed\n",
                            *io_npages);
                        swap_pager_full = 2;
                        swap_pager_almost_full = 1;
                }
                swdevhd = NULL;
        }
        mtx_unlock(&sw_dev_mtx);
        return (blk);
}

static bool
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 (swp_pager_isondev(bp->b_blkno, sp)) {
                        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 (swp_pager_isondev(blk, sp)) {
                        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)
{
        daddr_t addr, blk, n_free, s_free;
        int i, j, n;

        swp_pager_init_freerange(&s_free, &n_free);
        VM_OBJECT_WLOCK(object);
        for (i = 0; i < size; i += n) {
                n = size - i;
                blk = swp_pager_getswapspace(&n);
                if (blk == SWAPBLK_NONE) {
                        swp_pager_meta_free(object, start, i);
                        VM_OBJECT_WUNLOCK(object);
                        return (-1);
                }
                for (j = 0; j < n; ++j) {
                        addr = swp_pager_meta_build(object,
                            start + i + j, blk + j);
                        if (addr != SWAPBLK_NONE)
                                swp_pager_update_freerange(&s_free, &n_free,
                                    addr);
                }
        }
        swp_pager_freeswapspace(s_free, n_free);
        VM_OBJECT_WUNLOCK(object);
        return (0);
}

static bool
swp_pager_xfer_source(vm_object_t srcobject, vm_object_t dstobject,
    vm_pindex_t pindex, daddr_t addr)
{
        daddr_t dstaddr;

        KASSERT(srcobject->type == OBJT_SWAP,
            ("%s: Srcobject not swappable", __func__));
        if (dstobject->type == OBJT_SWAP &&
            swp_pager_meta_lookup(dstobject, pindex) != SWAPBLK_NONE) {
                /* Caller should destroy the source block. */
                return (false);
        }

        /*
         * Destination has no swapblk and is not resident, transfer source.
         * swp_pager_meta_build() can sleep.
         */
        VM_OBJECT_WUNLOCK(srcobject);
        dstaddr = swp_pager_meta_build(dstobject, pindex, addr);
        KASSERT(dstaddr == SWAPBLK_NONE,
            ("Unexpected destination swapblk"));
        VM_OBJECT_WLOCK(srcobject);

        return (true);
}

/*
 * 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().
 *
 *      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_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->flags & OBJ_ANON) == 0 &&
            srcobject->handle != NULL) {
                VM_OBJECT_WUNLOCK(srcobject);
                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_WLOCK(srcobject);
        }

        /*
         * Transfer source to destination.
         */
        swp_pager_meta_transfer(srcobject, dstobject, offset, dstobject->size);

        /*
         * 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);
        KASSERT(object->type == OBJT_SWAP,
            ("%s: object not swappable", __func__));

        /*
         * do we have good backing store at the requested index ?
         */
        blk0 = swp_pager_meta_lookup(object, pindex);
        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_lookup(object, pindex - i);
                        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_lookup(object, pindex + i);
                        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 may be locked.
 */
static void
swap_pager_unswapped(vm_page_t m)
{
        struct swblk *sb;
        vm_object_t obj;

        /*
         * Handle enqueing deferred frees first.  If we do not have the
         * object lock we wait for the page daemon to clear the space.
         */
        obj = m->object;
        if (!VM_OBJECT_WOWNED(obj)) {
                VM_PAGE_OBJECT_BUSY_ASSERT(m);
                /*
                 * The caller is responsible for synchronization but we
                 * will harmlessly handle races.  This is typically provided
                 * by only calling unswapped() when a page transitions from
                 * clean to dirty.
                 */
                if ((m->a.flags & (PGA_SWAP_SPACE | PGA_SWAP_FREE)) ==
                    PGA_SWAP_SPACE) {
                        vm_page_aflag_set(m, PGA_SWAP_FREE);
                        counter_u64_add(swap_free_deferred, 1);
                }
                return;
        }
        if ((m->a.flags & PGA_SWAP_FREE) != 0)
                counter_u64_add(swap_free_completed, 1);
        vm_page_aflag_clear(m, PGA_SWAP_FREE | PGA_SWAP_SPACE);

