/*
 * (MPSAFE)
 *
 * Copyright (c) 2010,2019 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.
 */

/*
 * Implement the swapcache daemon.  When enabled swap is assumed to be
 * configured on a fast storage device such as a SSD.  Swap is assigned
 * to clean vnode-backed pages in the inactive queue, clustered by object
 * if possible, and written out.  The swap assignment sticks around even
 * after the underlying pages have been recycled.
 *
 * The daemon manages write bandwidth based on sysctl settings to control
 * wear on the SSD.
 *
 * The vnode strategy code will check for the swap assignments and divert
 * reads to the swap device when the data is present in the swapcache.
 *
 * This operates on both regular files and the block device vnodes used by
 * filesystems to manage meta-data.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/kthread.h>
#include <sys/resourcevar.h>
#include <sys/signalvar.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/sysctl.h>
#include <sys/eventhandler.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <sys/lock.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>
#include <vm/vm_extern.h>

#include <sys/spinlock2.h>
#include <vm/vm_page2.h>

struct swmarker {
        struct vm_object dummy_obj;
        struct vm_object *save_obj;
        vm_ooffset_t save_off;
};

typedef struct swmarker swmarker_t;

/* the kernel process "vm_pageout"*/
static int vm_swapcached_flush (vm_page_t m, int isblkdev);
static int vm_swapcache_test(vm_page_t m);
static int vm_swapcache_writing_heuristic(void);
static int vm_swapcache_writing(vm_page_t marker, int count, int scount);
static void vm_swapcache_cleaning(swmarker_t *marker,
                        struct vm_object_hash **swindexp);
static void vm_swapcache_movemarker(swmarker_t *marker,
                        struct vm_object_hash *swindex, vm_object_t object);
struct thread *swapcached_thread;

SYSCTL_NODE(_vm, OID_AUTO, swapcache, CTLFLAG_RW, NULL, NULL);

int vm_swapcache_read_enable;
static long vm_swapcache_wtrigger;
static int vm_swapcache_sleep;
static int vm_swapcache_maxscan = PQ_L2_SIZE * 8;
static int vm_swapcache_maxlaunder = PQ_L2_SIZE * 4;
static int vm_swapcache_data_enable = 0;
static int vm_swapcache_meta_enable = 0;
static int vm_swapcache_maxswappct = 75;
static int vm_swapcache_hysteresis;
static int vm_swapcache_min_hysteresis;
int vm_swapcache_use_chflags = 0;       /* require chflags cache */
static int64_t vm_swapcache_minburst = 10000000LL;      /* 10MB */
static int64_t vm_swapcache_curburst = 4000000000LL;    /* 4G after boot */
static int64_t vm_swapcache_maxburst = 2000000000LL;    /* 2G nominal max */
static int64_t vm_swapcache_accrate = 100000LL;         /* 100K/s */
static int64_t vm_swapcache_write_count;
static int64_t vm_swapcache_maxfilesize;
static int64_t vm_swapcache_cleanperobj = 16*1024*1024;

SYSCTL_INT(_vm_swapcache, OID_AUTO, maxlaunder,
        CTLFLAG_RW, &vm_swapcache_maxlaunder, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, maxscan,
        CTLFLAG_RW, &vm_swapcache_maxscan, 0, "");

SYSCTL_INT(_vm_swapcache, OID_AUTO, data_enable,
        CTLFLAG_RW, &vm_swapcache_data_enable, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, meta_enable,
        CTLFLAG_RW, &vm_swapcache_meta_enable, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, read_enable,
        CTLFLAG_RW, &vm_swapcache_read_enable, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, maxswappct,
        CTLFLAG_RW, &vm_swapcache_maxswappct, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, hysteresis,
        CTLFLAG_RD, &vm_swapcache_hysteresis, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, min_hysteresis,
        CTLFLAG_RW, &vm_swapcache_min_hysteresis, 0, "");
SYSCTL_INT(_vm_swapcache, OID_AUTO, use_chflags,
        CTLFLAG_RW, &vm_swapcache_use_chflags, 0, "");

