Merge commit '7e934d3acc051b7ee3ef0d11571fd1225800a607'

[unleashed.git] / kernel / os / vm_pageout.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*      Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T */
/*        All Rights Reserved   */

/*
 * University Copyright- Copyright (c) 1982, 1986, 1988
 * The Regents of the University of California
 * All Rights Reserved
 *
 * University Acknowledgment- Portions of this document are derived from
 * software developed by the University of California, Berkeley, and its
 * contributors.
 */

#include <sys/types.h>
#include <sys/t_lock.h>
#include <sys/param.h>
#include <sys/buf.h>
#include <sys/uio.h>
#include <sys/proc.h>
#include <sys/systm.h>
#include <sys/mman.h>
#include <sys/cred.h>
#include <sys/vnode.h>
#include <sys/vm.h>
#include <sys/vmparam.h>
#include <sys/vtrace.h>
#include <sys/cmn_err.h>
#include <sys/cpuvar.h>
#include <sys/user.h>
#include <sys/kmem.h>
#include <sys/debug.h>
#include <sys/callb.h>
#include <sys/tnf_probe.h>
#include <sys/time.h>

#include <vm/hat.h>
#include <vm/as.h>
#include <vm/seg.h>
#include <vm/page.h>
#include <vm/pvn.h>
#include <vm/seg_kmem.h>

static int checkpage(page_t *, int);

/*
 * The following parameters control operation of the page replacement
 * algorithm.  They are initialized to 0, and then computed at boot time
 * based on the size of the system.  If they are patched non-zero in
 * a loaded vmunix they are left alone and may thus be changed per system
 * using adb on the loaded system.
 */
pgcnt_t         slowscan = 0;
pgcnt_t         fastscan = 0;

static pgcnt_t  handspreadpages = 0;
static int      loopfraction = 2;
static pgcnt_t  looppages;
static int      min_percent_cpu = 4;
static int      max_percent_cpu = 80;
static pgcnt_t  maxfastscan = 0;
static pgcnt_t  maxslowscan = 100;

pgcnt_t maxpgio = 0;
pgcnt_t minfree = 0;
pgcnt_t desfree = 0;
pgcnt_t lotsfree = 0;
pgcnt_t needfree = 0;
pgcnt_t throttlefree = 0;
pgcnt_t pageout_reserve = 0;

pgcnt_t deficit;
pgcnt_t nscan;
pgcnt_t desscan;

/*
 * Values for min_pageout_ticks, max_pageout_ticks and pageout_ticks
 * are the number of ticks in each wakeup cycle that gives the
 * equivalent of some underlying %CPU duty cycle.
 * When RATETOSCHEDPAGING is 4,  and hz is 100, pageout_scanner is
 * awakened every 25 clock ticks.  So, converting from %CPU to ticks
 * per wakeup cycle would be x% of 25, that is (x * 100) / 25.
 * So, for example, 4% == 1 tick and 80% == 20 ticks.
 *
 * min_pageout_ticks:
 *     ticks/wakeup equivalent of min_percent_cpu.
 *
 * max_pageout_ticks:
 *     ticks/wakeup equivalent of max_percent_cpu.
 *
 * pageout_ticks:
 *     Number of clock ticks budgeted for each wakeup cycle.
 *     Computed each time around by schedpaging().
 *     Varies between min_pageout_ticks .. max_pageout_ticks,
 *     depending on memory pressure.
 *
 * pageout_lbolt:
 *     Timestamp of the last time pageout_scanner woke up and started
 *     (or resumed) scanning for not recently referenced pages.
 */

static clock_t  min_pageout_ticks;
static clock_t  max_pageout_ticks;
static clock_t  pageout_ticks;
static clock_t  pageout_lbolt;

static uint_t   reset_hands;