        /*
         * The meta data only exists if the object is OBJT_SWAP
         * and even then might not be allocated yet.
         */
        KASSERT(m->object->type == OBJT_SWAP,
            ("Free object not swappable"));

        sb = SWAP_PCTRIE_LOOKUP(&m->object->un_pager.swp.swp_blks,
            rounddown(m->pindex, SWAP_META_PAGES));
        if (sb == NULL)
                return;
        if (sb->d[m->pindex % SWAP_META_PAGES] == SWAPBLK_NONE)
                return;
        swp_pager_freeswapspace(sb->d[m->pindex % SWAP_META_PAGES], 1);
        sb->d[m->pindex % SWAP_META_PAGES] = SWAPBLK_NONE;
        swp_pager_free_empty_swblk(m->object, sb);
}

/*
 * 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_locked(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;

        VM_OBJECT_ASSERT_WLOCKED(object);
        reqcount = count;

        KASSERT(object->type == OBJT_SWAP,
            ("%s: object not swappable", __func__));
        if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, &maxahead)) {
                VM_OBJECT_WUNLOCK(object);
                return (VM_PAGER_FAIL);
        }

        KASSERT(reqcount - 1 <= maxahead,
            ("page count %d extends beyond swap block", reqcount));

        /*
         * Do not transfer any pages other than those that are xbusied
         * when running during a split or collapse operation.  This
         * prevents clustering from re-creating pages which are being
         * moved into another object.
         */
        if ((object->flags & (OBJ_SPLIT | OBJ_DEAD)) != 0) {
                maxahead = reqcount - 1;
                maxbehind = 0;
        }

        /*
         * Clip the readahead and readbehind ranges to exclude resident pages.
         */
        if (rahead != NULL) {
                *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_lookup(object, pindex);
        KASSERT(blk != SWAPBLK_NONE,
            ("no swap blocking containing %p(%jx)", object, (uintmax_t)pindex));

        VM_OBJECT_WUNLOCK(object);
        bp = uma_zalloc(swrbuf_zone, M_WAITOK);
        /* 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);
        /* This could be implemented more efficiently with aflags */
        while ((ma[0]->oflags & VPO_SWAPINPROG) != 0) {
                ma[0]->oflags |= VPO_SWAPSLEEP;
                VM_CNT_INC(v_intrans);
                if (VM_OBJECT_SLEEP(object, &object->handle, 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);
                }
        }
        VM_OBJECT_WUNLOCK(object);

        /*
         * 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.
         */
}

static int
swap_pager_getpages(vm_object_t object, vm_page_t *ma, int count,
    int *rbehind, int *rahead)
{

        VM_OBJECT_WLOCK(object);
        return (swap_pager_getpages_locked(object, ma, count, rbehind, rahead));
}

/*
 *      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);
        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);

        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 whose 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)
{
        struct buf *bp;
        daddr_t addr, blk, n_free, s_free;
        vm_page_t mreq;
        int i, j, n;
        bool async;

        KASSERT(count == 0 || ma[0]->object == object,
            ("%s: object mismatch %p/%p",
            __func__, object, ma[0]->object));

        /*
         * Step 1
         *
         * Turn object into OBJT_SWAP.  Force sync if not a 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);
        async = curproc == pageproc && (flags & VM_PAGER_PUT_SYNC) == 0;
        swp_pager_init_freerange(&s_free, &n_free);

        /*
         * 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) {
                /* Maximum I/O size is limited by maximum swap block size. */
                n = min(count - i, nsw_cluster_max);

                if (async) {
                        mtx_lock(&swbuf_mtx);
                        while (nsw_wcount_async == 0)
                                msleep(&nsw_wcount_async, &swbuf_mtx, PVM,
                                    "swbufa", 0);
                        nsw_wcount_async--;
                        mtx_unlock(&swbuf_mtx);
                }

