kernel - Rewrite vnode ref-counting code to improve performance

[dragonfly.git] / sys / vm / swap_pager.c

/*
 * (MPSAFE)
 *
 * Copyright (c) 1998-2010 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project 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 COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS 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.
 * 
 * Copyright (c) 1994 John S. Dyson
 * Copyright (c) 1990 University of Utah.
 * Copyright (c) 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * the Systems Programming Group of the University of Utah Computer
 * Science Department.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *                              New Swap System
 *                              Matthew Dillon
 *
 * Radix Bitmap 'blists'.
 *
 *      - The new swapper uses the new radix bitmap code.  This should scale
 *        to arbitrarily small or arbitrarily large swap spaces and an almost
 *        arbitrary degree of fragmentation.
 *
 * Features:
 *
 *      - on the fly reallocation of swap during putpages.  The new system
 *        does not try to keep previously allocated swap blocks for dirty
 *        pages.  
 *
 *      - on the fly deallocation of swap
 *
 *      - No more garbage collection required.  Unnecessarily allocated swap
 *        blocks only exist for dirty vm_page_t's now and these are already
 *        cycled (in a high-load system) by the pager.  We also do on-the-fly
 *        removal of invalidated swap blocks when a page is destroyed
 *        or renamed.
 *
 * from: Utah $Hdr: swap_pager.c 1.4 91/04/30$
 * @(#)swap_pager.c     8.9 (Berkeley) 3/21/94
 * $FreeBSD: src/sys/vm/swap_pager.c,v 1.130.2.12 2002/08/31 21:15:55 dillon Exp $
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/conf.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/buf.h>
#include <sys/vnode.h>
#include <sys/malloc.h>
#include <sys/vmmeter.h>
#include <sys/sysctl.h>
#include <sys/blist.h>
#include <sys/lock.h>
#include <sys/thread2.h>

#include "opt_swap.h"
#include <vm/vm.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pager.h>
#include <vm/vm_pageout.h>
#include <vm/swap_pager.h>
#include <vm/vm_extern.h>
#include <vm/vm_zone.h>
#include <vm/vnode_pager.h>

#include <sys/buf2.h>
#include <vm/vm_page2.h>

#ifndef MAX_PAGEOUT_CLUSTER
#define MAX_PAGEOUT_CLUSTER     SWB_NPAGES
#endif

#define SWM_FREE        0x02    /* free, period                 */
#define SWM_POP         0x04    /* pop out                      */

#define SWBIO_READ      0x01
#define SWBIO_WRITE     0x02
#define SWBIO_SYNC      0x04

struct swfreeinfo {
        vm_object_t     object;
        vm_pindex_t     basei;
        vm_pindex_t     begi;
        vm_pindex_t     endi;   /* inclusive */
};

struct swswapoffinfo {
        vm_object_t     object;
        int             devidx;
        int             shared;
};

/*
 * vm_swap_size is in page-sized chunks now.  It was DEV_BSIZE'd chunks
 * in the old system.
 */

int swap_pager_full;            /* swap space exhaustion (task killing) */
int vm_swap_cache_use;
int vm_swap_anon_use;
static int vm_report_swap_allocs;

static int swap_pager_almost_full; /* swap space exhaustion (w/ hysteresis)*/
static int nsw_rcount;          /* free read buffers                    */
static int nsw_wcount_sync;     /* limit write buffers / synchronous    */
static int nsw_wcount_async;    /* limit write buffers / asynchronous   */
static int nsw_wcount_async_max;/* assigned maximum                     */
static int nsw_cluster_max;     /* maximum VOP I/O allowed              */

struct blist *swapblist;
static int swap_async_max = 4;  /* maximum in-progress async I/O's      */
static int swap_burst_read = 0; /* allow burst reading */
static swblk_t swapiterator;    /* linearize allocations */

/* from vm_swap.c */
extern struct vnode *swapdev_vp;
extern struct swdevt *swdevt;
extern int nswdev;

#define BLK2DEVIDX(blk) (nswdev > 1 ? blk / dmmax % nswdev : 0)

SYSCTL_INT(_vm, OID_AUTO, swap_async_max,
        CTLFLAG_RW, &swap_async_max, 0, "Maximum running async swap ops");
SYSCTL_INT(_vm, OID_AUTO, swap_burst_read,
        CTLFLAG_RW, &swap_burst_read, 0, "Allow burst reads for pageins");

SYSCTL_INT(_vm, OID_AUTO, swap_cache_use,
        CTLFLAG_RD, &vm_swap_cache_use, 0, "");
SYSCTL_INT(_vm, OID_AUTO, swap_anon_use,
        CTLFLAG_RD, &vm_swap_anon_use, 0, "");
SYSCTL_INT(_vm, OID_AUTO, swap_size,
        CTLFLAG_RD, &vm_swap_size, 0, "");
SYSCTL_INT(_vm, OID_AUTO, report_swap_allocs,
        CTLFLAG_RW, &vm_report_swap_allocs, 0, "");

vm_zone_t               swap_zone;

/*
 * Red-Black tree for swblock entries
 *
 * The caller must hold vm_token
 */
RB_GENERATE2(swblock_rb_tree, swblock, swb_entry, rb_swblock_compare,
             vm_pindex_t, swb_index);

int
rb_swblock_compare(struct swblock *swb1, struct swblock *swb2)
{
        if (swb1->swb_index < swb2->swb_index)
                return(-1);
        if (swb1->swb_index > swb2->swb_index)
                return(1);
        return(0);
}

static
int
rb_swblock_scancmp(struct swblock *swb, void *data)
{
        struct swfreeinfo *info = data;

        if (swb->swb_index < info->basei)
                return(-1);
        if (swb->swb_index > info->endi)
                return(1);
        return(0);
}

static
int
rb_swblock_condcmp(struct swblock *swb, void *data)
{
        struct swfreeinfo *info = data;

        if (swb->swb_index < info->basei)
                return(-1);
        return(0);
}

/*
 * 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 void     swap_pager_dealloc (vm_object_t object);
static int      swap_pager_getpage (vm_object_t, vm_page_t *, int);
static void     swap_chain_iodone(struct bio *biox);

struct pagerops swappagerops = {
        swap_pager_dealloc,     /* deallocate an OBJT_SWAP object       */
        swap_pager_getpage,     /* pagein                               */
        swap_pager_putpages,    /* pageout                              */
        swap_pager_haspage      /* get backing store status for page    */
};

/*
 * dmmax is in page-sized chunks with the new swap system.  It was
 * dev-bsized chunks in the old.  dmmax is always a power of 2.
 *
 * swap_*() routines are externally accessible.  swp_*() routines are
 * internal.
 */

int dmmax;
static int dmmax_mask;
int nswap_lowat = 128;          /* in pages, swap_pager_almost_full warn */
int nswap_hiwat = 512;          /* in pages, swap_pager_almost_full warn */

static __inline void    swp_sizecheck (void);
static void     swp_pager_async_iodone (struct bio *bio);

/*
 * Swap bitmap functions
 */

static __inline void    swp_pager_freeswapspace(vm_object_t object,
                                                swblk_t blk, int npages);
static __inline swblk_t swp_pager_getswapspace(vm_object_t object, int npages);

/*
 * Metadata functions
 */

static void swp_pager_meta_convert(vm_object_t);
static void swp_pager_meta_build(vm_object_t, vm_pindex_t, swblk_t);
static void swp_pager_meta_free(vm_object_t, vm_pindex_t, vm_pindex_t);
static void swp_pager_meta_free_all(vm_object_t);
static swblk_t swp_pager_meta_ctl(vm_object_t, vm_pindex_t, int);

/*
 * SWP_SIZECHECK() -    update swap_pager_full indication
 *      
 *      update the swap_pager_almost_full indication and warn when we are
 *      about to run out of swap space, using lowat/hiwat hysteresis.
 *
 *      Clear swap_pager_full ( task killing ) indication when lowat is met.
 *
 * No restrictions on call
 * This routine may not block.
 * SMP races are ok.
 */
static __inline void
swp_sizecheck(void)
{
        if (vm_swap_size < nswap_lowat) {
                if (swap_pager_almost_full == 0) {
                        kprintf("swap_pager: out of swap space\n");
                        swap_pager_almost_full = 1;
                }
        } else {
                swap_pager_full = 0;
                if (vm_swap_size > nswap_hiwat)
                        swap_pager_almost_full = 0;
        }
}

/*
 * SWAP_PAGER_INIT() -  initialize the swap pager!
 *
 *      Expected to be started from system init.  NOTE:  This code is run 
 *      before much else so be careful what you depend on.  Most of the VM
 *      system has yet to be initialized at this point.
 *
 * Called from the low level boot code only.
 */
static void
swap_pager_init(void *arg __unused)
{
        /*
         * Device Stripe, in PAGE_SIZE'd blocks
         */
        dmmax = SWB_NPAGES * 2;
        dmmax_mask = ~(dmmax - 1);
}
SYSINIT(vm_mem, SI_BOOT1_VM, SI_ORDER_THIRD, swap_pager_init, 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.
 *
 * Called from the low level boot code only.
 */
void
swap_pager_swap_init(void)
{
        int n, n2;

