Add one more comment to the OOM changes so that future readers of

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

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

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/conf.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/bio.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/sx.h>
#include <sys/vmmeter.h>

#ifndef MAX_PAGEOUT_CLUSTER
#define MAX_PAGEOUT_CLUSTER 16
#endif

#define SWB_NPAGES      MAX_PAGEOUT_CLUSTER

#include "opt_swap.h"
#include <vm/vm.h>
#include <vm/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_zone.h>
#include <vm/swap_pager.h>
#include <vm/vm_extern.h>

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

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

extern int vm_swap_size;        /* number of free swap blocks, in pages */

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

struct blist *swapblist;
static struct swblock **swhash;
static int swhash_mask;
static int swap_async_max = 4;  /* maximum in-progress async I/O's      */
static struct sx sw_alloc_sx;

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

SYSCTL_INT(_vm, OID_AUTO, swap_async_max,
        CTLFLAG_RW, &swap_async_max, 0, "Maximum running async swap ops");

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

/*
 * "named" and "unnamed" anon region objects.  Try to reduce the overhead
 * of searching a named list by hashing it just a little.
 */

#define NOBJLISTS               8

#define NOBJLIST(handle)        \
        (&swap_pager_object_list[((int)(intptr_t)handle >> 4) & (NOBJLISTS-1)])

static struct mtx sw_alloc_mtx; /* protect list manipulation */ 
static struct pagerlst  swap_pager_object_list[NOBJLISTS];
struct pagerlst         swap_pager_un_object_list;
vm_zone_t               swap_zone;

/*
 * pagerops for OBJT_SWAP - "swap pager".  Some ops are also global procedure
 * calls hooked from other parts of the VM system and do not appear here.
 * (see vm/swap_pager.h).
 */

static vm_object_t
                swap_pager_alloc __P((void *handle, vm_ooffset_t size,
                                      vm_prot_t prot, vm_ooffset_t offset));
static void     swap_pager_dealloc __P((vm_object_t object));
static int      swap_pager_getpages __P((vm_object_t, vm_page_t *, int, int));
static void     swap_pager_init __P((void));
static void     swap_pager_unswapped __P((vm_page_t));
static void     swap_pager_strategy __P((vm_object_t, struct bio *));

struct pagerops swappagerops = {
        swap_pager_init,        /* early system initialization of pager */
        swap_pager_alloc,       /* allocate an OBJT_SWAP object         */
        swap_pager_dealloc,     /* deallocate an OBJT_SWAP object       */
        swap_pager_getpages,    /* pagein                               */
        swap_pager_putpages,    /* pageout                              */
        swap_pager_haspage,     /* get backing store status for page    */
        swap_pager_unswapped,   /* remove swap related to page          */
        swap_pager_strategy     /* pager strategy call                  */
};

static struct buf *getchainbuf(struct bio *bp, struct vnode *vp, int flags);
static void flushchainbuf(struct buf *nbp);
static void waitchainbuf(struct bio *bp, int count, int done);

/*
 * 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 */

SYSCTL_INT(_vm, OID_AUTO, dmmax,
        CTLFLAG_RD, &dmmax, 0, "Maximum size of a swap block");

static __inline void    swp_sizecheck __P((void));
static void     swp_pager_sync_iodone __P((struct buf *bp));
static void     swp_pager_async_iodone __P((struct buf *bp));

/*
 * Swap bitmap functions
 */

static __inline void    swp_pager_freeswapspace __P((daddr_t blk, int npages));
static __inline daddr_t swp_pager_getswapspace __P((int npages));

/*
 * Metadata functions
 */

static void swp_pager_meta_build __P((vm_object_t, vm_pindex_t, daddr_t));
static void swp_pager_meta_free __P((vm_object_t, vm_pindex_t, daddr_t));
static void swp_pager_meta_free_all __P((vm_object_t));
static daddr_t swp_pager_meta_ctl __P((vm_object_t, vm_pindex_t, int));

/*
 * SWP_SIZECHECK() -    update swap_pager_full indication
 *      
 *      update the swap_pager_almost_full indication and warn when we are
 *      about to run out of swap space, using lowat/hiwat hysteresis.
 *
 *      Clear swap_pager_full ( task killing ) indication when lowat is met.
 *
 *      No restrictions on call
 *      This routine may not block.
 *      This routine must be called at splvm()
 */

static __inline void
swp_sizecheck()
{
        GIANT_REQUIRED;

        if (vm_swap_size < nswap_lowat) {
                if (swap_pager_almost_full == 0) {
                        printf("swap_pager: out of swap space\n");
                        swap_pager_almost_full = 1;
                }
        } else {
                swap_pager_full = 0;
                if (vm_swap_size > nswap_hiwat)
                        swap_pager_almost_full = 0;
        }
}

/*
 * SWAP_PAGER_INIT() -  initialize the swap pager!
 *
 *      Expected to be started from system init.  NOTE:  This code is run 
 *      before much else so be careful what you depend on.  Most of the VM
 *      system has yet to be initialized at this point.
 */

static void
swap_pager_init()
{
        /*
         * Initialize object lists
         */
        int i;

        for (i = 0; i < NOBJLISTS; ++i)
                TAILQ_INIT(&swap_pager_object_list[i]);
        TAILQ_INIT(&swap_pager_un_object_list);
        mtx_init(&sw_alloc_mtx, "swap_pager list", MTX_DEF);

        /*
         * Device Stripe, in PAGE_SIZE'd blocks
         */

        dmmax = SWB_NPAGES * 2;
        dmmax_mask = ~(dmmax - 1);
}

/*
 * SWAP_PAGER_SWAP_INIT() - swap pager initialization from pageout process
 *
 *      Expected to be started from pageout process once, prior to entering
 *      its main loop.
 */

void
swap_pager_swap_init()
{
        int n, n2;

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

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

        mtx_lock(&pbuf_mtx);
        nsw_rcount = (nswbuf + 1) / 2;
        nsw_wcount_sync = (nswbuf + 3) / 4;
        nsw_wcount_async = 4;
        nsw_wcount_async_max = nsw_wcount_async;
        mtx_unlock(&pbuf_mtx);

        /*
         * Initialize our zone.  Right now I'm just guessing on the number
         * we need based on the number of pages in the system.  Each swblock
         * can hold 16 pages, so this is probably overkill.  This reservation
         * is typically limited to around 70MB by default.
         */

        n = cnt.v_page_count;
        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("failed to zinit swap_zone.");
        if (n2 != n)
                printf("Swap zone entries reduced from %d to %d.\n", n2, n);
        n2 = n;