SYSCTL_QUAD(_vm_swapcache, OID_AUTO, minburst,
        CTLFLAG_RW, &vm_swapcache_minburst, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, curburst,
        CTLFLAG_RW, &vm_swapcache_curburst, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, maxburst,
        CTLFLAG_RW, &vm_swapcache_maxburst, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, maxfilesize,
        CTLFLAG_RW, &vm_swapcache_maxfilesize, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, accrate,
        CTLFLAG_RW, &vm_swapcache_accrate, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, write_count,
        CTLFLAG_RW, &vm_swapcache_write_count, 0, "");
SYSCTL_QUAD(_vm_swapcache, OID_AUTO, cleanperobj,
        CTLFLAG_RW, &vm_swapcache_cleanperobj, 0, "");

#define SWAPMAX(adj)    \
        ((int64_t)vm_swap_max * (vm_swapcache_maxswappct + (adj)) / 100)

/*
 * When shutting down the machine we want to stop swapcache operation
 * immediately so swap is not accessed after devices have been shuttered.
 */
static void
shutdown_swapcache(void *arg __unused)
{
        vm_swapcache_read_enable = 0;
        vm_swapcache_data_enable = 0;
        vm_swapcache_meta_enable = 0;
        wakeup(&vm_swapcache_sleep);    /* shortcut 5-second wait */
}

/*
 * vm_swapcached is the high level pageout daemon.
 *
 * No requirements.
 */
static void
vm_swapcached_thread(void)
{
        enum { SWAPC_WRITING, SWAPC_CLEANING } state = SWAPC_WRITING;
        enum { SWAPB_BURSTING, SWAPB_RECOVERING } burst = SWAPB_BURSTING;
        static struct vm_page page_marker[PQ_L2_SIZE];
        static swmarker_t swmarker;
        static struct vm_object_hash *swindex;
        int q;

        /*
         * Thread setup
         */
        curthread->td_flags |= TDF_SYSTHREAD;
        EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_kproc,
                              swapcached_thread, SHUTDOWN_PRI_FIRST);
        EVENTHANDLER_REGISTER(shutdown_pre_sync, shutdown_swapcache,
                              NULL, SHUTDOWN_PRI_SECOND);

        /*
         * Initialize our marker for the inactive scan (SWAPC_WRITING)
         */
        bzero(&page_marker, sizeof(page_marker));
        for (q = 0; q < PQ_L2_SIZE; ++q) {
                page_marker[q].flags = PG_FICTITIOUS | PG_MARKER;
                page_marker[q].busy_count = PBUSY_LOCKED;
                page_marker[q].queue = PQ_INACTIVE + q;
                page_marker[q].pc = q;
                page_marker[q].wire_count = 1;
                vm_page_queues_spin_lock(PQ_INACTIVE + q);
                TAILQ_INSERT_HEAD(
                        &vm_page_queues[PQ_INACTIVE + q].pl,
                        &page_marker[q], pageq);
                vm_page_queues_spin_unlock(PQ_INACTIVE + q);
        }

        vm_swapcache_min_hysteresis = 1024;
        vm_swapcache_hysteresis = vm_swapcache_min_hysteresis;
        vm_swapcache_wtrigger = -vm_swapcache_hysteresis;

        /*
         * Initialize our marker for the vm_object scan (SWAPC_CLEANING)
         */
        bzero(&swmarker, sizeof(swmarker));
        swmarker.dummy_obj.type = OBJT_MARKER;
        swindex = &vm_object_hash[0];
        lwkt_gettoken(&swindex->token);
        TAILQ_INSERT_HEAD(&swindex->list, &swmarker.dummy_obj, object_entry);
        lwkt_reltoken(&swindex->token);

        for (;;) {
                int reached_end;
                int scount;
                int count;

                /*
                 * Handle shutdown
                 */
                kproc_suspend_loop();