        /*
         * Number of in-transit swap bp operations.  Don't
         * exhaust the pbufs completely.  Make sure we
         * initialize workable values (0 will work for hysteresis
         * but it isn't very efficient).
         *
         * The nsw_cluster_max is constrained by the number of pages an XIO
         * holds, i.e., (MAXPHYS/PAGE_SIZE) and our locally defined
         * MAX_PAGEOUT_CLUSTER.   Also be aware that swap ops are
         * constrained by the swap device interleave stripe size.
         *
         * Currently we hardwire nsw_wcount_async to 4.  This limit is 
         * designed to prevent other I/O from having high latencies due to
         * our pageout I/O.  The value 4 works well for one or two active swap
         * devices but is probably a little low if you have more.  Even so,
         * a higher value would probably generate only a limited improvement
         * with three or four active swap devices since the system does not
         * typically have to pageout at extreme bandwidths.   We will want
         * at least 2 per swap devices, and 4 is a pretty good value if you
         * have one NFS swap device due to the command/ack latency over NFS.
         * So it all works out pretty well.
         */

        nsw_cluster_max = min((MAXPHYS/PAGE_SIZE), MAX_PAGEOUT_CLUSTER);

        nsw_rcount = (nswbuf + 1) / 2;
        nsw_wcount_sync = (nswbuf + 3) / 4;
        nsw_wcount_async = 4;
        nsw_wcount_async_max = nsw_wcount_async;

        /*
         * The zone is dynamically allocated so generally size it to
         * maxswzone (32MB to 512MB of KVM).  Set a minimum size based
         * on physical memory of around 8x (each swblock can hold 16 pages).
         *
         * With the advent of SSDs (vs HDs) the practical (swap:memory) ratio
         * has increased dramatically.
         */
        n = vmstats.v_page_count / 2;
        if (maxswzone && n < maxswzone / sizeof(struct swblock))
                n = maxswzone / sizeof(struct swblock);
        n2 = n;

        do {
                swap_zone = zinit(
                        "SWAPMETA", 
                        sizeof(struct swblock), 
                        n,
                        ZONE_INTERRUPT, 
                        1);
                if (swap_zone != NULL)
                        break;
                /*
                 * if the allocation failed, try a zone two thirds the
                 * size of the previous attempt.
                 */
                n -= ((n + 2) / 3);
        } while (n > 0);

        if (swap_zone == NULL)
                panic("swap_pager_swap_init: swap_zone == NULL");
        if (n2 != n)
                kprintf("Swap zone entries reduced from %d to %d.\n", n2, n);
}

/*
 * SWAP_PAGER_ALLOC() - allocate a new OBJT_SWAP VM object and instantiate
 *                      its metadata structures.
 *
 *      This routine is called from the mmap and fork code to create a new
 *      OBJT_SWAP object.  We do this by creating an OBJT_DEFAULT object
 *      and then converting it with swp_pager_meta_convert().
 *
 *      We only support unnamed objects.
 *
 * No restrictions.
 */
vm_object_t
swap_pager_alloc(void *handle, off_t size, vm_prot_t prot, off_t offset)
{
        vm_object_t object;

        KKASSERT(handle == NULL);
        object = vm_object_allocate_hold(OBJT_DEFAULT,
                                         OFF_TO_IDX(offset + PAGE_MASK + size));
        swp_pager_meta_convert(object);
        vm_object_drop(object);

        return (object);
}

/*
 * SWAP_PAGER_DEALLOC() -       remove swap metadata from object
 *
 *      The swap backing for the object is destroyed.  The code is 
 *      designed such that we can reinstantiate it later, but this
 *      routine is typically called only when the entire object is
 *      about to be destroyed.
 *
 * The object must be locked or unreferenceable.
 * No other requirements.
 */
static void
swap_pager_dealloc(vm_object_t object)
{
        vm_object_hold(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);
        vm_object_drop(object);
}

/************************************************************************
 *                      SWAP PAGER BITMAP ROUTINES                      *
 ************************************************************************/

/*
 * SWP_PAGER_GETSWAPSPACE() -   allocate raw swap space
 *
 *      Allocate swap for the requested number of pages.  The starting
 *      swap block number (a page index) is returned or SWAPBLK_NONE
 *      if the allocation failed.
 *
 *      Also has the side effect of advising that somebody made a mistake
 *      when they configured swap and didn't configure enough.
 *
 * The caller must hold the object.
 * This routine may not block.
 */
static __inline swblk_t
swp_pager_getswapspace(vm_object_t object, int npages)
{
        swblk_t blk;

        lwkt_gettoken(&vm_token);
        blk = blist_allocat(swapblist, npages, swapiterator);
        if (blk == SWAPBLK_NONE)
                blk = blist_allocat(swapblist, npages, 0);
        if (blk == SWAPBLK_NONE) {
                if (swap_pager_full != 2) {
                        kprintf("swap_pager_getswapspace: failed alloc=%d\n",
                                npages);
                        swap_pager_full = 2;
                        swap_pager_almost_full = 1;
                }
        } else {
                /* swapiterator = blk; disable for now, doesn't work well */
                swapacctspace(blk, -npages);
                if (object->type == OBJT_SWAP)
                        vm_swap_anon_use += npages;
                else
                        vm_swap_cache_use += npages;
                swp_sizecheck();
        }
        lwkt_reltoken(&vm_token);
        return(blk);
}

/*
 * SWP_PAGER_FREESWAPSPACE() -  free raw swap space 
 *
 *      This routine returns the specified swap blocks back to the bitmap.
 *
 *      Note:  This routine may not block (it could in the old swap code),
 *      and through the use of the new blist routines it does not block.
 *
 *      We must be called at splvm() to avoid races with bitmap frees from
 *      vm_page_remove() aka swap_pager_page_removed().
 *
 * This routine may not block.
 */

static __inline void
swp_pager_freeswapspace(vm_object_t object, swblk_t blk, int npages)
{
        struct swdevt *sp = &swdevt[BLK2DEVIDX(blk)];

        lwkt_gettoken(&vm_token);
        sp->sw_nused -= npages;
        if (object->type == OBJT_SWAP)
                vm_swap_anon_use -= npages;
        else
                vm_swap_cache_use -= npages;

        if (sp->sw_flags & SW_CLOSING) {
                lwkt_reltoken(&vm_token);
                return;
        }

        blist_free(swapblist, blk, npages);
        vm_swap_size += npages;
        swp_sizecheck();
        lwkt_reltoken(&vm_token);
}

/*
 * 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.
 *
 * No requirements.
 */
void
swap_pager_freespace(vm_object_t object, vm_pindex_t start, vm_pindex_t size)
{
        vm_object_hold(object);
        swp_pager_meta_free(object, start, size);
        vm_object_drop(object);
}

/*
 * No requirements.
 */
void
swap_pager_freespace_all(vm_object_t object)
{
        vm_object_hold(object);
        swp_pager_meta_free_all(object);
        vm_object_drop(object);
}

/*
 * This function conditionally frees swap cache swap starting at
 * (*basei) in the object.  (count) swap blocks will be nominally freed.
 * The actual number of blocks freed can be more or less than the
 * requested number.
 *
 * This function nominally returns the number of blocks freed.  However,
 * the actual number of blocks freed may be less then the returned value.
 * If the function is unable to exhaust the object or if it is able to
 * free (approximately) the requested number of blocks it returns
 * a value n > count.
 *
 * If we exhaust the object we will return a value n <= count.
 *
 * The caller must hold the object.
 *
 * WARNING!  If count == 0 then -1 can be returned as a degenerate case,
 *           callers should always pass a count value > 0.
 */
static int swap_pager_condfree_callback(struct swblock *swap, void *data);

int
swap_pager_condfree(vm_object_t object, vm_pindex_t *basei, int count)
{
        struct swfreeinfo info;
        int n;
        int t;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        info.object = object;
        info.basei = *basei;    /* skip up to this page index */
        info.begi = count;      /* max swap pages to destroy */
        info.endi = count * 8;  /* max swblocks to scan */

        swblock_rb_tree_RB_SCAN(&object->swblock_root, rb_swblock_condcmp,
                                swap_pager_condfree_callback, &info);
        *basei = info.basei;

        /*
         * Take the higher difference swblocks vs pages
         */
        n = count - (int)info.begi;
        t = count * 8 - (int)info.endi;
        if (n < t)
                n = t;
        if (n < 1)
                n = 1;
        return(n);
}

/*
 * The idea is to free whole meta-block to avoid fragmenting
 * the swap space or disk I/O.  We only do this if NO VM pages
 * are present.
 *
 * We do not have to deal with clearing PG_SWAPPED in related VM
 * pages because there are no related VM pages.
 *
 * The caller must hold the object.
 */
static int
swap_pager_condfree_callback(struct swblock *swap, void *data)
{
        struct swfreeinfo *info = data;
        vm_object_t object = info->object;
        int i;

        for (i = 0; i < SWAP_META_PAGES; ++i) {
                if (vm_page_lookup(object, swap->swb_index + i))
                        break;
        }
        info->basei = swap->swb_index + SWAP_META_PAGES;
        if (i == SWAP_META_PAGES) {
                info->begi -= swap->swb_count;
                swap_pager_freespace(object, swap->swb_index, SWAP_META_PAGES);
        }
        --info->endi;
        if ((int)info->begi < 0 || (int)info->endi < 0)
                return(-1);
        lwkt_yield();
        return(0);
}