        /*
         * Initialize our meta-data hash table.  The swapper does not need to
         * be quite as efficient as the VM system, so we do not use an 
         * oversized hash table.
         *
         *      n:              size of hash table, must be power of 2
         *      swhash_mask:    hash table index mask
         */

        for (n = 1; n < n2 / 8; n *= 2)
                ;

        swhash = malloc(sizeof(struct swblock *) * n, M_VMPGDATA, M_WAITOK | M_ZERO);

        swhash_mask = n - 1;
}

/*
 * SWAP_PAGER_ALLOC() - allocate a new OBJT_SWAP VM object and instantiate
 *                      its metadata structures.
 *
 *      This routine is called from the mmap and fork code to create a new
 *      OBJT_SWAP object.  We do this by creating an OBJT_DEFAULT object
 *      and then converting it with swp_pager_meta_build().
 *
 *      This routine may block in vm_object_allocate() and create a named
 *      object lookup race, so we must interlock.   We must also run at
 *      splvm() for the object lookup to handle races with interrupts, but
 *      we do not have to maintain splvm() in between the lookup and the
 *      add because (I believe) it is not possible to attempt to create
 *      a new swap object w/handle when a default object with that handle
 *      already exists.
 */

static vm_object_t
swap_pager_alloc(void *handle, vm_ooffset_t size, vm_prot_t prot,
                 vm_ooffset_t offset)
{
        vm_object_t object;

        GIANT_REQUIRED;

        if (handle) {
                /*
                 * Reference existing named region or allocate new one.  There
                 * should not be a race here against swp_pager_meta_build()
                 * as called from vm_page_remove() in regards to the lookup
                 * of the handle.
                 */
                sx_xlock(&sw_alloc_sx);
                object = vm_pager_object_lookup(NOBJLIST(handle), handle);

                if (object != NULL) {
                        vm_object_reference(object);
                } else {
                        object = vm_object_allocate(OBJT_DEFAULT,
                                OFF_TO_IDX(offset + PAGE_MASK + size));
                        object->handle = handle;

                        swp_pager_meta_build(object, 0, SWAPBLK_NONE);
                }
                sx_xunlock(&sw_alloc_sx);
        } else {
                object = vm_object_allocate(OBJT_DEFAULT,
                        OFF_TO_IDX(offset + PAGE_MASK + size));

                swp_pager_meta_build(object, 0, SWAPBLK_NONE);
        }

        return (object);
}

/*
 * SWAP_PAGER_DEALLOC() -       remove swap metadata from object
 *
 *      The swap backing for the object is destroyed.  The code is 
 *      designed such that we can reinstantiate it later, but this
 *      routine is typically called only when the entire object is
 *      about to be destroyed.
 *
 *      This routine may block, but no longer does. 
 *
 *      The object must be locked or unreferenceable.
 */

static void
swap_pager_dealloc(object)
        vm_object_t object;
{
        int s;

        GIANT_REQUIRED;

        /*
         * Remove from list right away so lookups will fail if we block for
         * pageout completion.
         */
        mtx_lock(&sw_alloc_mtx);
        if (object->handle == NULL) {
                TAILQ_REMOVE(&swap_pager_un_object_list, object, pager_object_list);
        } else {
                TAILQ_REMOVE(NOBJLIST(object->handle), object, pager_object_list);
        }
        mtx_unlock(&sw_alloc_mtx);

        vm_object_pip_wait(object, "swpdea");

        /*
         * Free all remaining metadata.  We only bother to free it from 
         * the swap meta data.  We do not attempt to free swapblk's still
         * associated with vm_page_t's for this object.  We do not care
         * if paging is still in progress on some objects.
         */
        s = splvm();
        swp_pager_meta_free_all(object);
        splx(s);
}

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

/*
 * SWP_PAGER_GETSWAPSPACE() -   allocate raw swap space
 *
 *      Allocate swap for the requested number of pages.  The starting
 *      swap block number (a page index) is returned or SWAPBLK_NONE
 *      if the allocation failed.
 *
 *      Also has the side effect of advising that somebody made a mistake
 *      when they configured swap and didn't configure enough.
 *
 *      Must be called at splvm() to avoid races with bitmap frees from
 *      vm_page_remove() aka swap_pager_page_removed().
 *
 *      This routine may not block
 *      This routine must be called at splvm().
 */

static __inline daddr_t
swp_pager_getswapspace(npages)
        int npages;
{
        daddr_t blk;

        GIANT_REQUIRED;

        if ((blk = blist_alloc(swapblist, npages)) == SWAPBLK_NONE) {
                if (swap_pager_full != 2) {
                        printf("swap_pager_getswapspace: failed\n");
                        swap_pager_full = 2;
                        swap_pager_almost_full = 1;
                }
        } else {
                vm_swap_size -= npages;
                /* per-swap area stats */
                swdevt[BLK2DEVIDX(blk)].sw_used += npages;
                swp_sizecheck();
        }
        return(blk);
}

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

static __inline void
swp_pager_freeswapspace(blk, npages)
        daddr_t blk;
        int npages;
{
        GIANT_REQUIRED;

        blist_free(swapblist, blk, npages);
        vm_swap_size += npages;
        /* per-swap area stats */
        swdevt[BLK2DEVIDX(blk)].sw_used -= npages;
        swp_sizecheck();
}

/*
 * SWAP_PAGER_FREESPACE() -     frees swap blocks associated with a page
 *                              range within an object.
 *
 *      This is a globally accessible routine.
 *
 *      This routine removes swapblk assignments from swap metadata.
 *
 *      The external callers of this routine typically have already destroyed 
 *      or renamed vm_page_t's associated with this range in the object so 
 *      we should be ok.
 *
 *      This routine may be called at any spl.  We up our spl to splvm temporarily
 *      in order to perform the metadata removal.
 */

void
swap_pager_freespace(object, start, size)
        vm_object_t object;
        vm_pindex_t start;
        vm_size_t size;
{
        int s = splvm();

        GIANT_REQUIRED;
        swp_pager_meta_free(object, start, size);
        splx(s);
}

/*
 * SWAP_PAGER_RESERVE() - reserve swap blocks in object
 *
 *      Assigns swap blocks to the specified range within the object.  The 
 *      swap blocks are not zerod.  Any previous swap assignment is destroyed.
 *
 *      Returns 0 on success, -1 on failure.
 */

int
swap_pager_reserve(vm_object_t object, vm_pindex_t start, vm_size_t size)
{
        int s;
        int n = 0;
        daddr_t blk = SWAPBLK_NONE;
        vm_pindex_t beg = start;        /* save start index */