                /*
                 * Check every 5 seconds when not enabled or if no swap
                 * is present.
                 */
                if ((vm_swapcache_data_enable == 0 &&
                     vm_swapcache_meta_enable == 0 &&
                     vm_swap_cache_use <= SWAPMAX(0)) ||
                    vm_swap_max == 0) {
                        tsleep(&vm_swapcache_sleep, 0, "csleep", hz * 5);
                        continue;
                }

                /*
                 * Polling rate when enabled is approximately 10 hz.
                 */
                tsleep(&vm_swapcache_sleep, 0, "csleep", hz / 10);

                /*
                 * State hysteresis.  Generate write activity up to 75% of
                 * swap, then clean out swap assignments down to 70%, then
                 * repeat.
                 */
                if (state == SWAPC_WRITING) {
                        if (vm_swap_cache_use > SWAPMAX(0))
                                state = SWAPC_CLEANING;
                } else {
                        if (vm_swap_cache_use < SWAPMAX(-10))
                                state = SWAPC_WRITING;
                }

                /*
                 * We are allowed to continue accumulating burst value
                 * in either state.  Allow the user to set curburst > maxburst
                 * for the initial load-in.
                 */
                if (vm_swapcache_curburst < vm_swapcache_maxburst) {
                        vm_swapcache_curburst += vm_swapcache_accrate / 10;
                        if (vm_swapcache_curburst > vm_swapcache_maxburst)
                                vm_swapcache_curburst = vm_swapcache_maxburst;
                }

                /*
                 * We don't want to nickle-and-dime the scan as that will
                 * create unnecessary fragmentation.  The minimum burst
                 * is one-seconds worth of accumulation.
                 */
                if (state != SWAPC_WRITING) {
                        vm_swapcache_cleaning(&swmarker, &swindex);
                        continue;
                }
                if (vm_swapcache_curburst < vm_swapcache_accrate)
                        continue;

                reached_end = 0;
                count = vm_swapcache_maxlaunder / PQ_L2_SIZE + 2;
                scount = vm_swapcache_maxscan / PQ_L2_SIZE + 2;

                if (burst == SWAPB_BURSTING) {
                        if (vm_swapcache_writing_heuristic()) {
                                for (q = 0; q < PQ_L2_SIZE; ++q) {
                                        reached_end +=
                                                vm_swapcache_writing(
                                                        &page_marker[q],
                                                        count,
                                                        scount);
                                }
                        }
                        if (vm_swapcache_curburst <= 0)
                                burst = SWAPB_RECOVERING;
                } else if (vm_swapcache_curburst > vm_swapcache_minburst) {
                        if (vm_swapcache_writing_heuristic()) {
                                for (q = 0; q < PQ_L2_SIZE; ++q) {
                                        reached_end +=
                                                vm_swapcache_writing(
                                                        &page_marker[q],
                                                        count,
                                                        scount);
                                }
                        }
                        burst = SWAPB_BURSTING;
                }
                if (reached_end == PQ_L2_SIZE) {
                        vm_swapcache_wtrigger = -vm_swapcache_hysteresis;
                }
        }

        /*
         * Cleanup (NOT REACHED)
         */
        for (q = 0; q < PQ_L2_SIZE; ++q) {
                vm_page_queues_spin_lock(PQ_INACTIVE + q);
                TAILQ_REMOVE(
                        &vm_page_queues[PQ_INACTIVE + q].pl,
                        &page_marker[q], pageq);
                vm_page_queues_spin_unlock(PQ_INACTIVE + q);
        }

        lwkt_gettoken(&swindex->token);
        TAILQ_REMOVE(&swindex->list, &swmarker.dummy_obj, object_entry);
        lwkt_reltoken(&swindex->token);
}

static struct kproc_desc swpc_kp = {
        "swapcached",
        vm_swapcached_thread,
        &swapcached_thread
};
SYSINIT(swapcached, SI_SUB_KTHREAD_PAGE, SI_ORDER_SECOND, kproc_start, &swpc_kp);