/*
 * Called by vm_page_alloc() when a new VM page is inserted
 * into a VM object.  Checks whether swap has been assigned to
 * the page and sets PG_SWAPPED as necessary.
 *
 * No requirements.
 */
void
swap_pager_page_inserted(vm_page_t m)
{
        if (m->object->swblock_count) {
                vm_object_hold(m->object);
                if (swp_pager_meta_ctl(m->object, m->pindex, 0) != SWAPBLK_NONE)
                        vm_page_flag_set(m, PG_SWAPPED);
                vm_object_drop(m->object);
        }
}

/*
 * SWAP_PAGER_RESERVE() - reserve swap blocks in object
 *
 *      Assigns swap blocks to the specified range within the object.  The 
 *      swap blocks are not zerod.  Any previous swap assignment is destroyed.
 *
 *      Returns 0 on success, -1 on failure.
 *
 * The caller is responsible for avoiding races in the specified range.
 * No other requirements.
 */
int
swap_pager_reserve(vm_object_t object, vm_pindex_t start, vm_size_t size)
{
        int n = 0;
        swblk_t blk = SWAPBLK_NONE;
        vm_pindex_t beg = start;        /* save start index */

        vm_object_hold(object);

        while (size) {
                if (n == 0) {
                        n = BLIST_MAX_ALLOC;
                        while ((blk = swp_pager_getswapspace(object, n)) ==
                               SWAPBLK_NONE)
                        {
                                n >>= 1;
                                if (n == 0) {
                                        swp_pager_meta_free(object, beg,
                                                            start - beg);
                                        vm_object_drop(object);
                                        return(-1);
                                }
                        }
                }
                swp_pager_meta_build(object, start, blk);
                --size;
                ++start;
                ++blk;
                --n;
        }
        swp_pager_meta_free(object, start, n);
        vm_object_drop(object);
        return(0);
}

/*
 * SWAP_PAGER_COPY() -  copy blocks from source pager to destination pager
 *                      and destroy the source.
 *
 *      Copy any valid swapblks from the source to the destination.  In
 *      cases where both the source and destination have a valid swapblk,
 *      we keep the destination's.
 *
 *      This routine is allowed to block.  It may block allocating metadata
 *      indirectly through swp_pager_meta_build() or if paging is still in
 *      progress on the source. 
 *
 *      XXX vm_page_collapse() kinda expects us not to block because we 
 *      supposedly do not need to allocate memory, but for the moment we
 *      *may* have to get a little memory from the zone allocator, but
 *      it is taken from the interrupt memory.  We should be ok. 
 *
 *      The source object contains no vm_page_t's (which is just as well)
 *      The source object is of type OBJT_SWAP.
 *
 *      The source and destination objects must be held by the caller.
 */
void
swap_pager_copy(vm_object_t srcobject, vm_object_t dstobject,
                vm_pindex_t base_index, int destroysource)
{
        vm_pindex_t i;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(srcobject));
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(dstobject));

        /*
         * transfer source to destination.
         */
        for (i = 0; i < dstobject->size; ++i) {
                swblk_t dstaddr;

                /*
                 * Locate (without changing) the swapblk on the destination,
                 * unless it is invalid in which case free it silently, or
                 * if the destination is a resident page, in which case the
                 * source is thrown away.
                 */
                dstaddr = swp_pager_meta_ctl(dstobject, i, 0);

                if (dstaddr == SWAPBLK_NONE) {
                        /*
                         * Destination has no swapblk and is not resident,
                         * copy source.
                         */
                        swblk_t srcaddr;

                        srcaddr = swp_pager_meta_ctl(srcobject,
                                                     base_index + i, SWM_POP);

                        if (srcaddr != SWAPBLK_NONE)
                                swp_pager_meta_build(dstobject, i, srcaddr);
                } else {
                        /*
                         * Destination has valid swapblk or it is represented
                         * by a resident page.  We destroy the sourceblock.
                         */
                        swp_pager_meta_ctl(srcobject, base_index + i, SWM_FREE);
                }
        }

        /*
         * Free left over swap blocks in source.
         *
         * We have to revert the type to OBJT_DEFAULT so we do not accidently
         * double-remove the object from the swap queues.
         */
        if (destroysource) {
                /*
                 * 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.
                 */
                swp_pager_meta_free_all(srcobject);
                if (srcobject->type == OBJT_SWAP)
                        srcobject->type = OBJT_DEFAULT;
        }
}

/*
 * SWAP_PAGER_HASPAGE() -       determine if we have good backing store for
 *                              the requested page.
 *
 *      We determine whether good backing store exists for the requested
 *      page and return TRUE if it does, FALSE if it doesn't.
 *
 *      If TRUE, we also try to determine how much valid, contiguous backing
 *      store exists before and after the requested page within a reasonable
 *      distance.  We do not try to restrict it to the swap device stripe
 *      (that is handled in getpages/putpages).  It probably isn't worth
 *      doing here.
 *
 * No requirements.
 */
boolean_t
swap_pager_haspage(vm_object_t object, vm_pindex_t pindex)
{
        swblk_t blk0;

        /*
         * do we have good backing store at the requested index ?
         */
        vm_object_hold(object);
        blk0 = swp_pager_meta_ctl(object, pindex, 0);

        if (blk0 == SWAPBLK_NONE) {
                vm_object_drop(object);
                return (FALSE);
        }
        vm_object_drop(object);
        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 operates on any VM object, not just OBJT_SWAP
 * objects.
 *
 * 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.
 *
 * The page must be busied or soft-busied.
 * The caller can hold the object to avoid blocking, else we might block.
 * No other requirements.
 */
void
swap_pager_unswapped(vm_page_t m)
{
        if (m->flags & PG_SWAPPED) {
                vm_object_hold(m->object);
                KKASSERT(m->flags & PG_SWAPPED);
                swp_pager_meta_ctl(m->object, m->pindex, SWM_FREE);
                vm_page_flag_clear(m, PG_SWAPPED);
                vm_object_drop(m->object);
        }
}

/*
 * SWAP_PAGER_STRATEGY() - read, write, free blocks
 *
 * This implements a VM OBJECT strategy function using swap backing store.
 * This can operate on any VM OBJECT type, not necessarily just OBJT_SWAP
 * types.
 *
 * This is intended to be a cacheless interface (i.e. caching occurs at
 * higher levels), and is also used as a swap-based SSD cache for vnode
 * and device objects.
 *
 * All I/O goes directly to and from the swap device.
 *      
 * We currently attempt to run I/O synchronously or asynchronously as
 * the caller requests.  This isn't perfect because we loose error
 * sequencing when we run multiple ops in parallel to satisfy a request.
 * But this is swap, so we let it all hang out.
 *
 * No requirements.
 */
void
swap_pager_strategy(vm_object_t object, struct bio *bio)
{
        struct buf *bp = bio->bio_buf;
        struct bio *nbio;
        vm_pindex_t start;
        vm_pindex_t biox_blkno = 0;
        int count;
        char *data;
        struct bio *biox;
        struct buf *bufx;
#if 0
        struct bio_track *track;
#endif

#if 0
        /*
         * tracking for swapdev vnode I/Os
         */
        if (bp->b_cmd == BUF_CMD_READ)
                track = &swapdev_vp->v_track_read;
        else
                track = &swapdev_vp->v_track_write;
#endif

        if (bp->b_bcount & PAGE_MASK) {
                bp->b_error = EINVAL;
                bp->b_flags |= B_ERROR | B_INVAL;
                biodone(bio);
                kprintf("swap_pager_strategy: bp %p offset %lld size %d, "
                        "not page bounded\n",
                        bp, (long long)bio->bio_offset, (int)bp->b_bcount);
                return;
        }

        /*
         * Clear error indication, initialize page index, count, data pointer.
         */
        bp->b_error = 0;
        bp->b_flags &= ~B_ERROR;
        bp->b_resid = bp->b_bcount;

        start = (vm_pindex_t)(bio->bio_offset >> PAGE_SHIFT);
        count = howmany(bp->b_bcount, PAGE_SIZE);
        data = bp->b_data;

        /*
         * Deal with BUF_CMD_FREEBLKS
         */
        if (bp->b_cmd == BUF_CMD_FREEBLKS) {
                /*
                 * FREE PAGE(s) - destroy underlying swap that is no longer
                 *                needed.
                 */
                vm_object_hold(object);
                swp_pager_meta_free(object, start, count);
                vm_object_drop(object);
                bp->b_resid = 0;
                biodone(bio);
                return;
        }