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

/*
 * SWAP_PAGER_COPY() -  copy blocks from source pager to destination pager
 *                      and destroy the source.
 *
 *      Copy any valid swapblks from the source to the destination.  In
 *      cases where both the source and destination have a valid swapblk,
 *      we keep the destination's.
 *
 *      This routine is allowed to block.  It may block allocating metadata
 *      indirectly through swp_pager_meta_build() or if paging is still in
 *      progress on the source. 
 *
 *      This routine can be called at any spl
 *
 *      XXX vm_page_collapse() kinda expects us not to block because we 
 *      supposedly do not need to allocate memory, but for the moment we
 *      *may* have to get a little memory from the zone allocator, but
 *      it is taken from the interrupt memory.  We should be ok. 
 *
 *      The source object contains no vm_page_t's (which is just as well)
 *
 *      The source object is of type OBJT_SWAP.
 *
 *      The source and destination objects must be locked or 
 *      inaccessible (XXX are they ?)
 */

void
swap_pager_copy(srcobject, dstobject, offset, destroysource)
        vm_object_t srcobject;
        vm_object_t dstobject;
        vm_pindex_t offset;
        int destroysource;
{
        vm_pindex_t i;
        int s;

        GIANT_REQUIRED;

        s = splvm();
        /*
         * If destroysource is set, we remove the source object from the 
         * swap_pager internal queue now. 
         */

        if (destroysource) {
                mtx_lock(&sw_alloc_mtx);
                if (srcobject->handle == NULL) {
                        TAILQ_REMOVE(
                            &swap_pager_un_object_list, 
                            srcobject, 
                            pager_object_list
                        );
                } else {
                        TAILQ_REMOVE(
                            NOBJLIST(srcobject->handle),
                            srcobject,
                            pager_object_list
                        );
                }
                mtx_unlock(&sw_alloc_mtx);
        }

        /*
         * transfer source to destination.
         */

        for (i = 0; i < dstobject->size; ++i) {
                daddr_t dstaddr;

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

                dstaddr = swp_pager_meta_ctl(dstobject, i, 0);

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

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

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

        /*
         * Free left over swap blocks in source.
         *
         * We have to revert the type to OBJT_DEFAULT so we do not accidently
         * double-remove the object from the swap queues.
         */

        if (destroysource) {
                swp_pager_meta_free_all(srcobject);
                /*
                 * Reverting the type is not necessary, the caller is going
                 * to destroy srcobject directly, but I'm doing it here
                 * for consistency since we've removed the object from its
                 * queues.
                 */
                srcobject->type = OBJT_DEFAULT;
        }
        splx(s);
}

/*
 * SWAP_PAGER_HASPAGE() -       determine if we have good backing store for
 *                              the requested page.
 *
 *      We determine whether good backing store exists for the requested
 *      page and return TRUE if it does, FALSE if it doesn't.
 *
 *      If TRUE, we also try to determine how much valid, contiguous backing
 *      store exists before and after the requested page within a reasonable
 *      distance.  We do not try to restrict it to the swap device stripe
 *      (that is handled in getpages/putpages).  It probably isn't worth
 *      doing here.
 */

boolean_t
swap_pager_haspage(object, pindex, before, after)
        vm_object_t object;
        vm_pindex_t pindex;
        int *before;
        int *after;
{
        daddr_t blk0;
        int s;

        /*
         * do we have good backing store at the requested index ?
         */

        s = splvm();
        blk0 = swp_pager_meta_ctl(object, pindex, 0);

        if (blk0 == SWAPBLK_NONE) {
                splx(s);
                if (before)
                        *before = 0;
                if (after)
                        *after = 0;
                return (FALSE);
        }

        /*
         * find backwards-looking contiguous good backing store
         */

        if (before != NULL) {
                int i;

                for (i = 1; i < (SWB_NPAGES/2); ++i) {
                        daddr_t blk;

                        if (i > pindex)
                                break;
                        blk = swp_pager_meta_ctl(object, pindex - i, 0);
                        if (blk != blk0 - i)
                                break;
                }
                *before = (i - 1);
        }

        /*
         * find forward-looking contiguous good backing store
         */

        if (after != NULL) {
                int i;

                for (i = 1; i < (SWB_NPAGES/2); ++i) {
                        daddr_t blk;

                        blk = swp_pager_meta_ctl(object, pindex + i, 0);
                        if (blk != blk0 + i)
                                break;
                }
                *after = (i - 1);
        }
        splx(s);
        return (TRUE);
}

/*
 * SWAP_PAGER_PAGE_UNSWAPPED() - remove swap backing store related to page
 *
 *      This removes any associated swap backing store, whether valid or
 *      not, from the page.  
 *
 *      This routine is typically called when a page is made dirty, at
 *      which point any associated swap can be freed.  MADV_FREE also
 *      calls us in a special-case situation
 *
 *      NOTE!!!  If the page is clean and the swap was valid, the caller
 *      should make the page dirty before calling this routine.  This routine
 *      does NOT change the m->dirty status of the page.  Also: MADV_FREE
 *      depends on it.
 *
 *      This routine may not block
 *      This routine must be called at splvm()
 */

static void
swap_pager_unswapped(m)
        vm_page_t m;
{
        swp_pager_meta_ctl(m->object, m->pindex, SWM_FREE);
}

/*
 * SWAP_PAGER_STRATEGY() - read, write, free blocks
 *
 *      This implements the vm_pager_strategy() interface to swap and allows
 *      other parts of the system to directly access swap as backing store
 *      through vm_objects of type OBJT_SWAP.  This is intended to be a 
 *      cacheless interface ( i.e. caching occurs at higher levels ).
 *      Therefore we do not maintain any resident pages.  All I/O goes
 *      directly to and from the swap device.
 *      
 *      Note that b_blkno is scaled for PAGE_SIZE
 *
 *      We currently attempt to run I/O synchronously or asynchronously as
 *      the caller requests.  This isn't perfect because we loose error
 *      sequencing when we run multiple ops in parallel to satisfy a request.
 *      But this is swap, so we let it all hang out.
 */

static void     
swap_pager_strategy(vm_object_t object, struct bio *bp)
{
        vm_pindex_t start;
        int count;
        int s;
        char *data;
        struct buf *nbp = NULL;