/*
 * Deal with an overflow of the heuristic counter or if the user
 * manually changes the hysteresis.
 *
 * Try to avoid small incremental pageouts by waiting for enough
 * pages to buildup in the inactive queue to hopefully get a good
 * burst in.  This heuristic is bumped by the VM system and reset
 * when our scan hits the end of the queue.
 *
 * Return TRUE if we need to take a writing pass.
 */
static int
vm_swapcache_writing_heuristic(void)
{
        int hyst;
        int q;
        long adds;

        hyst = vmstats.v_inactive_count / 4;
        if (hyst < vm_swapcache_min_hysteresis)
                hyst = vm_swapcache_min_hysteresis;
        cpu_ccfence();
        vm_swapcache_hysteresis = hyst;

        adds = 0;
        for (q = PQ_INACTIVE; q < PQ_INACTIVE + PQ_L2_SIZE; ++q) {
                adds += atomic_swap_long(&vm_page_queues[q].adds, 0);
        }
        vm_swapcache_wtrigger += adds;
        if (vm_swapcache_wtrigger < -hyst)
                vm_swapcache_wtrigger = -hyst;
        return (vm_swapcache_wtrigger >= 0);
}

/*
 * Take a writing pass on one of the inactive queues, return non-zero if
 * we hit the end of the queue.
 */
static int
vm_swapcache_writing(vm_page_t marker, int count, int scount)
{
        vm_object_t object;
        struct vnode *vp;
        vm_page_t m;
        int isblkdev;

        /*
         * Scan the inactive queue from our marker to locate
         * suitable pages to push to the swap cache.
         *
         * We are looking for clean vnode-backed pages.
         */
        vm_page_queues_spin_lock(marker->queue);
        while ((m = TAILQ_NEXT(marker, pageq)) != NULL &&
               count > 0 && scount-- > 0) {
                KKASSERT(m->queue == marker->queue);

                /*
                 * Stop using swap if paniced, dumping, or dumped.
                 * Don't try to write if our curburst has been exhausted.
                 */
                if (panicstr || dumping)
                        break;
                if (vm_swapcache_curburst < 0)
                        break;

                /*
                 * Move marker
                 */
                TAILQ_REMOVE(
                        &vm_page_queues[marker->queue].pl, marker, pageq);
                TAILQ_INSERT_AFTER(
                        &vm_page_queues[marker->queue].pl, m, marker, pageq);

                /*
                 * Ignore markers and ignore pages that already have a swap
                 * assignment.
                 */
                if (m->flags & (PG_MARKER | PG_SWAPPED))
                        continue;
                if (vm_page_busy_try(m, TRUE))
                        continue;
                vm_page_queues_spin_unlock(marker->queue);

                if ((object = m->object) == NULL) {
                        vm_page_wakeup(m);
                        vm_page_queues_spin_lock(marker->queue);
                        continue;
                }
                vm_object_hold(object);
                if (m->object != object) {
                        vm_object_drop(object);
                        vm_page_wakeup(m);
                        vm_page_queues_spin_lock(marker->queue);
                        continue;
                }
                if (vm_swapcache_test(m)) {
                        vm_object_drop(object);
                        vm_page_wakeup(m);
                        vm_page_queues_spin_lock(marker->queue);
                        continue;
                }

                vp = object->handle;
                if (vp == NULL) {
                        vm_object_drop(object);
                        vm_page_wakeup(m);
                        vm_page_queues_spin_lock(marker->queue);
                        continue;
                }

                switch(vp->v_type) {
                case VREG:
                        /*
                         * PG_NOTMETA generically means 'don't swapcache this',
                         * and HAMMER will set this for regular data buffers
                         * (and leave it unset for meta-data buffers) as
                         * appropriate when double buffering is enabled.
                         */
                        if (m->flags & PG_NOTMETA) {
                                vm_object_drop(object);
                                vm_page_wakeup(m);
                                vm_page_queues_spin_lock(marker->queue);
                                continue;
                        }