        /*
         * We need to be able to create a new cluster of I/O's.  We cannot
         * use the caller fields of the passed bio so push a new one.
         *
         * Because nbio is just a placeholder for the cluster links,
         * we can biodone() the original bio instead of nbio to make
         * things a bit more efficient.
         */
        nbio = push_bio(bio);
        nbio->bio_offset = bio->bio_offset;
        nbio->bio_caller_info1.cluster_head = NULL;
        nbio->bio_caller_info2.cluster_tail = NULL;

        biox = NULL;
        bufx = NULL;

        /*
         * Execute read or write
         */
        vm_object_hold(object);

        while (count > 0) {
                swblk_t blk;

                /*
                 * Obtain block.  If block not found and writing, allocate a
                 * new block and build it into the object.
                 */
                blk = swp_pager_meta_ctl(object, start, 0);
                if ((blk == SWAPBLK_NONE) && bp->b_cmd != BUF_CMD_READ) {
                        blk = swp_pager_getswapspace(object, 1);
                        if (blk == SWAPBLK_NONE) {
                                bp->b_error = ENOMEM;
                                bp->b_flags |= B_ERROR;
                                break;
                        }
                        swp_pager_meta_build(object, start, blk);
                }
                        
                /*
                 * Do we have to flush our current collection?  Yes if:
                 *
                 *      - no swap block at this index
                 *      - swap block is not contiguous
                 *      - we cross a physical disk boundry in the
                 *        stripe.
                 */
                if (
                    biox && (biox_blkno + btoc(bufx->b_bcount) != blk ||
                     ((biox_blkno ^ blk) & dmmax_mask)
                    )
                ) {
                        if (bp->b_cmd == BUF_CMD_READ) {
                                ++mycpu->gd_cnt.v_swapin;
                                mycpu->gd_cnt.v_swappgsin += btoc(bufx->b_bcount);
                        } else {
                                ++mycpu->gd_cnt.v_swapout;
                                mycpu->gd_cnt.v_swappgsout += btoc(bufx->b_bcount);
                                bufx->b_dirtyend = bufx->b_bcount;
                        }

                        /*
                         * Finished with this buf.
                         */
                        KKASSERT(bufx->b_bcount != 0);
                        if (bufx->b_cmd != BUF_CMD_READ)
                                bufx->b_dirtyend = bufx->b_bcount;
                        biox = NULL;
                        bufx = NULL;
                }

                /*
                 * Add new swapblk to biox, instantiating biox if necessary.
                 * Zero-fill reads are able to take a shortcut.
                 */
                if (blk == SWAPBLK_NONE) {
                        /*
                         * We can only get here if we are reading.  Since
                         * we are at splvm() we can safely modify b_resid,
                         * even if chain ops are in progress.
                         */
                        bzero(data, PAGE_SIZE);
                        bp->b_resid -= PAGE_SIZE;
                } else {
                        if (biox == NULL) {
                                /* XXX chain count > 4, wait to <= 4 */

                                bufx = getpbuf(NULL);
                                biox = &bufx->b_bio1;
                                cluster_append(nbio, bufx);
                                bufx->b_flags |= (bp->b_flags & B_ORDERED);
                                bufx->b_cmd = bp->b_cmd;
                                biox->bio_done = swap_chain_iodone;
                                biox->bio_offset = (off_t)blk << PAGE_SHIFT;
                                biox->bio_caller_info1.cluster_parent = nbio;
                                biox_blkno = blk;
                                bufx->b_bcount = 0;
                                bufx->b_data = data;
                        }
                        bufx->b_bcount += PAGE_SIZE;
                }
                --count;
                ++start;
                data += PAGE_SIZE;
        }

        vm_object_drop(object);

        /*
         *  Flush out last buffer
         */
        if (biox) {
                if (bufx->b_cmd == BUF_CMD_READ) {
                        ++mycpu->gd_cnt.v_swapin;
                        mycpu->gd_cnt.v_swappgsin += btoc(bufx->b_bcount);
                } else {
                        ++mycpu->gd_cnt.v_swapout;
                        mycpu->gd_cnt.v_swappgsout += btoc(bufx->b_bcount);
                        bufx->b_dirtyend = bufx->b_bcount;
                }
                KKASSERT(bufx->b_bcount);
                if (bufx->b_cmd != BUF_CMD_READ)
                        bufx->b_dirtyend = bufx->b_bcount;
                /* biox, bufx = NULL */
        }

        /*
         * Now initiate all the I/O.  Be careful looping on our chain as
         * I/O's may complete while we are still initiating them.
         *
         * If the request is a 100% sparse read no bios will be present
         * and we just biodone() the buffer.
         */
        nbio->bio_caller_info2.cluster_tail = NULL;
        bufx = nbio->bio_caller_info1.cluster_head;

        if (bufx) {
                while (bufx) {
                        biox = &bufx->b_bio1;
                        BUF_KERNPROC(bufx);
                        bufx = bufx->b_cluster_next;
                        vn_strategy(swapdev_vp, biox);
                }
        } else {
                biodone(bio);
        }

        /*
         * Completion of the cluster will also call biodone_chain(nbio).
         * We never call biodone(nbio) so we don't have to worry about
         * setting up a bio_done callback.  It's handled in the sub-IO.
         */
        /**/
}

/*
 * biodone callback
 *
 * No requirements.
 */
static void
swap_chain_iodone(struct bio *biox)
{
        struct buf **nextp;
        struct buf *bufx;       /* chained sub-buffer */
        struct bio *nbio;       /* parent nbio with chain glue */
        struct buf *bp;         /* original bp associated with nbio */
        int chain_empty;

        bufx = biox->bio_buf;
        nbio = biox->bio_caller_info1.cluster_parent;
        bp = nbio->bio_buf;

        /*
         * Update the original buffer
         */
        KKASSERT(bp != NULL);
        if (bufx->b_flags & B_ERROR) {
                atomic_set_int(&bufx->b_flags, B_ERROR);
                bp->b_error = bufx->b_error;    /* race ok */
        } else if (bufx->b_resid != 0) {
                atomic_set_int(&bufx->b_flags, B_ERROR);
                bp->b_error = EINVAL;           /* race ok */
        } else {
                atomic_subtract_int(&bp->b_resid, bufx->b_bcount);
        }

        /*
         * Remove us from the chain.
         */
        spin_lock(&bp->b_lock.lk_spinlock);
        nextp = &nbio->bio_caller_info1.cluster_head;
        while (*nextp != bufx) {
                KKASSERT(*nextp != NULL);
                nextp = &(*nextp)->b_cluster_next;
        }
        *nextp = bufx->b_cluster_next;
        chain_empty = (nbio->bio_caller_info1.cluster_head == NULL);
        spin_unlock(&bp->b_lock.lk_spinlock);

        /*
         * Clean up bufx.  If the chain is now empty we finish out
         * the parent.  Note that we may be racing other completions
         * so we must use the chain_empty status from above.
         */
        if (chain_empty) {
                if (bp->b_resid != 0 && !(bp->b_flags & B_ERROR)) {
                        atomic_set_int(&bp->b_flags, B_ERROR);
                        bp->b_error = EINVAL;
                }
                biodone_chain(nbio);
        }
        relpbuf(bufx, NULL);
}

/*
 * SWAP_PAGER_GETPAGES() - bring page in from swap
 *
 * The requested page may have to be brought in from swap.  Calculate the
 * swap block and bring in additional pages if possible.  All pages must
 * have contiguous swap block assignments and reside in the same object.
 *
 * The caller has a single vm_object_pip_add() reference prior to
 * calling us and we should return with the same.
 *
 * The caller has BUSY'd the page.  We should return with (*mpp) left busy,
 * and any additinal pages unbusied.
 *
 * If the caller encounters a PG_RAM page it will pass it to us even though
 * it may be valid and dirty.  We cannot overwrite the page in this case!
 * The case is used to allow us to issue pure read-aheads.
 *
 * NOTE! XXX This code does not entirely pipeline yet due to the fact that
 *       the PG_RAM page is validated at the same time as mreq.  What we
 *       really need to do is issue a separate read-ahead pbuf.
 *
 * No requirements.
 */
static int
swap_pager_getpage(vm_object_t object, vm_page_t *mpp, int seqaccess)
{
        struct buf *bp;
        struct bio *bio;
        vm_page_t mreq;
        vm_page_t m;
        vm_offset_t kva;
        swblk_t blk;
        int i;
        int j;
        int raonly;
        int error;
        u_int32_t flags;
        vm_page_t marray[XIO_INTERNAL_PAGES];

        mreq = *mpp;

        vm_object_hold(object);
        if (mreq->object != object) {
                panic("swap_pager_getpages: object mismatch %p/%p", 
                    object, 
                    mreq->object
                );
        }