        GIANT_REQUIRED;

        /* XXX: KASSERT instead ? */
        if (bp->bio_bcount & PAGE_MASK) {
                biofinish(bp, NULL, EINVAL);
                printf("swap_pager_strategy: bp %p blk %d size %d, not page bounded\n", bp, (int)bp->bio_pblkno, (int)bp->bio_bcount);
                return;
        }

        /*
         * Clear error indication, initialize page index, count, data pointer.
         */

        bp->bio_error = 0;
        bp->bio_flags &= ~BIO_ERROR;
        bp->bio_resid = bp->bio_bcount;
        *(u_int *) &bp->bio_driver1 = 0;

        start = bp->bio_pblkno;
        count = howmany(bp->bio_bcount, PAGE_SIZE);
        data = bp->bio_data;

        s = splvm();

        /*
         * Deal with BIO_DELETE
         */

        if (bp->bio_cmd == BIO_DELETE) {
                /*
                 * FREE PAGE(s) - destroy underlying swap that is no longer
                 *                needed.
                 */
                swp_pager_meta_free(object, start, count);
                splx(s);
                bp->bio_resid = 0;
                biodone(bp);
                return;
        }

        /*
         * Execute read or write
         */
        while (count > 0) {
                daddr_t blk;

                /*
                 * Obtain block.  If block not found and writing, allocate a
                 * new block and build it into the object.
                 */

                blk = swp_pager_meta_ctl(object, start, 0);
                if ((blk == SWAPBLK_NONE) && (bp->bio_cmd == BIO_WRITE)) {
                        blk = swp_pager_getswapspace(1);
                        if (blk == SWAPBLK_NONE) {
                                bp->bio_error = ENOMEM;
                                bp->bio_flags |= BIO_ERROR;
                                break;
                        }
                        swp_pager_meta_build(object, start, blk);
                }
                        
                /*
                 * Do we have to flush our current collection?  Yes if:
                 *
                 *      - no swap block at this index
                 *      - swap block is not contiguous
                 *      - we cross a physical disk boundry in the
                 *        stripe.
                 */

                if (
                    nbp && (nbp->b_blkno + btoc(nbp->b_bcount) != blk ||
                     ((nbp->b_blkno ^ blk) & dmmax_mask)
                    )
                ) {
                        splx(s);
                        if (bp->bio_cmd == BIO_READ) {
                                ++cnt.v_swapin;
                                cnt.v_swappgsin += btoc(nbp->b_bcount);
                        } else {
                                ++cnt.v_swapout;
                                cnt.v_swappgsout += btoc(nbp->b_bcount);
                                nbp->b_dirtyend = nbp->b_bcount;
                        }
                        flushchainbuf(nbp);
                        s = splvm();
                        nbp = NULL;
                }

                /*
                 * Add new swapblk to nbp, instantiating nbp if necessary.
                 * Zero-fill reads are able to take a shortcut.
                 */

                if (blk == SWAPBLK_NONE) {
                        /*
                         * We can only get here if we are reading.  Since
                         * we are at splvm() we can safely modify b_resid,
                         * even if chain ops are in progress.
                         */
                        bzero(data, PAGE_SIZE);
                        bp->bio_resid -= PAGE_SIZE;
                } else {
                        if (nbp == NULL) {
                                nbp = getchainbuf(bp, swapdev_vp, B_ASYNC);
                                nbp->b_blkno = blk;
                                nbp->b_bcount = 0;
                                nbp->b_data = data;
                        }
                        nbp->b_bcount += PAGE_SIZE;
                }
                --count;
                ++start;
                data += PAGE_SIZE;
        }

        /*
         *  Flush out last buffer
         */

        splx(s);

        if (nbp) {
                if (nbp->b_iocmd == BIO_READ) {
                        ++cnt.v_swapin;
                        cnt.v_swappgsin += btoc(nbp->b_bcount);
                } else {
                        ++cnt.v_swapout;
                        cnt.v_swappgsout += btoc(nbp->b_bcount);
                        nbp->b_dirtyend = nbp->b_bcount;
                }
                flushchainbuf(nbp);
                /* nbp = NULL; */
        }
        /*
         * Wait for completion.
         */

        waitchainbuf(bp, 0, 1);
}

/*
 * SWAP_PAGER_GETPAGES() - bring pages in from swap
 *
 *      Attempt to retrieve (m, count) pages from backing store, but make
 *      sure we retrieve at least m[reqpage].  We try to load in as large
 *      a chunk surrounding m[reqpage] as is contiguous in swap and which
 *      belongs to the same object.
 *
 *      The code is designed for asynchronous operation and 
 *      immediate-notification of 'reqpage' but tends not to be
 *      used that way.  Please do not optimize-out this algorithmic
 *      feature, I intend to improve on it in the future.
 *
 *      The parent has a single vm_object_pip_add() reference prior to
 *      calling us and we should return with the same.
 *
 *      The parent has BUSY'd the pages.  We should return with 'm'
 *      left busy, but the others adjusted.
 */

static int
swap_pager_getpages(object, m, count, reqpage)
        vm_object_t object;
        vm_page_t *m;
        int count, reqpage;
{
        struct buf *bp;
        vm_page_t mreq;
        int s;
        int i;
        int j;
        daddr_t blk;
        vm_offset_t kva;
        vm_pindex_t lastpindex;

        GIANT_REQUIRED;

        mreq = m[reqpage];

        if (mreq->object != object) {
                panic("swap_pager_getpages: object mismatch %p/%p", 
                    object, 
                    mreq->object
                );
        }
        /*
         * Calculate range to retrieve.  The pages have already been assigned
         * their swapblks.  We require a *contiguous* range that falls entirely
         * within a single device stripe.   If we do not supply it, bad things
         * happen.  Note that blk, iblk & jblk can be SWAPBLK_NONE, but the 
         * loops are set up such that the case(s) are handled implicitly.
         *
         * The swp_*() calls must be made at splvm().  vm_page_free() does
         * not need to be, but it will go a little faster if it is.
         */

        s = splvm();
        blk = swp_pager_meta_ctl(mreq->object, mreq->pindex, 0);

        for (i = reqpage - 1; i >= 0; --i) {
                daddr_t iblk;

                iblk = swp_pager_meta_ctl(m[i]->object, m[i]->pindex, 0);
                if (blk != iblk + (reqpage - i))
                        break;
                if ((blk ^ iblk) & dmmax_mask)
                        break;
        }
        ++i;

        for (j = reqpage + 1; j < count; ++j) {
                daddr_t jblk;

                jblk = swp_pager_meta_ctl(m[j]->object, m[j]->pindex, 0);
                if (blk != jblk - (j - reqpage))
                        break;
                if ((blk ^ jblk) & dmmax_mask)
                        break;
        }