                        /*
                         * If data_enable is 0 do not try to swapcache data.
                         * If use_chflags is set then only swapcache data for
                         * VSWAPCACHE marked vnodes, otherwise any vnode.
                         */
                        if (vm_swapcache_data_enable == 0 ||
                            ((vp->v_flag & VSWAPCACHE) == 0 &&
                             vm_swapcache_use_chflags)) {
                                vm_object_drop(object);
                                vm_page_wakeup(m);
                                vm_page_queues_spin_lock(marker->queue);
                                continue;
                        }
                        if (vm_swapcache_maxfilesize &&
                            object->size >
                            (vm_swapcache_maxfilesize >> PAGE_SHIFT)) {
                                vm_object_drop(object);
                                vm_page_wakeup(m);
                                vm_page_queues_spin_lock(marker->queue);
                                continue;
                        }
                        isblkdev = 0;
                        break;
                case VCHR:
                        /*
                         * PG_NOTMETA generically means 'don't swapcache this',
                         * and HAMMER will set this for regular data buffers
                         * (and leave it unset for meta-data buffers) as
                         * appropriate when double buffering is enabled.
                         */
                        if (m->flags & PG_NOTMETA) {
                                vm_object_drop(object);
                                vm_page_wakeup(m);
                                vm_page_queues_spin_lock(marker->queue);
                                continue;
                        }
                        if (vm_swapcache_meta_enable == 0) {
                                vm_object_drop(object);
                                vm_page_wakeup(m);
                                vm_page_queues_spin_lock(marker->queue);
                                continue;
                        }
                        isblkdev = 1;
                        break;
                default:
                        vm_object_drop(object);
                        vm_page_wakeup(m);
                        vm_page_queues_spin_lock(marker->queue);
                        continue;
                }


                /*
                 * Assign swap and initiate I/O.
                 *
                 * (adjust for the --count which also occurs in the loop)
                 */
                count -= vm_swapcached_flush(m, isblkdev);

                /*
                 * Setup for next loop using marker.
                 */
                vm_object_drop(object);
                vm_page_queues_spin_lock(marker->queue);
        }

        /*
         * The marker could wind up at the end, which is ok.  If we hit the
         * end of the list adjust the heuristic.
         *
         * Earlier inactive pages that were dirty and become clean
         * are typically moved to the end of PQ_INACTIVE by virtue
         * of vfs_vmio_release() when they become unwired from the
         * buffer cache.
         */
        vm_page_queues_spin_unlock(marker->queue);

        /*
         * m invalid but can be used to test for NULL
         */
        return (m == NULL);
}

/*
 * Flush the specified page using the swap_pager.  The page
 * must be busied by the caller and its disposition will become
 * the responsibility of this function.
 *
 * Try to collect surrounding pages, including pages which may
 * have already been assigned swap.  Try to cluster within a
 * contiguous aligned SMAP_META_PAGES (typ 16 x PAGE_SIZE) block
 * to match what swap_pager_putpages() can do.
 *
 * We also want to try to match against the buffer cache blocksize
 * but we don't really know what it is here.  Since the buffer cache
 * wires and unwires pages in groups the fact that we skip wired pages
 * should be sufficient.
 *
 * Returns a count of pages we might have flushed (minimum 1)
 */
static
int
vm_swapcached_flush(vm_page_t m, int isblkdev)
{
        vm_object_t object;
        vm_page_t marray[SWAP_META_PAGES];
        vm_pindex_t basei;
        int rtvals[SWAP_META_PAGES];
        int x;
        int i;
        int j;
        int count;
        int error;

        vm_page_io_start(m);
        vm_page_protect(m, VM_PROT_READ);
        object = m->object;
        vm_object_hold(object);