        /*
         * We don't want to overwrite a fully valid page as it might be
         * dirty.  This case can occur when e.g. vm_fault hits a perfectly
         * valid page with PG_RAM set.
         *
         * In this case we see if the next page is a suitable page-in
         * candidate and if it is we issue read-ahead.  PG_RAM will be
         * set on the last page of the read-ahead to continue the pipeline.
         */
        if (mreq->valid == VM_PAGE_BITS_ALL) {
                if (swap_burst_read == 0 || mreq->pindex + 1 >= object->size) {
                        vm_object_drop(object);
                        return(VM_PAGER_OK);
                }
                blk = swp_pager_meta_ctl(object, mreq->pindex + 1, 0);
                if (blk == SWAPBLK_NONE) {
                        vm_object_drop(object);
                        return(VM_PAGER_OK);
                }
                m = vm_page_lookup_busy_try(object, mreq->pindex + 1,
                                            TRUE, &error);
                if (error) {
                        vm_object_drop(object);
                        return(VM_PAGER_OK);
                } else if (m == NULL) {
                        /*
                         * Use VM_ALLOC_QUICK to avoid blocking on cache
                         * page reuse.
                         */
                        m = vm_page_alloc(object, mreq->pindex + 1,
                                          VM_ALLOC_QUICK);
                        if (m == NULL) {
                                vm_object_drop(object);
                                return(VM_PAGER_OK);
                        }
                } else {
                        if (m->valid) {
                                vm_page_wakeup(m);
                                vm_object_drop(object);
                                return(VM_PAGER_OK);
                        }
                        vm_page_unqueue_nowakeup(m);
                }
                /* page is busy */
                mreq = m;
                raonly = 1;
        } else {
                raonly = 0;
        }

        /*
         * Try to block-read contiguous pages from swap if sequential,
         * otherwise just read one page.  Contiguous pages from swap must
         * reside within a single device stripe because the I/O cannot be
         * broken up across multiple stripes.
         *
         * Note that blk and iblk can be SWAPBLK_NONE but the loop is
         * set up such that the case(s) are handled implicitly.
         */
        blk = swp_pager_meta_ctl(mreq->object, mreq->pindex, 0);
        marray[0] = mreq;

        for (i = 1; swap_burst_read &&
                    i < XIO_INTERNAL_PAGES &&
                    mreq->pindex + i < object->size; ++i) {
                swblk_t iblk;

                iblk = swp_pager_meta_ctl(object, mreq->pindex + i, 0);
                if (iblk != blk + i)
                        break;
                if ((blk ^ iblk) & dmmax_mask)
                        break;
                m = vm_page_lookup_busy_try(object, mreq->pindex + i,
                                            TRUE, &error);
                if (error) {
                        break;
                } else if (m == NULL) {
                        /*
                         * Use VM_ALLOC_QUICK to avoid blocking on cache
                         * page reuse.
                         */
                        m = vm_page_alloc(object, mreq->pindex + i,
                                          VM_ALLOC_QUICK);
                        if (m == NULL)
                                break;
                } else {
                        if (m->valid) {
                                vm_page_wakeup(m);
                                break;
                        }
                        vm_page_unqueue_nowakeup(m);
                }
                /* page is busy */
                marray[i] = m;
        }
        if (i > 1)
                vm_page_flag_set(marray[i - 1], PG_RAM);

        /*
         * If mreq is the requested page and we have nothing to do return
         * VM_PAGER_FAIL.  If raonly is set mreq is just another read-ahead
         * page and must be cleaned up.
         */
        if (blk == SWAPBLK_NONE) {
                KKASSERT(i == 1);
                if (raonly) {
                        vnode_pager_freepage(mreq);
                        vm_object_drop(object);
                        return(VM_PAGER_OK);
                } else {
                        vm_object_drop(object);
                        return(VM_PAGER_FAIL);
                }
        }

        /*
         * map our page(s) into kva for input
         */
        bp = getpbuf_kva(&nsw_rcount);
        bio = &bp->b_bio1;
        kva = (vm_offset_t) bp->b_kvabase;
        bcopy(marray, bp->b_xio.xio_pages, i * sizeof(vm_page_t));
        pmap_qenter(kva, bp->b_xio.xio_pages, i);

        bp->b_data = (caddr_t)kva;
        bp->b_bcount = PAGE_SIZE * i;
        bp->b_xio.xio_npages = i;
        bio->bio_done = swp_pager_async_iodone;
        bio->bio_offset = (off_t)blk << PAGE_SHIFT;
        bio->bio_caller_info1.index = SWBIO_READ;

        /*
         * Set index.  If raonly set the index beyond the array so all
         * the pages are treated the same, otherwise the original mreq is
         * at index 0.
         */
        if (raonly)
                bio->bio_driver_info = (void *)(intptr_t)i;
        else
                bio->bio_driver_info = (void *)(intptr_t)0;

        for (j = 0; j < i; ++j)
                vm_page_flag_set(bp->b_xio.xio_pages[j], PG_SWAPINPROG);

        mycpu->gd_cnt.v_swapin++;
        mycpu->gd_cnt.v_swappgsin += bp->b_xio.xio_npages;

        /*
         * We still hold the lock on mreq, and our automatic completion routine
         * does not remove it.
         */
        vm_object_pip_add(object, bp->b_xio.xio_npages);

        /*
         * perform the I/O.  NOTE!!!  bp cannot be considered valid after
         * this point because we automatically release it on completion.
         * Instead, we look at the one page we are interested in which we
         * still hold a lock on even through the I/O completion.
         *
         * The other pages in our m[] array are also released on completion,
         * so we cannot assume they are valid anymore either.
         */
        bp->b_cmd = BUF_CMD_READ;
        BUF_KERNPROC(bp);
        vn_strategy(swapdev_vp, bio);

        /*
         * Wait for the page we want to complete.  PG_SWAPINPROG is always
         * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
         * is set in the meta-data.
         *
         * If this is a read-ahead only we return immediately without
         * waiting for I/O.
         */
        if (raonly) {
                vm_object_drop(object);
                return(VM_PAGER_OK);
        }

        /*
         * Read-ahead includes originally requested page case.
         */
        for (;;) {
                flags = mreq->flags;
                cpu_ccfence();
                if ((flags & PG_SWAPINPROG) == 0)
                        break;
                tsleep_interlock(mreq, 0);
                if (!atomic_cmpset_int(&mreq->flags, flags,
                                       flags | PG_WANTED | PG_REFERENCED)) {
                        continue;
                }
                mycpu->gd_cnt.v_intrans++;
                if (tsleep(mreq, PINTERLOCKED, "swread", hz*20)) {
                        kprintf(
                            "swap_pager: indefinite wait buffer: "
                                " offset: %lld, size: %ld\n",
                            (long long)bio->bio_offset,
                            (long)bp->b_bcount
                        );
                }
        }

        /*
         * mreq is left bussied after completion, but all the other pages
         * are freed.  If we had an unrecoverable read error the page will
         * not be valid.
         */
        vm_object_drop(object);
        if (mreq->valid != VM_PAGE_BITS_ALL)
                return(VM_PAGER_ERROR);
        else
                return(VM_PAGER_OK);

        /*
         * A final note: in a low swap situation, we cannot deallocate swap
         * and mark a page dirty here because the caller is likely to mark
         * the page clean when we return, causing the page to possibly revert 
         * to all-zero's later.
         */
}

/*
 *      swap_pager_putpages: 
 *
 *      Assign swap (if necessary) and initiate I/O on the specified pages.
 *
 *      We support both OBJT_DEFAULT and OBJT_SWAP objects.  DEFAULT objects
 *      are automatically converted to SWAP objects.
 *
 *      In a low memory situation we may block in vn_strategy(), but the new 
 *      vm_page reservation system coupled with properly written VFS devices 
 *      should ensure that no low-memory deadlock occurs.  This is an area
 *      which needs work.
 *
 *      The parent has N vm_object_pip_add() references prior to
 *      calling us and will remove references for rtvals[] that are
 *      not set to VM_PAGER_PEND.  We need to remove the rest on I/O
 *      completion.
 *
 *      The parent has soft-busy'd the pages it passes us and will unbusy
 *      those whos rtvals[] entry is not set to VM_PAGER_PEND on return.
 *      We need to unbusy the rest on I/O completion.
 *
 * No requirements.
 */
void
swap_pager_putpages(vm_object_t object, vm_page_t *m, int count,
                    boolean_t sync, int *rtvals)
{
        int i;
        int n = 0;

        vm_object_hold(object);

        if (count && m[0]->object != object) {
                panic("swap_pager_getpages: object mismatch %p/%p", 
                    object, 
                    m[0]->object
                );
        }

        /*
         * Step 1
         *
         * Turn object into OBJT_SWAP
         * check for bogus sysops
         * force sync if not pageout process
         */
        if (object->type == OBJT_DEFAULT) {
                if (object->type == OBJT_DEFAULT)
                        swp_pager_meta_convert(object);
        }

        if (curthread != pagethread)
                sync = TRUE;

        /*
         * Step 2
         *
         * Update nsw parameters from swap_async_max sysctl values.  
         * Do not let the sysop crash the machine with bogus numbers.
         */
        if (swap_async_max != nsw_wcount_async_max) {
                int n;

                /*
                 * limit range
                 */
                if ((n = swap_async_max) > nswbuf / 2)
                        n = nswbuf / 2;
                if (n < 1)
                        n = 1;
                swap_async_max = n;