        /*
         * free pages outside our collection range.   Note: we never free
         * mreq, it must remain busy throughout.
         */

        {
                int k;

                for (k = 0; k < i; ++k)
                        vm_page_free(m[k]);
                for (k = j; k < count; ++k)
                        vm_page_free(m[k]);
        }
        splx(s);


        /*
         * Return VM_PAGER_FAIL if we have nothing to do.  Return mreq 
         * still busy, but the others unbusied.
         */

        if (blk == SWAPBLK_NONE)
                return(VM_PAGER_FAIL);

        /*
         * Get a swap buffer header to perform the IO
         */

        bp = getpbuf(&nsw_rcount);
        kva = (vm_offset_t) bp->b_data;

        /*
         * map our page(s) into kva for input
         *
         * NOTE: B_PAGING is set by pbgetvp()
         */

        pmap_qenter(kva, m + i, j - i);

        bp->b_iocmd = BIO_READ;
        bp->b_iodone = swp_pager_async_iodone;
        bp->b_rcred = crhold(proc0.p_ucred);
        bp->b_wcred = crhold(proc0.p_ucred);
        bp->b_data = (caddr_t) kva;
        bp->b_blkno = blk - (reqpage - i);
        bp->b_bcount = PAGE_SIZE * (j - i);
        bp->b_bufsize = PAGE_SIZE * (j - i);
        bp->b_pager.pg_reqpage = reqpage - i;

        {
                int k;

                for (k = i; k < j; ++k) {
                        bp->b_pages[k - i] = m[k];
                        vm_page_flag_set(m[k], PG_SWAPINPROG);
                }
        }
        bp->b_npages = j - i;

        pbgetvp(swapdev_vp, bp);

        cnt.v_swapin++;
        cnt.v_swappgsin += bp->b_npages;

        /*
         * We still hold the lock on mreq, and our automatic completion routine
         * does not remove it.
         */

        vm_object_pip_add(mreq->object, bp->b_npages);
        lastpindex = m[j-1]->pindex;

        /*
         * perform the I/O.  NOTE!!!  bp cannot be considered valid after
         * this point because we automatically release it on completion.
         * Instead, we look at the one page we are interested in which we
         * still hold a lock on even through the I/O completion.
         *
         * The other pages in our m[] array are also released on completion,
         * so we cannot assume they are valid anymore either.
         *
         * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
         */
        BUF_KERNPROC(bp);
        BUF_STRATEGY(bp);

        /*
         * wait for the page we want to complete.  PG_SWAPINPROG is always
         * cleared on completion.  If an I/O error occurs, SWAPBLK_NONE
         * is set in the meta-data.
         */

        s = splvm();

        while ((mreq->flags & PG_SWAPINPROG) != 0) {
                vm_page_flag_set(mreq, PG_WANTED | PG_REFERENCED);
                cnt.v_intrans++;
                if (tsleep(mreq, PSWP, "swread", hz*20)) {
                        printf(
                            "swap_pager: indefinite wait buffer: device:"
                                " %s, blkno: %ld, size: %ld\n",
                            devtoname(bp->b_dev), (long)bp->b_blkno,
                            bp->b_bcount
                        );
                }
        }

        splx(s);

        /*
         * mreq is left bussied after completion, but all the other pages
         * are freed.  If we had an unrecoverable read error the page will
         * not be valid.
         */

        if (mreq->valid != VM_PAGE_BITS_ALL) {
                return(VM_PAGER_ERROR);
        } else {
                return(VM_PAGER_OK);
        }

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

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

void
swap_pager_putpages(object, m, count, sync, rtvals)
        vm_object_t object;
        vm_page_t *m;
        int count;
        boolean_t sync;
        int *rtvals;
{
        int i;
        int n = 0;

        GIANT_REQUIRED;
        if (count && m[0]->object != object) {
                panic("swap_pager_getpages: object mismatch %p/%p", 
                    object, 
                    m[0]->object
                );
        }
        /*
         * Step 1
         *
         * Turn object into OBJT_SWAP
         * check for bogus sysops
         * force sync if not pageout process
         */

        if (object->type != OBJT_SWAP)
                swp_pager_meta_build(object, 0, SWAPBLK_NONE);

        if (curproc != pageproc)
                sync = TRUE;

        /*
         * Step 2
         *
         * Update nsw parameters from swap_async_max sysctl values.  
         * Do not let the sysop crash the machine with bogus numbers.
         */

        mtx_lock(&pbuf_mtx);
        if (swap_async_max != nsw_wcount_async_max) {
                int n;
                int s;

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

                /*
                 * Adjust difference ( if possible ).  If the current async
                 * count is too low, we may not be able to make the adjustment
                 * at this time.
                 */
                s = splvm();
                n -= nsw_wcount_async_max;
                if (nsw_wcount_async + n >= 0) {
                        nsw_wcount_async += n;
                        nsw_wcount_async_max += n;
                        wakeup(&nsw_wcount_async);
                }
                splx(s);
        }
        mtx_unlock(&pbuf_mtx);

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

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

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

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

                s = splvm();

                /*
                 * Get biggest block of swap we can.  If we fail, fall
                 * back and try to allocate a smaller block.  Don't go
                 * overboard trying to allocate space if it would overly
                 * fragment swap.
                 */
                while (
                    (blk = swp_pager_getswapspace(n)) == SWAPBLK_NONE &&
                    n > 4
                ) {
                        n >>= 1;
                }
                if (blk == SWAPBLK_NONE) {
                        for (j = 0; j < n; ++j)
                                rtvals[i+j] = VM_PAGER_FAIL;
                        splx(s);
                        continue;
                }

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

                /*
                 * All I/O parameters have been satisfied, build the I/O
                 * request and assign the swap space.
                 *
                 * NOTE: B_PAGING is set by pbgetvp()
                 */

                if (sync == TRUE) {
                        bp = getpbuf(&nsw_wcount_sync);
                } else {
                        bp = getpbuf(&nsw_wcount_async);
                        bp->b_flags = B_ASYNC;
                }
                bp->b_iocmd = BIO_WRITE;
                bp->b_spc = NULL;       /* not used, but NULL-out anyway */