        /*
         * Try to cluster around (m), keeping in mind that the swap pager
         * can only do SMAP_META_PAGES worth of continguous write.
         */
        x = (int)m->pindex & SWAP_META_MASK;
        marray[x] = m;
        basei = m->pindex;
        vm_page_wakeup(m);

        for (i = x - 1; i >= 0; --i) {
                m = vm_page_lookup_busy_try(object, basei - x + i,
                                            TRUE, &error);
                if (error || m == NULL)
                        break;
                if (vm_swapcache_test(m)) {
                        vm_page_wakeup(m);
                        break;
                }
                if (isblkdev && (m->flags & PG_NOTMETA)) {
                        vm_page_wakeup(m);
                        break;
                }
                vm_page_io_start(m);
                vm_page_protect(m, VM_PROT_READ);
                if (m->queue - m->pc == PQ_CACHE) {
                        vm_page_unqueue_nowakeup(m);
                        vm_page_deactivate(m);
                }
                marray[i] = m;
                vm_page_wakeup(m);
        }
        ++i;

        for (j = x + 1; j < SWAP_META_PAGES; ++j) {
                m = vm_page_lookup_busy_try(object, basei - x + j,
                                            TRUE, &error);
                if (error || m == NULL)
                        break;
                if (vm_swapcache_test(m)) {
                        vm_page_wakeup(m);
                        break;
                }
                if (isblkdev && (m->flags & PG_NOTMETA)) {
                        vm_page_wakeup(m);
                        break;
                }
                vm_page_io_start(m);
                vm_page_protect(m, VM_PROT_READ);
                if (m->queue - m->pc == PQ_CACHE) {
                        vm_page_unqueue_nowakeup(m);
                        vm_page_deactivate(m);
                }
                marray[j] = m;
                vm_page_wakeup(m);
        }

        count = j - i;
        vm_object_pip_add(object, count);
        swap_pager_putpages(object, marray + i, count, FALSE, rtvals + i);
        vm_swapcache_write_count += count * PAGE_SIZE;
        vm_swapcache_curburst -= count * PAGE_SIZE;

        while (i < j) {
                if (rtvals[i] != VM_PAGER_PEND) {
                        vm_page_busy_wait(marray[i], FALSE, "swppgfd");
                        vm_page_io_finish(marray[i]);
                        vm_page_wakeup(marray[i]);
                        vm_object_pip_wakeup(object);
                }
                ++i;
        }
        vm_object_drop(object);
        return(count);
}

/*
 * Test whether a VM page is suitable for writing to the swapcache.
 * Does not test m->queue, PG_MARKER, or PG_SWAPPED.
 *
 * Returns 0 on success, 1 on failure
 */
static int
vm_swapcache_test(vm_page_t m)
{
        vm_object_t object;

        if (m->flags & (PG_UNQUEUED | PG_FICTITIOUS))
                return(1);
        if (m->hold_count || m->wire_count)
                return(1);
        if (m->valid != VM_PAGE_BITS_ALL)
                return(1);
        if (m->dirty & m->valid)
                return(1);
        if ((object = m->object) == NULL)
                return(1);
        if (object->type != OBJT_VNODE ||
            (object->flags & OBJ_DEAD)) {
                return(1);
        }
        vm_page_test_dirty(m);
        if (m->dirty & m->valid)
                return(1);
        return(0);
}

/*
 * Cleaning pass.
 *
 * We clean whole objects up to 16MB
 */
static
void
vm_swapcache_cleaning(swmarker_t *marker, struct vm_object_hash **swindexp)
{
        vm_object_t object;
        struct vnode *vp;
        int count;
        int scount;
        int n;
        int didmove;

        count = vm_swapcache_maxlaunder;
        scount = vm_swapcache_maxscan;

        /*
         * Look for vnode objects
         */
        lwkt_gettoken(&(*swindexp)->token);

        didmove = 0;
outerloop:
        while ((object = TAILQ_NEXT(&marker->dummy_obj,
                                    object_entry)) != NULL) {
                /*
                 * We have to skip markers.  We cannot hold/drop marker
                 * objects!
                 */
                if (object->type == OBJT_MARKER) {
                        vm_swapcache_movemarker(marker, *swindexp, object);
                        didmove = 1;
                        continue;
                }