                /*
                 * Adjust difference ( if possible ).  If the current async
                 * count is too low, we may not be able to make the adjustment
                 * at this time.
                 *
                 * vm_token needed for nsw_wcount sleep interlock
                 */
                lwkt_gettoken(&vm_token);
                n -= nsw_wcount_async_max;
                if (nsw_wcount_async + n >= 0) {
                        nsw_wcount_async_max += n;
                        pbuf_adjcount(&nsw_wcount_async, n);
                }
                lwkt_reltoken(&vm_token);
        }

        /*
         * Step 3
         *
         * Assign swap blocks and issue I/O.  We reallocate swap on the fly.
         * The page is left dirty until the pageout operation completes
         * successfully.
         */

        for (i = 0; i < count; i += n) {
                struct buf *bp;
                struct bio *bio;
                swblk_t blk;
                int j;

                /*
                 * Maximum I/O size is limited by a number of factors.
                 */

                n = min(BLIST_MAX_ALLOC, count - i);
                n = min(n, nsw_cluster_max);

                lwkt_gettoken(&vm_token);

                /*
                 * Get biggest block of swap we can.  If we fail, fall
                 * back and try to allocate a smaller block.  Don't go
                 * overboard trying to allocate space if it would overly
                 * fragment swap.
                 */
                while (
                    (blk = swp_pager_getswapspace(object, n)) == SWAPBLK_NONE &&
                    n > 4
                ) {
                        n >>= 1;
                }
                if (blk == SWAPBLK_NONE) {
                        for (j = 0; j < n; ++j)
                                rtvals[i+j] = VM_PAGER_FAIL;
                        lwkt_reltoken(&vm_token);
                        continue;
                }
                if (vm_report_swap_allocs > 0) {
                        kprintf("swap_alloc %08jx,%d\n", (intmax_t)blk, n);
                        --vm_report_swap_allocs;
                }

                /*
                 * The I/O we are constructing cannot cross a physical
                 * disk boundry in the swap stripe.  Note: we are still
                 * at splvm().
                 */
                if ((blk ^ (blk + n)) & dmmax_mask) {
                        j = ((blk + dmmax) & dmmax_mask) - blk;
                        swp_pager_freeswapspace(object, blk + j, n - j);
                        n = j;
                }

                /*
                 * All I/O parameters have been satisfied, build the I/O
                 * request and assign the swap space.
                 */
                if (sync == TRUE)
                        bp = getpbuf_kva(&nsw_wcount_sync);
                else
                        bp = getpbuf_kva(&nsw_wcount_async);
                bio = &bp->b_bio1;

                lwkt_reltoken(&vm_token);

                pmap_qenter((vm_offset_t)bp->b_data, &m[i], n);

                bp->b_bcount = PAGE_SIZE * n;
                bio->bio_offset = (off_t)blk << PAGE_SHIFT;

                for (j = 0; j < n; ++j) {
                        vm_page_t mreq = m[i+j];

                        swp_pager_meta_build(mreq->object, mreq->pindex,
                                             blk + j);
                        if (object->type == OBJT_SWAP)
                                vm_page_dirty(mreq);
                        rtvals[i+j] = VM_PAGER_OK;

                        vm_page_flag_set(mreq, PG_SWAPINPROG);
                        bp->b_xio.xio_pages[j] = mreq;
                }
                bp->b_xio.xio_npages = n;

                mycpu->gd_cnt.v_swapout++;
                mycpu->gd_cnt.v_swappgsout += bp->b_xio.xio_npages;

                bp->b_dirtyoff = 0;             /* req'd for NFS */
                bp->b_dirtyend = bp->b_bcount;  /* req'd for NFS */
                bp->b_cmd = BUF_CMD_WRITE;
                bio->bio_caller_info1.index = SWBIO_WRITE;

                /*
                 * asynchronous
                 */
                if (sync == FALSE) {
                        bio->bio_done = swp_pager_async_iodone;
                        BUF_KERNPROC(bp);
                        vn_strategy(swapdev_vp, bio);

                        for (j = 0; j < n; ++j)
                                rtvals[i+j] = VM_PAGER_PEND;
                        continue;
                }

                /*
                 * Issue synchrnously.
                 *
                 * Wait for the sync I/O to complete, then update rtvals.
                 * We just set the rtvals[] to VM_PAGER_PEND so we can call
                 * our async completion routine at the end, thus avoiding a
                 * double-free.
                 */
                bio->bio_caller_info1.index |= SWBIO_SYNC;
                bio->bio_done = biodone_sync;
                bio->bio_flags |= BIO_SYNC;
                vn_strategy(swapdev_vp, bio);
                biowait(bio, "swwrt");

                for (j = 0; j < n; ++j)
                        rtvals[i+j] = VM_PAGER_PEND;

                /*
                 * Now that we are through with the bp, we can call the
                 * normal async completion, which frees everything up.
                 */
                swp_pager_async_iodone(bio);
        }
        vm_object_drop(object);
}

/*
 * No requirements.
 */
void
swap_pager_newswap(void)
{
        swp_sizecheck();
}

/*
 *      swp_pager_async_iodone:
 *
 *      Completion routine for asynchronous reads and writes from/to swap.
 *      Also called manually by synchronous code to finish up a bp.
 *
 *      For READ operations, the pages are PG_BUSY'd.  For WRITE operations, 
 *      the pages are vm_page_t->busy'd.  For READ operations, we PG_BUSY 
 *      unbusy all pages except the 'main' request page.  For WRITE 
 *      operations, we vm_page_t->busy'd unbusy all pages ( we can do this 
 *      because we marked them all VM_PAGER_PEND on return from putpages ).
 *
 *      This routine may not block.
 *
 * No requirements.
 */
static void
swp_pager_async_iodone(struct bio *bio)
{
        struct buf *bp = bio->bio_buf;
        vm_object_t object = NULL;
        int i;
        int *nswptr;

        /*
         * report error
         */
        if (bp->b_flags & B_ERROR) {
                kprintf(
                    "swap_pager: I/O error - %s failed; offset %lld,"
                        "size %ld, error %d\n",
                    ((bio->bio_caller_info1.index & SWBIO_READ) ?
                        "pagein" : "pageout"),
                    (long long)bio->bio_offset,
                    (long)bp->b_bcount,
                    bp->b_error
                );
        }

        /*
         * set object, raise to splvm().
         */
        if (bp->b_xio.xio_npages)
                object = bp->b_xio.xio_pages[0]->object;

        /*
         * remove the mapping for kernel virtual
         */
        pmap_qremove((vm_offset_t)bp->b_data, bp->b_xio.xio_npages);

        /*
         * cleanup pages.  If an error occurs writing to swap, we are in
         * very serious trouble.  If it happens to be a disk error, though,
         * we may be able to recover by reassigning the swap later on.  So
         * in this case we remove the m->swapblk assignment for the page 
         * but do not free it in the rlist.  The errornous block(s) are thus
         * never reallocated as swap.  Redirty the page and continue.
         */
        for (i = 0; i < bp->b_xio.xio_npages; ++i) {
                vm_page_t m = bp->b_xio.xio_pages[i];

                if (bp->b_flags & B_ERROR) {
                        /*
                         * If an error occurs I'd love to throw the swapblk
                         * away without freeing it back to swapspace, so it
                         * can never be used again.  But I can't from an 
                         * interrupt.
                         */

                        if (bio->bio_caller_info1.index & SWBIO_READ) {
                                /*
                                 * When reading, reqpage needs to stay
                                 * locked for the parent, but all other
                                 * pages can be freed.  We still want to
                                 * wakeup the parent waiting on the page,
                                 * though.  ( also: pg_reqpage can be -1 and 
                                 * not match anything ).
                                 *
                                 * We have to wake specifically requested pages
                                 * up too because we cleared PG_SWAPINPROG and
                                 * someone may be waiting for that.
                                 *
                                 * NOTE: for reads, m->dirty will probably
                                 * be overridden by the original caller of
                                 * getpages so don't play cute tricks here.
                                 *
                                 * NOTE: We can't actually free the page from
                                 * here, because this is an interrupt.  It
                                 * is not legal to mess with object->memq
                                 * from an interrupt.  Deactivate the page
                                 * instead.
                                 */

                                m->valid = 0;
                                vm_page_flag_clear(m, PG_ZERO);
                                vm_page_flag_clear(m, PG_SWAPINPROG);