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

                bp->b_rcred = crhold(proc0.p_ucred);
                bp->b_wcred = crhold(proc0.p_ucred);
                bp->b_bcount = PAGE_SIZE * n;
                bp->b_bufsize = PAGE_SIZE * n;
                bp->b_blkno = blk;

                pbgetvp(swapdev_vp, bp);

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

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

                        vm_page_flag_set(mreq, PG_SWAPINPROG);
                        bp->b_pages[j] = mreq;
                }
                bp->b_npages = n;
                /*
                 * Must set dirty range for NFS to work.
                 */
                bp->b_dirtyoff = 0;
                bp->b_dirtyend = bp->b_bcount;

                cnt.v_swapout++;
                cnt.v_swappgsout += bp->b_npages;
                swapdev_vp->v_numoutput++;

                splx(s);

                /*
                 * asynchronous
                 *
                 * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
                 */

                if (sync == FALSE) {
                        bp->b_iodone = swp_pager_async_iodone;
                        BUF_KERNPROC(bp);
                        BUF_STRATEGY(bp);

                        for (j = 0; j < n; ++j)
                                rtvals[i+j] = VM_PAGER_PEND;
                        /* restart outter loop */
                        continue;
                }

                /*
                 * synchronous
                 *
                 * NOTE: b_blkno is destroyed by the call to VOP_STRATEGY
                 */

                bp->b_iodone = swp_pager_sync_iodone;
                BUF_STRATEGY(bp);

                /*
                 * Wait for the sync I/O to complete, then update rtvals.
                 * We just set the rtvals[] to VM_PAGER_PEND so we can call
                 * our async completion routine at the end, thus avoiding a
                 * double-free.
                 */
                s = splbio();

                while ((bp->b_flags & B_DONE) == 0) {
                        tsleep(bp, PVM, "swwrt", 0);
                }

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

                /*
                 * Now that we are through with the bp, we can call the
                 * normal async completion, which frees everything up.
                 */

                swp_pager_async_iodone(bp);
                splx(s);
        }
}

/*
 *      swap_pager_sync_iodone:
 *
 *      Completion routine for synchronous reads and writes from/to swap.
 *      We just mark the bp is complete and wake up anyone waiting on it.
 *
 *      This routine may not block.  This routine is called at splbio() or better.
 */

static void
swp_pager_sync_iodone(bp)
        struct buf *bp;
{
        bp->b_flags |= B_DONE;
        bp->b_flags &= ~B_ASYNC;
        wakeup(bp);
}

/*
 *      swp_pager_async_iodone:
 *
 *      Completion routine for asynchronous reads and writes from/to swap.
 *      Also called manually by synchronous code to finish up a bp.
 *
 *      For READ operations, the pages are PG_BUSY'd.  For WRITE operations, 
 *      the pages are vm_page_t->busy'd.  For READ operations, we PG_BUSY 
 *      unbusy all pages except the 'main' request page.  For WRITE 
 *      operations, we vm_page_t->busy'd unbusy all pages ( we can do this 
 *      because we marked them all VM_PAGER_PEND on return from putpages ).
 *
 *      This routine may not block.
 *      This routine is called at splbio() or better
 *
 *      We up ourselves to splvm() as required for various vm_page related
 *      calls.
 */

static void
swp_pager_async_iodone(bp)
        struct buf *bp;
{
        int s;
        int i;
        vm_object_t object = NULL;

        GIANT_REQUIRED;

        bp->b_flags |= B_DONE;

        /*
         * report error
         */

        if (bp->b_ioflags & BIO_ERROR) {
                printf(
                    "swap_pager: I/O error - %s failed; blkno %ld,"
                        "size %ld, error %d\n",
                    ((bp->b_iocmd == BIO_READ) ? "pagein" : "pageout"),
                    (long)bp->b_blkno, 
                    (long)bp->b_bcount,
                    bp->b_error
                );
        }

        /*
         * set object, raise to splvm().
         */

        if (bp->b_npages)
                object = bp->b_pages[0]->object;
        s = splvm();

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

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

        for (i = 0; i < bp->b_npages; ++i) {
                vm_page_t m = bp->b_pages[i];

                vm_page_flag_clear(m, PG_SWAPINPROG);

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

                        if (bp->b_iocmd == BIO_READ) {
                                /*
                                 * When reading, reqpage needs to stay
                                 * locked for the parent, but all other
                                 * pages can be freed.  We still want to
                                 * wakeup the parent waiting on the page,
                                 * though.  ( also: pg_reqpage can be -1 and 
                                 * not match anything ).
                                 *
                                 * We have to wake specifically requested pages
                                 * up too because we cleared PG_SWAPINPROG and
                                 * someone may be waiting for that.
                                 *
                                 * NOTE: for reads, m->dirty will probably
                                 * be overridden by the original caller of
                                 * getpages so don't play cute tricks here.
                                 *
                                 * XXX IT IS NOT LEGAL TO FREE THE PAGE HERE
                                 * AS THIS MESSES WITH object->memq, and it is
                                 * not legal to mess with object->memq from an
                                 * interrupt.
                                 */

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

                                if (i != bp->b_pager.pg_reqpage)
                                        vm_page_free(m);
                                else
                                        vm_page_flash(m);
                                /*
                                 * If i == bp->b_pager.pg_reqpage, do not wake 
                                 * the page up.  The caller needs to.
                                 */
                        } else {
                                /*
                                 * If a write error occurs, reactivate page
                                 * so it doesn't clog the inactive list,
                                 * then finish the I/O.
                                 */
                                vm_page_dirty(m);
                                vm_page_activate(m);
                                vm_page_io_finish(m);
                        }
                } else if (bp->b_iocmd == BIO_READ) {
                        /*
                         * For read success, clear dirty bits.  Nobody should
                         * have this page mapped but don't take any chances,
                         * make sure the pmap modify bits are also cleared.
                         *
                         * NOTE: for reads, m->dirty will probably be 
                         * overridden by the original caller of getpages so
                         * we cannot set them in order to free the underlying
                         * swap in a low-swap situation.  I don't think we'd
                         * want to do that anyway, but it was an optimization
                         * that existed in the old swapper for a time before
                         * it got ripped out due to precisely this problem.
                         *
                         * clear PG_ZERO in page.
                         *
                         * If not the requested page then deactivate it.
                         *
                         * Note that the requested page, reqpage, is left
                         * busied, but we still have to wake it up.  The
                         * other pages are released (unbusied) by 
                         * vm_page_wakeup().  We do not set reqpage's
                         * valid bits here, it is up to the caller.
                         */