                /* Get a block of swap of size up to size n. */
                VM_OBJECT_WLOCK(object);
                blk = swp_pager_getswapspace(&n);
                if (blk == SWAPBLK_NONE) {
                        VM_OBJECT_WUNLOCK(object);
                        mtx_lock(&swbuf_mtx);
                        if (++nsw_wcount_async == 1)
                                wakeup(&nsw_wcount_async);
                        mtx_unlock(&swbuf_mtx);
                        for (j = 0; j < n; ++j)
                                rtvals[i + j] = VM_PAGER_FAIL;
                        continue;
                }
                for (j = 0; j < n; ++j) {
                        mreq = ma[i + j];
                        vm_page_aflag_clear(mreq, PGA_SWAP_FREE);
                        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;
                }
                VM_OBJECT_WUNLOCK(object);

                bp = uma_zalloc(swwbuf_zone, M_WAITOK);
                if (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;
                for (j = 0; j < n; j++)
                        bp->b_pages[j] = ma[i + j];
                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 (async) {
                        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);
        }
        swp_pager_freeswapspace(s_free, n_free);
        VM_OBJECT_WLOCK(object);
}

/*
 *      swp_pager_async_iodone:
 *
 *      Completion routine for asynchronous reads and writes from/to swap.
 *      Also called manually by synchronous code to finish up a bp.
 *
 *      This routine may not sleep.
 */
static void
swp_pager_async_iodone(struct buf *bp)
{
        int i;
        vm_object_t object = NULL;

        /*
         * Report error - unless we ran out of memory, in which case
         * we've already logged it in swapgeom_strategy().
         */
        if (bp->b_ioflags & BIO_ERROR && bp->b_error != ENOMEM) {
                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->handle);
                }

                /* We always have space after I/O, successful or not. */
                vm_page_aflag_set(m, PGA_SWAP_SPACE);

                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.
                                 */
                                vm_page_invalid(m);
                        } 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);

                                /* PQ_UNSWAPPABLE? */
                                vm_page_activate(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));

                        vm_page_valid(m);
                        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_deactivate_noreuse(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
         */
        if (bp->b_flags & B_ASYNC) {
                mtx_lock(&swbuf_mtx);
                if (++nsw_wcount_async == 1)
                        wakeup(&nsw_wcount_async);
                mtx_unlock(&swbuf_mtx);
        }
        uma_zfree((bp->b_iocmd == BIO_READ) ? swrbuf_zone : swwbuf_zone, bp);
}

int
swap_pager_nswapdev(void)
{

        return (nswapdev);
}

static void
swp_pager_force_dirty(vm_page_t m)
{

        vm_page_dirty(m);
        swap_pager_unswapped(m);
        vm_page_launder(m);
}

/*
 *      swap_pager_swapoff_object:
 *
 *      Page in all of the pages that have been paged out for an object
 *      to a swap device.
 */
static void
swap_pager_swapoff_object(struct swdevt *sp, vm_object_t object)
{
        struct swblk *sb;
        vm_page_t m;
        vm_pindex_t pi;
        daddr_t blk;
        int i, nv, rahead, rv;

        KASSERT(object->type == OBJT_SWAP,
            ("%s: Object not swappable", __func__));

        for (pi = 0; (sb = SWAP_PCTRIE_LOOKUP_GE(
            &object->un_pager.swp.swp_blks, pi)) != NULL; ) {
                if ((object->flags & OBJ_DEAD) != 0) {
                        /*
                         * Make sure that pending writes finish before
                         * returning.
                         */
                        vm_object_pip_wait(object, "swpoff");
                        swp_pager_meta_free_all(object);
                        break;
                }
                for (i = 0; i < SWAP_META_PAGES; i++) {
                        /*
                         * Count the number of contiguous valid blocks.
                         */
                        for (nv = 0; nv < SWAP_META_PAGES - i; nv++) {
                                blk = sb->d[i + nv];
                                if (!swp_pager_isondev(blk, sp) ||
                                    blk == SWAPBLK_NONE)
                                        break;
                        }
                        if (nv == 0)
                                continue;