                                /*
                                 * bio_driver_info holds the requested page
                                 * index.
                                 */
                                if (i != (int)(intptr_t)bio->bio_driver_info) {
                                        vm_page_deactivate(m);
                                        vm_page_wakeup(m);
                                } else {
                                        vm_page_flash(m);
                                }
                                /*
                                 * If i == bp->b_pager.pg_reqpage, do not wake 
                                 * the page up.  The caller needs to.
                                 */
                        } else {
                                /*
                                 * If a write error occurs remove the swap
                                 * assignment (note that PG_SWAPPED may or
                                 * may not be set depending on prior activity).
                                 *
                                 * Re-dirty OBJT_SWAP pages as there is no
                                 * other backing store, we can't throw the
                                 * page away.
                                 *
                                 * Non-OBJT_SWAP pages (aka swapcache) must
                                 * not be dirtied since they may not have
                                 * been dirty in the first place, and they
                                 * do have backing store (the vnode).
                                 */
                                vm_page_busy_wait(m, FALSE, "swadpg");
                                swp_pager_meta_ctl(m->object, m->pindex,
                                                   SWM_FREE);
                                vm_page_flag_clear(m, PG_SWAPPED);
                                if (m->object->type == OBJT_SWAP) {
                                        vm_page_dirty(m);
                                        vm_page_activate(m);
                                }
                                vm_page_flag_clear(m, PG_SWAPINPROG);
                                vm_page_io_finish(m);
                                vm_page_wakeup(m);
                        }
                } else if (bio->bio_caller_info1.index & SWBIO_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.
                         *
                         * clear PG_ZERO in page.
                         *
                         * If not the requested page then deactivate it.
                         *
                         * Note that the requested page, reqpage, is left
                         * busied, but we still have to wake it up.  The
                         * other pages are released (unbusied) by 
                         * vm_page_wakeup().  We do not set reqpage's
                         * valid bits here, it is up to the caller.
                         */

                        /* 
                         * NOTE: can't call pmap_clear_modify(m) from an
                         * interrupt thread, the pmap code may have to map
                         * non-kernel pmaps and currently asserts the case.
                         */
                        /*pmap_clear_modify(m);*/
                        m->valid = VM_PAGE_BITS_ALL;
                        vm_page_undirty(m);
                        vm_page_flag_clear(m, PG_ZERO | PG_SWAPINPROG);
                        vm_page_flag_set(m, PG_SWAPPED);

                        /*
                         * We have to wake specifically requested pages
                         * up too because we cleared PG_SWAPINPROG and
                         * could be waiting for it in getpages.  However,
                         * be sure to not unbusy getpages specifically
                         * requested page - getpages expects it to be 
                         * left busy.
                         *
                         * bio_driver_info holds the requested page
                         */
                        if (i != (int)(intptr_t)bio->bio_driver_info) {
                                vm_page_deactivate(m);
                                vm_page_wakeup(m);
                        } else {
                                vm_page_flash(m);
                        }
                } else {
                        /*
                         * Mark the page clean but do not mess with the
                         * pmap-layer's modified state.  That state should
                         * also be clear since the caller protected the
                         * page VM_PROT_READ, but allow the case.
                         *
                         * We are in an interrupt, avoid pmap operations.
                         *
                         * If we have a severe page deficit, deactivate the
                         * page.  Do not try to cache it (which would also
                         * involve a pmap op), because the page might still
                         * be read-heavy.
                         *
                         * When using the swap to cache clean vnode pages
                         * we do not mess with the page dirty bits.
                         */
                        vm_page_busy_wait(m, FALSE, "swadpg");
                        if (m->object->type == OBJT_SWAP)
                                vm_page_undirty(m);
                        vm_page_flag_clear(m, PG_SWAPINPROG);
                        vm_page_flag_set(m, PG_SWAPPED);
                        if (vm_page_count_severe())
                                vm_page_deactivate(m);
#if 0
                        if (!vm_page_count_severe() || !vm_page_try_to_cache(m))
                                vm_page_protect(m, VM_PROT_READ);
#endif
                        vm_page_io_finish(m);
                        vm_page_wakeup(m);
                }
        }

        /*
         * adjust pip.  NOTE: the original parent may still have its own
         * pip refs on the object.
         */

        if (object)
                vm_object_pip_wakeup_n(object, bp->b_xio.xio_npages);

        /*
         * Release the physical I/O buffer.
         *
         * NOTE: Due to synchronous operations in the write case b_cmd may
         *       already be set to BUF_CMD_DONE and BIO_SYNC may have already
         *       been cleared.
         *
         * Use vm_token to interlock nsw_rcount/wcount wakeup?
         */
        lwkt_gettoken(&vm_token);
        if (bio->bio_caller_info1.index & SWBIO_READ)
                nswptr = &nsw_rcount;
        else if (bio->bio_caller_info1.index & SWBIO_SYNC)
                nswptr = &nsw_wcount_sync;
        else
                nswptr = &nsw_wcount_async;
        bp->b_cmd = BUF_CMD_DONE;
        relpbuf(bp, nswptr);
        lwkt_reltoken(&vm_token);
}

/*
 * Fault-in a potentially swapped page and remove the swap reference.
 * (used by swapoff code)
 *
 * object must be held.
 */
static __inline void
swp_pager_fault_page(vm_object_t object, int *sharedp, vm_pindex_t pindex)
{
        struct vnode *vp;
        vm_page_t m;
        int error;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        if (object->type == OBJT_VNODE) {
                /*
                 * Any swap related to a vnode is due to swapcache.  We must
                 * vget() the vnode in case it is not active (otherwise
                 * vref() will panic).  Calling vm_object_page_remove() will
                 * ensure that any swap ref is removed interlocked with the
                 * page.  clean_only is set to TRUE so we don't throw away
                 * dirty pages.
                 */
                vp = object->handle;
                error = vget(vp, LK_SHARED | LK_RETRY | LK_CANRECURSE);
                if (error == 0) {
                        vm_object_page_remove(object, pindex, pindex + 1, TRUE);
                        vput(vp);
                }
        } else {
                /*
                 * Otherwise it is a normal OBJT_SWAP object and we can
                 * fault the page in and remove the swap.
                 */
                m = vm_fault_object_page(object, IDX_TO_OFF(pindex),
                                         VM_PROT_NONE,
                                         VM_FAULT_DIRTY | VM_FAULT_UNSWAP,
                                         sharedp, &error);
                if (m)
                        vm_page_unhold(m);
        }
}

/*
 * This removes all swap blocks related to a particular device.  We have
 * to be careful of ripups during the scan.
 */
static int swp_pager_swapoff_callback(struct swblock *swap, void *data);

int
swap_pager_swapoff(int devidx)
{
        struct vm_object marker;
        vm_object_t object;
        struct swswapoffinfo info;

        bzero(&marker, sizeof(marker));
        marker.type = OBJT_MARKER;

        lwkt_gettoken(&vmobj_token);
        TAILQ_INSERT_HEAD(&vm_object_list, &marker, object_list);

        while ((object = TAILQ_NEXT(&marker, object_list)) != NULL) {
                if (object->type == OBJT_MARKER)
                        goto skip;
                if (object->type != OBJT_SWAP && object->type != OBJT_VNODE)
                        goto skip;
                vm_object_hold(object);
                if (object->type != OBJT_SWAP && object->type != OBJT_VNODE) {
                        vm_object_drop(object);
                        goto skip;
                }
                info.object = object;
                info.shared = 0;
                info.devidx = devidx;
                swblock_rb_tree_RB_SCAN(&object->swblock_root,
                                        NULL,
                                        swp_pager_swapoff_callback,
                                        &info);
                vm_object_drop(object);
skip:
                if (object == TAILQ_NEXT(&marker, object_list)) {
                        TAILQ_REMOVE(&vm_object_list, &marker, object_list);
                        TAILQ_INSERT_AFTER(&vm_object_list, object,
                                           &marker, object_list);
                }
        }
        TAILQ_REMOVE(&vm_object_list, &marker, object_list);
        lwkt_reltoken(&vmobj_token);

        /*
         * If we fail to locate all swblocks we just fail gracefully and
         * do not bother to restore paging on the swap device.  If the
         * user wants to retry the user can retry.
         */
        if (swdevt[devidx].sw_nused)
                return (1);
        else
                return (0);
}

static
int
swp_pager_swapoff_callback(struct swblock *swap, void *data)
{
        struct swswapoffinfo *info = data;
        vm_object_t object = info->object;
        vm_pindex_t index;
        swblk_t v;
        int i;

        index = swap->swb_index;
        for (i = 0; i < SWAP_META_PAGES; ++i) {
                /*
                 * Make sure we don't race a dying object.  This will
                 * kill the scan of the object's swap blocks entirely.
                 */
                if (object->flags & OBJ_DEAD)
                        return(-1);

                /*
                 * Fault the page, which can obviously block.  If the swap
                 * structure disappears break out.
                 */
                v = swap->swb_pages[i];
                if (v != SWAPBLK_NONE && BLK2DEVIDX(v) == info->devidx) {
                        swp_pager_fault_page(object, &info->shared,
                                             swap->swb_index + i);
                        /* swap ptr might go away */
                        if (RB_LOOKUP(swblock_rb_tree,
                                      &object->swblock_root, index) != swap) {
                                break;
                        }
                }
        }
        return(0);
}

/************************************************************************
 *                              SWAP META DATA                          *
 ************************************************************************
 *
 *      These routines manipulate the swap metadata stored in the 
 *      OBJT_SWAP object.  All swp_*() routines must be called at
 *      splvm() because swap can be freed up by the low level vm_page
 *      code which might be called from interrupts beyond what splbio() covers.
 *
 *      Swap metadata is implemented with a global hash and not directly
 *      linked into the object.  Instead the object simply contains
 *      appropriate tracking counters.
 */