                        pmap_clear_modify(m);
                        m->valid = VM_PAGE_BITS_ALL;
                        vm_page_undirty(m);
                        vm_page_flag_clear(m, PG_ZERO);

                        /*
                         * We have to wake specifically requested pages
                         * up too because we cleared PG_SWAPINPROG and
                         * could be waiting for it in getpages.  However,
                         * be sure to not unbusy getpages specifically
                         * requested page - getpages expects it to be 
                         * left busy.
                         */
                        if (i != bp->b_pager.pg_reqpage) {
                                vm_page_deactivate(m);
                                vm_page_wakeup(m);
                        } else {
                                vm_page_flash(m);
                        }
                } else {
                        /*
                         * For write success, clear the modify and dirty 
                         * status, then finish the I/O ( which decrements the 
                         * busy count and possibly wakes waiter's up ).
                         */
                        pmap_clear_modify(m);
                        vm_page_undirty(m);
                        vm_page_io_finish(m);
                        if (!vm_page_count_severe() || !vm_page_try_to_cache(m))
                                vm_page_protect(m, VM_PROT_READ);
                }
        }

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

        if (object)
                vm_object_pip_wakeupn(object, bp->b_npages);

        /*
         * release the physical I/O buffer
         */

        relpbuf(
            bp, 
            ((bp->b_iocmd == BIO_READ) ? &nsw_rcount : 
                ((bp->b_flags & B_ASYNC) ? 
                    &nsw_wcount_async : 
                    &nsw_wcount_sync
                )
            )
        );
        splx(s);
}

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

/*
 * SWP_PAGER_HASH() -   hash swap meta data
 *
 *      This is an inline helper function which hashes the swapblk given
 *      the object and page index.  It returns a pointer to a pointer
 *      to the object, or a pointer to a NULL pointer if it could not
 *      find a swapblk.
 *
 *      This routine must be called at splvm().
 */

static __inline struct swblock **
swp_pager_hash(vm_object_t object, vm_pindex_t index)
{
        struct swblock **pswap;
        struct swblock *swap;

        index &= ~SWAP_META_MASK;
        pswap = &swhash[(index ^ (int)(intptr_t)object) & swhash_mask];

        while ((swap = *pswap) != NULL) {
                if (swap->swb_object == object &&
                    swap->swb_index == index
                ) {
                        break;
                }
                pswap = &swap->swb_hnext;
        }
        return(pswap);
}

/*
 * SWP_PAGER_META_BUILD() -     add swap block to swap meta data for object
 *
 *      We first convert the object to a swap object if it is a default
 *      object.
 *
 *      The specified swapblk is added to the object's swap metadata.  If
 *      the swapblk is not valid, it is freed instead.  Any previously
 *      assigned swapblk is freed.
 *
 *      This routine must be called at splvm(), except when used to convert
 *      an OBJT_DEFAULT object into an OBJT_SWAP object.
 */

static void
swp_pager_meta_build(
        vm_object_t object, 
        vm_pindex_t index,
        daddr_t swapblk
) {
        struct swblock *swap;
        struct swblock **pswap;

        GIANT_REQUIRED;
        /*
         * Convert default object to swap object if necessary
         */

        if (object->type != OBJT_SWAP) {
                object->type = OBJT_SWAP;
                object->un_pager.swp.swp_bcount = 0;

                mtx_lock(&sw_alloc_mtx);
                if (object->handle != NULL) {
                        TAILQ_INSERT_TAIL(
                            NOBJLIST(object->handle),
                            object, 
                            pager_object_list
                        );
                } else {
                        TAILQ_INSERT_TAIL(
                            &swap_pager_un_object_list,
                            object, 
                            pager_object_list
                        );
                }
                mtx_unlock(&sw_alloc_mtx);
        }
        
        /*
         * Locate hash entry.  If not found create, but if we aren't adding
         * anything just return.  If we run out of space in the map we wait
         * and, since the hash table may have changed, retry.
         */

retry:
        pswap = swp_pager_hash(object, index);

        if ((swap = *pswap) == NULL) {
                int i;

                if (swapblk == SWAPBLK_NONE)
                        return;

                swap = *pswap = zalloc(swap_zone);
                if (swap == NULL) {
                        VM_WAIT;
                        goto retry;
                }
                swap->swb_hnext = NULL;
                swap->swb_object = object;
                swap->swb_index = index & ~SWAP_META_MASK;
                swap->swb_count = 0;

                ++object->un_pager.swp.swp_bcount;

                for (i = 0; i < SWAP_META_PAGES; ++i)
                        swap->swb_pages[i] = SWAPBLK_NONE;
        }

        /*
         * Delete prior contents of metadata
         */

        index &= SWAP_META_MASK;

        if (swap->swb_pages[index] != SWAPBLK_NONE) {
                swp_pager_freeswapspace(swap->swb_pages[index], 1);
                --swap->swb_count;
        }

        /*
         * Enter block into metadata
         */

        swap->swb_pages[index] = swapblk;
        if (swapblk != SWAPBLK_NONE)
                ++swap->swb_count;
}

/*
 * SWP_PAGER_META_FREE() - free a range of blocks in the object's swap metadata
 *
 *      The requested range of blocks is freed, with any associated swap 
 *      returned to the swap bitmap.
 *
 *      This routine will free swap metadata structures as they are cleaned 
 *      out.  This routine does *NOT* operate on swap metadata associated
 *      with resident pages.
 *
 *      This routine must be called at splvm()
 */

static void
swp_pager_meta_free(vm_object_t object, vm_pindex_t index, daddr_t count)
{
        GIANT_REQUIRED;

        if (object->type != OBJT_SWAP)
                return;

        while (count > 0) {
                struct swblock **pswap;
                struct swblock *swap;

                pswap = swp_pager_hash(object, index);

                if ((swap = *pswap) != NULL) {
                        daddr_t v = swap->swb_pages[index & SWAP_META_MASK];

                        if (v != SWAPBLK_NONE) {
                                swp_pager_freeswapspace(v, 1);
                                swap->swb_pages[index & SWAP_META_MASK] =
                                        SWAPBLK_NONE;
                                if (--swap->swb_count == 0) {
                                        *pswap = swap->swb_hnext;
                                        zfree(swap_zone, swap);
                                        --object->un_pager.swp.swp_bcount;
                                }
                        }
                        --count;
                        ++index;
                } else {
                        int n = SWAP_META_PAGES - (index & SWAP_META_MASK);
                        count -= n;
                        index += n;
                }
        }
}