                        /*
                         * Look for a page corresponding to the first
                         * valid block and ensure that any pending paging
                         * operations on it are complete.  If the page is valid,
                         * mark it dirty and free the swap block.  Try to batch
                         * this operation since it may cause sp to be freed,
                         * meaning that we must restart the scan.  Avoid busying
                         * valid pages since we may block forever on kernel
                         * stack pages.
                         */
                        m = vm_page_lookup(object, sb->p + i);
                        if (m == NULL) {
                                m = vm_page_alloc(object, sb->p + i,
                                    VM_ALLOC_NORMAL | VM_ALLOC_WAITFAIL);
                                if (m == NULL)
                                        break;
                        } else {
                                if ((m->oflags & VPO_SWAPINPROG) != 0) {
                                        m->oflags |= VPO_SWAPSLEEP;
                                        VM_OBJECT_SLEEP(object, &object->handle,
                                            PSWP, "swpoff", 0);
                                        break;
                                }
                                if (vm_page_all_valid(m)) {
                                        do {
                                                swp_pager_force_dirty(m);
                                        } while (--nv > 0 &&
                                            (m = vm_page_next(m)) != NULL &&
                                            vm_page_all_valid(m) &&
                                            (m->oflags & VPO_SWAPINPROG) == 0);
                                        break;
                                }
                                if (!vm_page_busy_acquire(m, VM_ALLOC_WAITFAIL))
                                        break;
                        }

                        vm_object_pip_add(object, 1);
                        rahead = SWAP_META_PAGES;
                        rv = swap_pager_getpages_locked(object, &m, 1, NULL,
                            &rahead);
                        if (rv != VM_PAGER_OK)
                                panic("%s: read from swap failed: %d",
                                    __func__, rv);
                        vm_object_pip_wakeupn(object, 1);
                        VM_OBJECT_WLOCK(object);
                        vm_page_xunbusy(m);

                        /*
                         * The object lock was dropped so we must restart the
                         * scan of this swap block.  Pages paged in during this
                         * iteration will be marked dirty in a future iteration.
                         */
                        break;
                }
                if (i == SWAP_META_PAGES)
                        pi = sb->p + SWAP_META_PAGES;
        }
}

/*
 *      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)
{
        vm_object_t object;
        int 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;

                swap_pager_swapoff_object(sp, object);
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_FREE_EMPTY_SWBLK() - frees if a block is free
 *
 *  Nothing is done if the block is still in use.
 */
static void
swp_pager_free_empty_swblk(vm_object_t object, struct swblk *sb)
{

        if (swp_pager_swblk_empty(sb, 0, SWAP_META_PAGES)) {
                SWAP_PCTRIE_REMOVE(&object->un_pager.swp.swp_blks, sb->p);
                uma_zfree(swblk_zone, sb);
        }
}
   
/*
 * 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;
                object->un_pager.swp.writemappings = 0;
                KASSERT((object->flags & OBJ_ANON) != 0 ||
                    object->handle == NULL,
                    ("default pager %p with handle %p",
                    object, object->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_free_empty_swblk(object, sb);
        return (prev_swapblk);
}

/*
 * SWP_PAGER_META_TRANSFER() - free a range of blocks in the srcobject's swap
 * metadata, or transfer it into dstobject.
 *
 *      This routine will free swap metadata structures as they are cleaned
 *      out.
 */
static void
swp_pager_meta_transfer(vm_object_t srcobject, vm_object_t dstobject,
    vm_pindex_t pindex, vm_pindex_t count)
{
        struct swblk *sb;
        daddr_t n_free, s_free;
        vm_pindex_t offset, last;
        int i, limit, start;

        VM_OBJECT_ASSERT_WLOCKED(srcobject);
        if (srcobject->type != OBJT_SWAP || count == 0)
                return;

        swp_pager_init_freerange(&s_free, &n_free);
        offset = pindex;
        last = pindex + count;
        for (;;) {
                sb = SWAP_PCTRIE_LOOKUP_GE(&srcobject->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;
                        if (dstobject == NULL ||
                            !swp_pager_xfer_source(srcobject, dstobject, 
                            sb->p + i - offset, sb->d[i])) {
                                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(&srcobject->un_pager.swp.swp_blks,
                            sb->p);
                        uma_zfree(swblk_zone, sb);
                }
        }
        swp_pager_freeswapspace(s_free, n_free);
}