/*
 * Lookup the swblock containing the specified swap block index.
 *
 * The caller must hold the object.
 */
static __inline
struct swblock *
swp_pager_lookup(vm_object_t object, vm_pindex_t index)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        index &= ~(vm_pindex_t)SWAP_META_MASK;
        return (RB_LOOKUP(swblock_rb_tree, &object->swblock_root, index));
}

/*
 * Remove a swblock from the RB tree.
 *
 * The caller must hold the object.
 */
static __inline
void
swp_pager_remove(vm_object_t object, struct swblock *swap)
{
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));
        RB_REMOVE(swblock_rb_tree, &object->swblock_root, swap);
}

/*
 * Convert default object to swap object if necessary
 *
 * The caller must hold the object.
 */
static void
swp_pager_meta_convert(vm_object_t object)
{
        if (object->type == OBJT_DEFAULT) {
                object->type = OBJT_SWAP;
                KKASSERT(object->swblock_count == 0);
        }
}

/*
 * 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.  Vnode objects do not need to be converted.
 *
 *      The specified swapblk is added to the object's swap metadata.  If
 *      the swapblk is not valid, it is freed instead.  Any previously
 *      assigned swapblk is freed.
 *
 * The caller must hold the object.
 */
static void
swp_pager_meta_build(vm_object_t object, vm_pindex_t index, swblk_t swapblk)
{
        struct swblock *swap;
        struct swblock *oswap;
        vm_pindex_t v;

        KKASSERT(swapblk != SWAPBLK_NONE);
        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        /*
         * Convert object if necessary
         */
        if (object->type == OBJT_DEFAULT)
                swp_pager_meta_convert(object);
        
        /*
         * Locate swblock.  If not found create, but if we aren't adding
         * anything just return.  If we run out of space in the map we wait
         * and, since the hash table may have changed, retry.
         */
retry:
        swap = swp_pager_lookup(object, index);

        if (swap == NULL) {
                int i;

                swap = zalloc(swap_zone);
                if (swap == NULL) {
                        vm_wait(0);
                        goto retry;
                }
                swap->swb_index = index & ~(vm_pindex_t)SWAP_META_MASK;
                swap->swb_count = 0;

                ++object->swblock_count;

                for (i = 0; i < SWAP_META_PAGES; ++i)
                        swap->swb_pages[i] = SWAPBLK_NONE;
                oswap = RB_INSERT(swblock_rb_tree, &object->swblock_root, swap);
                KKASSERT(oswap == NULL);
        }

        /*
         * Delete prior contents of metadata.
         *
         * NOTE: Decrement swb_count after the freeing operation (which
         *       might block) to prevent racing destruction of the swblock.
         */
        index &= SWAP_META_MASK;

        while ((v = swap->swb_pages[index]) != SWAPBLK_NONE) {
                swap->swb_pages[index] = SWAPBLK_NONE;
                /* can block */
                swp_pager_freeswapspace(object, v, 1);
                --swap->swb_count;
                --mycpu->gd_vmtotal.t_vm;
        }

        /*
         * Enter block into metadata
         */
        swap->swb_pages[index] = swapblk;
        if (swapblk != SWAPBLK_NONE) {
                ++swap->swb_count;
                ++mycpu->gd_vmtotal.t_vm;
        }
}

/*
 * 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.
 *
 * The caller must hold the object.
 */
static int swp_pager_meta_free_callback(struct swblock *swb, void *data);

static void
swp_pager_meta_free(vm_object_t object, vm_pindex_t index, vm_pindex_t count)
{
        struct swfreeinfo info;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        /*
         * Nothing to do
         */
        if (object->swblock_count == 0) {
                KKASSERT(RB_EMPTY(&object->swblock_root));
                return;
        }
        if (count == 0)
                return;

        /*
         * Setup for RB tree scan.  Note that the pindex range can be huge
         * due to the 64 bit page index space so we cannot safely iterate.
         */
        info.object = object;
        info.basei = index & ~(vm_pindex_t)SWAP_META_MASK;
        info.begi = index;
        info.endi = index + count - 1;
        swblock_rb_tree_RB_SCAN(&object->swblock_root, rb_swblock_scancmp,
                                swp_pager_meta_free_callback, &info);
}

/*
 * The caller must hold the object.
 */
static
int
swp_pager_meta_free_callback(struct swblock *swap, void *data)
{
        struct swfreeinfo *info = data;
        vm_object_t object = info->object;
        int index;
        int eindex;

        /*
         * Figure out the range within the swblock.  The wider scan may
         * return edge-case swap blocks when the start and/or end points
         * are in the middle of a block.
         */
        if (swap->swb_index < info->begi)
                index = (int)info->begi & SWAP_META_MASK;
        else
                index = 0;

        if (swap->swb_index + SWAP_META_PAGES > info->endi)
                eindex = (int)info->endi & SWAP_META_MASK;
        else
                eindex = SWAP_META_MASK;

        /*
         * Scan and free the blocks.  The loop terminates early
         * if (swap) runs out of blocks and could be freed.
         *
         * NOTE: Decrement swb_count after swp_pager_freeswapspace()
         *       to deal with a zfree race.
         */
        while (index <= eindex) {
                swblk_t v = swap->swb_pages[index];

                if (v != SWAPBLK_NONE) {
                        swap->swb_pages[index] = SWAPBLK_NONE;
                        /* can block */
                        swp_pager_freeswapspace(object, v, 1);
                        --mycpu->gd_vmtotal.t_vm;
                        if (--swap->swb_count == 0) {
                                swp_pager_remove(object, swap);
                                zfree(swap_zone, swap);
                                --object->swblock_count;
                                break;
                        }
                }
                ++index;
        }

        /* swap may be invalid here due to zfree above */
        lwkt_yield();

        return(0);
}

/*
 * SWP_PAGER_META_FREE_ALL() - destroy all swap metadata associated with object
 *
 *      This routine locates and destroys all swap metadata associated with
 *      an object.
 *
 * NOTE: Decrement swb_count after the freeing operation (which
 *       might block) to prevent racing destruction of the swblock.
 *
 * The caller must hold the object.
 */
static void
swp_pager_meta_free_all(vm_object_t object)
{
        struct swblock *swap;
        int i;

        ASSERT_LWKT_TOKEN_HELD(vm_object_token(object));

        while ((swap = RB_ROOT(&object->swblock_root)) != NULL) {
                swp_pager_remove(object, swap);
                for (i = 0; i < SWAP_META_PAGES; ++i) {
                        swblk_t v = swap->swb_pages[i];
                        if (v != SWAPBLK_NONE) {
                                /* can block */
                                swp_pager_freeswapspace(object, v, 1);
                                --swap->swb_count;
                                --mycpu->gd_vmtotal.t_vm;
                        }
                }
                if (swap->swb_count != 0)
                        panic("swap_pager_meta_free_all: swb_count != 0");
                zfree(swap_zone, swap);
                --object->swblock_count;
                lwkt_yield();
        }
        KKASSERT(object->swblock_count == 0);
}

/*
 * SWP_PAGER_METACTL() -  misc control of swap and vm_page_t meta data.
 *
 *      This routine is capable of looking up, popping, or freeing
 *      swapblk assignments in the swap meta data or in the vm_page_t.
 *      The routine typically returns the swapblk being looked-up, or popped,
 *      or SWAPBLK_NONE if the block was freed, or SWAPBLK_NONE if the block
 *      was invalid.  This routine will automatically free any invalid 
 *      meta-data swapblks.
 *
 *      It is not possible to store invalid swapblks in the swap meta data
 *      (other then a literal 'SWAPBLK_NONE'), so we don't bother checking.
 *
 *      When acting on a busy resident page and paging is in progress, we 
 *      have to wait until paging is complete but otherwise can act on the 
 *      busy page.
 *
 *      SWM_FREE        remove and free swap block from metadata
 *      SWM_POP         remove from meta data but do not free.. pop it out
 *
 * The caller must hold the object.
 */
static swblk_t
swp_pager_meta_ctl(vm_object_t object, vm_pindex_t index, int flags)
{
        struct swblock *swap;
        swblk_t r1;

        if (object->swblock_count == 0)
                return(SWAPBLK_NONE);

        r1 = SWAPBLK_NONE;
        swap = swp_pager_lookup(object, index);

        if (swap != NULL) {
                index &= SWAP_META_MASK;
                r1 = swap->swb_pages[index];

                if (r1 != SWAPBLK_NONE) {
                        if (flags & (SWM_FREE|SWM_POP)) {
                                swap->swb_pages[index] = SWAPBLK_NONE;
                                --mycpu->gd_vmtotal.t_vm;
                                if (--swap->swb_count == 0) {
                                        swp_pager_remove(object, swap);
                                        zfree(swap_zone, swap);
                                        --object->swblock_count;
                                }
                        } 
                        /* swap ptr may be invalid */
                        if (flags & SWM_FREE) {
                                swp_pager_freeswapspace(object, r1, 1);
                                r1 = SWAPBLK_NONE;
                        }
                }
                /* swap ptr may be invalid */
        }
        return(r1);
}