                /*
                 * Safety, or in case there are millions of VM objects
                 * without swapcache backing.
                 */
                if (--scount <= 0)
                        goto breakout;

                /*
                 * We must hold the object before potentially yielding.
                 */
                vm_object_hold(object);
                lwkt_yield();

                /* 
                 * Only operate on live VNODE objects that are either
                 * VREG or VCHR (VCHR for meta-data).
                 */
                if ((object->type != OBJT_VNODE) ||
                    ((object->flags & OBJ_DEAD) ||
                     object->swblock_count == 0) ||
                    ((vp = object->handle) == NULL) ||
                    (vp->v_type != VREG && vp->v_type != VCHR)) {
                        vm_object_drop(object);
                        /* object may be invalid now */
                        vm_swapcache_movemarker(marker, *swindexp, object);
                        didmove = 1;
                        continue;
                }

                /*
                 * Reset the object pindex stored in the marker if the
                 * working object has changed.
                 */
                if (marker->save_obj != object || didmove) {
                        marker->dummy_obj.size = 0;
                        marker->save_off = 0;
                        marker->save_obj = object;
                        didmove = 0;
                }

                /*
                 * Look for swblocks starting at our iterator.
                 *
                 * The swap_pager_condfree() function attempts to free
                 * swap space starting at the specified index.  The index
                 * will be updated on return.  The function will return
                 * a scan factor (NOT the number of blocks freed).
                 *
                 * If it must cut its scan of the object short due to an
                 * excessive number of swblocks, or is able to free the
                 * requested number of blocks, it will return n >= count
                 * and we break and pick it back up on a future attempt.
                 *
                 * Scan the object linearly and try to batch large sets of
                 * blocks that are likely to clean out entire swap radix
                 * tree leafs.
                 */
                lwkt_token_swap();
                lwkt_reltoken(&(*swindexp)->token);

                n = swap_pager_condfree(object, &marker->dummy_obj.size,
                                    (count + SWAP_META_MASK) & ~SWAP_META_MASK);

                vm_object_drop(object);         /* object may be invalid now */
                lwkt_gettoken(&(*swindexp)->token);

                /*
                 * If we have exhausted the object or deleted our per-pass
                 * page limit then move us to the next object.  Note that
                 * the current object may no longer be on the vm_object_entry.
                 */
                if (n <= 0 ||
                    marker->save_off > vm_swapcache_cleanperobj) {
                        vm_swapcache_movemarker(marker, *swindexp, object);
                        didmove = 1;
                }

                /*
                 * If we have exhausted our max-launder stop for now.
                 */
                count -= n;
                marker->save_off += n * PAGE_SIZE;
                if (count < 0)
                        goto breakout;
        }

        /*
         * Iterate vm_object_hash[] hash table
         */
        TAILQ_REMOVE(&(*swindexp)->list, &marker->dummy_obj, object_entry);
        lwkt_reltoken(&(*swindexp)->token);
        if (++*swindexp >= &vm_object_hash[VMOBJ_HSIZE])
                *swindexp = &vm_object_hash[0];
        lwkt_gettoken(&(*swindexp)->token);
        TAILQ_INSERT_HEAD(&(*swindexp)->list, &marker->dummy_obj, object_entry);

        if (*swindexp != &vm_object_hash[0])
                goto outerloop;

breakout:
        lwkt_reltoken(&(*swindexp)->token);
}

/*
 * Move the marker past the current object.  Object can be stale, but we
 * still need it to determine if the marker has to be moved.  If the object
 * is still the 'current object' (object after the marker), we hop-scotch
 * the marker past it.
 */
static void
vm_swapcache_movemarker(swmarker_t *marker, struct vm_object_hash *swindex,
                        vm_object_t object)
{
        if (TAILQ_NEXT(&marker->dummy_obj, object_entry) == object) {
                TAILQ_REMOVE(&swindex->list, &marker->dummy_obj, object_entry);
                TAILQ_INSERT_AFTER(&swindex->list, object,
                                   &marker->dummy_obj, object_entry);
        }
}