/*
 * 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)
{
        swp_pager_meta_transfer(object, NULL, pindex, count);
}

/*
 * 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.
 */
static daddr_t
swp_pager_meta_lookup(vm_object_t object, vm_pindex_t pindex)
{
        struct swblk *sb;

        VM_OBJECT_ASSERT_LOCKED(object);

        /*
         * The meta data only exists if the object is OBJT_SWAP
         * and even then might not be allocated yet.
         */
        KASSERT(object->type == OBJT_SWAP,
            ("Lookup object not swappable"));

        sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
            rounddown(pindex, SWAP_META_PAGES));
        if (sb == NULL)
                return (SWAPBLK_NONE);
        return (sb->d[pindex % SWAP_META_PAGES]);
}

/*
 * 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)
{

        /* recommend using no more than half that amount */
        if (swap_total > swap_maxpages / 2) {
                printf("warning: total configured swap (%lu pages) "
                    "exceeds maximum recommended amount (%lu pages).\n",
                    swap_total, swap_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;
        daddr_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_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 blocks in order to avoid overwriting
         * any bsd label at the front of the partition
         */
        blist_free(sp->sw_blist, howmany(BBSIZE, PAGE_SIZE),
            nblks - howmany(BBSIZE, PAGE_SIZE));

        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 - howmany(BBSIZE, PAGE_SIZE);
        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);
        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

#if defined(__amd64__) && defined(COMPAT_FREEBSD32)
struct xswdev32 {
        u_int   xsw_version;
        u_int   xsw_dev1, xsw_dev2;
        int     xsw_flags;
        int     xsw_nblks;
        int     xsw_used;
};
#endif

static int
sysctl_vm_swap_info(SYSCTL_HANDLER_ARGS)
{
        struct xswdev xs;
#if defined(__amd64__) && defined(COMPAT_FREEBSD32)
        struct xswdev32 xs32;
#endif
#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(__amd64__) && defined(COMPAT_FREEBSD32)
        if (req->oldlen == sizeof(xs32)) {
                xs32.xsw_version = XSWDEV_VERSION;
                xs32.xsw_dev1 = xs.xsw_dev;
                xs32.xsw_dev2 = xs.xsw_dev >> 32;
                xs32.xsw_flags = xs.xsw_flags;
                xs32.xsw_nblks = xs.xsw_nblks;
                xs32.xsw_used = xs.xsw_used;
                error = SYSCTL_OUT(req, &xs32, sizeof(xs32));
                return (error);
        }
#endif
#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));
                return (error);
        }
#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;

        VM_MAP_ENTRY_FOREACH(cur, map) {
                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;
        bp->b_caller1 = NULL;
        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;
                printf("swap_pager: cannot allocate bio\n");
                bufdone(bp);
                return;
        }

        bp->b_caller1 = bio;
        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 || VN_IS_DOOMED(vp)) {
                error = ENOENT;
        } else {
                g_topology_lock();
                error = swapongeom_locked(vp->v_rdev, vp);
                g_topology_unlock();
        }
        VOP_UNLOCK(vp);
        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);
        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(&swbuf_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, &swbuf_mtx, PSWP,
                            "swpsysctl", 0);
                }
        }
        mtx_unlock(&swbuf_mtx);

        return (0);
}

static void
swap_pager_update_writecount(vm_object_t object, vm_offset_t start,
    vm_offset_t end)
{

        VM_OBJECT_WLOCK(object);
        KASSERT((object->flags & OBJ_ANON) == 0,
            ("Splittable object with writecount"));
        object->un_pager.swp.writemappings += (vm_ooffset_t)end - start;
        VM_OBJECT_WUNLOCK(object);
}

static void
swap_pager_release_writecount(vm_object_t object, vm_offset_t start,
    vm_offset_t end)
{

        VM_OBJECT_WLOCK(object);
        KASSERT((object->flags & OBJ_ANON) == 0,
            ("Splittable object with writecount"));
        object->un_pager.swp.writemappings -= (vm_ooffset_t)end - start;
        VM_OBJECT_WUNLOCK(object);
}