/*
 * SWP_PAGER_META_FREE_ALL() - destroy all swap metadata associated with object
 *
 *      This routine locates and destroys all swap metadata associated with
 *      an object.
 *
 *      This routine must be called at splvm()
 */

static void
swp_pager_meta_free_all(vm_object_t object)
{
        daddr_t index = 0;

        GIANT_REQUIRED;
        
        if (object->type != OBJT_SWAP)
                return;

        while (object->un_pager.swp.swp_bcount) {
                struct swblock **pswap;
                struct swblock *swap;

                pswap = swp_pager_hash(object, index);
                if ((swap = *pswap) != NULL) {
                        int i;

                        for (i = 0; i < SWAP_META_PAGES; ++i) {
                                daddr_t v = swap->swb_pages[i];
                                if (v != SWAPBLK_NONE) {
                                        --swap->swb_count;
                                        swp_pager_freeswapspace(v, 1);
                                }
                        }
                        if (swap->swb_count != 0)
                                panic("swap_pager_meta_free_all: swb_count != 0");
                        *pswap = swap->swb_hnext;
                        zfree(swap_zone, swap);
                        --object->un_pager.swp.swp_bcount;
                }
                index += SWAP_META_PAGES;
                if (index > 0x20000000)
                        panic("swp_pager_meta_free_all: failed to locate all swap meta blocks");
        }
}

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

static daddr_t
swp_pager_meta_ctl(
        vm_object_t object,
        vm_pindex_t index,
        int flags
) {
        struct swblock **pswap;
        struct swblock *swap;
        daddr_t r1;

        GIANT_REQUIRED;
        /*
         * The meta data only exists of the object is OBJT_SWAP 
         * and even then might not be allocated yet.
         */

        if (object->type != OBJT_SWAP)
                return(SWAPBLK_NONE);

        r1 = SWAPBLK_NONE;
        pswap = swp_pager_hash(object, index);

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

                if (r1 != SWAPBLK_NONE) {
                        if (flags & SWM_FREE) {
                                swp_pager_freeswapspace(r1, 1);
                                r1 = SWAPBLK_NONE;
                        }
                        if (flags & (SWM_FREE|SWM_POP)) {
                                swap->swb_pages[index] = SWAPBLK_NONE;
                                if (--swap->swb_count == 0) {
                                        *pswap = swap->swb_hnext;
                                        zfree(swap_zone, swap);
                                        --object->un_pager.swp.swp_bcount;
                                }
                        } 
                }
        }
        return(r1);
}

/********************************************************
 *              CHAINING FUNCTIONS                      *
 ********************************************************
 *
 *      These functions support recursion of I/O operations
 *      on bp's, typically by chaining one or more 'child' bp's
 *      to the parent.  Synchronous, asynchronous, and semi-synchronous
 *      chaining is possible.
 */

/*
 *      vm_pager_chain_iodone:
 *
 *      io completion routine for child bp.  Currently we fudge a bit
 *      on dealing with b_resid.   Since users of these routines may issue
 *      multiple children simultaneously, sequencing of the error can be lost.
 */

static void
vm_pager_chain_iodone(struct buf *nbp)
{
        struct bio *bp;
        u_int *count;

        bp = nbp->b_caller1;
        count = (u_int *)&(bp->bio_driver1);
        if (bp != NULL) {
                if (nbp->b_ioflags & BIO_ERROR) {
                        bp->bio_flags |= BIO_ERROR;
                        bp->bio_error = nbp->b_error;
                } else if (nbp->b_resid != 0) {
                        bp->bio_flags |= BIO_ERROR;
                        bp->bio_error = EINVAL;
                } else {
                        bp->bio_resid -= nbp->b_bcount;
                }
                nbp->b_caller1 = NULL;
                --(*count);
                if (bp->bio_flags & BIO_FLAG1) {
                        bp->bio_flags &= ~BIO_FLAG1;
                        wakeup(bp);
                }
        }
        nbp->b_flags |= B_DONE;
        nbp->b_flags &= ~B_ASYNC;
        relpbuf(nbp, NULL);
}

/*
 *      getchainbuf:
 *
 *      Obtain a physical buffer and chain it to its parent buffer.  When
 *      I/O completes, the parent buffer will be B_SIGNAL'd.  Errors are
 *      automatically propagated to the parent
 */

struct buf *
getchainbuf(struct bio *bp, struct vnode *vp, int flags)
{
        struct buf *nbp;
        u_int *count;

        GIANT_REQUIRED;
        nbp = getpbuf(NULL);
        count = (u_int *)&(bp->bio_driver1);

        nbp->b_caller1 = bp;
        ++(*count);

        if (*count > 4)
                waitchainbuf(bp, 4, 0);

        nbp->b_iocmd = bp->bio_cmd;
        nbp->b_ioflags = bp->bio_flags & BIO_ORDERED;
        nbp->b_flags = flags;
        nbp->b_rcred = crhold(proc0.p_ucred);
        nbp->b_wcred = crhold(proc0.p_ucred);
        nbp->b_iodone = vm_pager_chain_iodone;

        if (vp)
                pbgetvp(vp, nbp);
        return(nbp);
}

void
flushchainbuf(struct buf *nbp)
{
        GIANT_REQUIRED;
        if (nbp->b_bcount) {
                nbp->b_bufsize = nbp->b_bcount;
                if (nbp->b_iocmd == BIO_WRITE)
                        nbp->b_dirtyend = nbp->b_bcount;
                BUF_KERNPROC(nbp);
                BUF_STRATEGY(nbp);
        } else {
                bufdone(nbp);
        }
}

static void
waitchainbuf(struct bio *bp, int limit, int done)
{
        int s;
        u_int *count;

        GIANT_REQUIRED;
        count = (u_int *)&(bp->bio_driver1);
        s = splbio();
        while (*count > limit) {
                bp->bio_flags |= BIO_FLAG1;
                tsleep(bp, PRIBIO + 4, "bpchain", 0);
        }
        if (done) {
                if (bp->bio_resid != 0 && !(bp->bio_flags & BIO_ERROR)) {
                        bp->bio_flags |= BIO_ERROR;
                        bp->bio_error = EINVAL;
                }
                biodone(bp);
        }
        splx(s);
}