#define PAGES_POLL_MASK 1023

/*
 * pageout_sample_lim:
 *     The limit on the number of samples needed to establish a value
 *     for new pageout parameters, fastscan, slowscan, and handspreadpages.
 *
 * pageout_sample_cnt:
 *     Current sample number.  Once the sample gets large enough,
 *     set new values for handspreadpages, fastscan and slowscan.
 *
 * pageout_sample_pages:
 *     The accumulated number of pages scanned during sampling.
 *
 * pageout_sample_ticks:
 *     The accumulated clock ticks for the sample.
 *
 * pageout_rate:
 *     Rate in pages/nanosecond, computed at the end of sampling.
 *
 * pageout_new_spread:
 *     The new value to use for fastscan and handspreadpages.
 *     Calculated after enough samples have been taken.
 */

typedef hrtime_t hrrate_t;

static uint64_t pageout_sample_lim = 4;
static uint64_t pageout_sample_cnt = 0;
static pgcnt_t  pageout_sample_pages = 0;
static hrrate_t pageout_rate = 0;
static pgcnt_t  pageout_new_spread = 0;

static clock_t  pageout_cycle_ticks;
static hrtime_t sample_start, sample_end;
static hrtime_t pageout_sample_etime = 0;

/*
 * Record number of times a pageout_scanner wakeup cycle finished because it
 * timed out (exceeded its CPU budget), rather than because it visited
 * its budgeted number of pages.
 */
uint64_t pageout_timeouts = 0;

#ifdef VM_STATS
static struct pageoutvmstats_str {
        ulong_t checkpage[3];
} pageoutvmstats;
#endif /* VM_STATS */

/*
 * Threads waiting for free memory use this condition variable and lock until
 * memory becomes available.
 */
kmutex_t        memavail_lock;
kcondvar_t      memavail_cv;

/*
 * The size of the clock loop.
 */
#define LOOPPAGES       total_pages

/*
 * Set up the paging constants for the clock algorithm.
 * Called after the system is initialized and the amount of memory
 * and number of paging devices is known.
 *
 * lotsfree is 1/64 of memory, but at least 512K.
 * desfree is 1/2 of lotsfree.
 * minfree is 1/2 of desfree.
 *
 * Note: to revert to the paging algorithm of Solaris 2.4/2.5, set:
 *
 *      lotsfree = btop(512K)
 *      desfree = btop(200K)
 *      minfree = btop(100K)
 *      throttlefree = INT_MIN
 *      max_percent_cpu = 4
 */
void
setupclock(int recalc)
{

        static spgcnt_t init_lfree, init_dfree, init_mfree;
        static spgcnt_t init_tfree, init_preserve, init_mpgio;
        static spgcnt_t init_mfscan, init_fscan, init_sscan, init_hspages;

        looppages = LOOPPAGES;

        /*
         * setupclock can now be called to recalculate the paging
         * parameters in the case of dynamic addition of memory.
         * So to make sure we make the proper calculations, if such a
         * situation should arise, we save away the initial values
         * of each parameter so we can recall them when needed. This
         * way we don't lose the settings an admin might have made
         * through the /etc/system file.
         */

        if (!recalc) {
                init_lfree = lotsfree;
                init_dfree = desfree;
                init_mfree = minfree;
                init_tfree = throttlefree;
                init_preserve = pageout_reserve;
                init_mpgio = maxpgio;
                init_mfscan = maxfastscan;
                init_fscan = fastscan;
                init_sscan = slowscan;
                init_hspages = handspreadpages;
        }

        /*
         * Set up thresholds for paging:
         */

        /*
         * Lotsfree is threshold where paging daemon turns on.
         */
        if (init_lfree == 0 || init_lfree >= looppages)
                lotsfree = MAX(looppages / 64, btop(512 * 1024));
        else
                lotsfree = init_lfree;

        /*
         * Desfree is amount of memory desired free.
         * If less than this for extended period, start swapping.
         */
        if (init_dfree == 0 || init_dfree >= lotsfree)
                desfree = lotsfree / 2;
        else
                desfree = init_dfree;

        /*
         * Minfree is minimal amount of free memory which is tolerable.
         */
        if (init_mfree == 0 || init_mfree >= desfree)
                minfree = desfree / 2;
        else
                minfree = init_mfree;

        /*
         * Throttlefree is the point at which we start throttling
         * PG_WAIT requests until enough memory becomes available.
         */
        if (init_tfree == 0 || init_tfree >= desfree)
                throttlefree = minfree;
        else
                throttlefree = init_tfree;

        /*
         * Pageout_reserve is the number of pages that we keep in
         * stock for pageout's own use.  Having a few such pages
         * provides insurance against system deadlock due to
         * pageout needing pages.  When freemem < pageout_reserve,
         * non-blocking allocations are denied to any threads
         * other than pageout and sched.  (At some point we might
         * want to consider a per-thread flag like T_PUSHING_PAGES
         * to indicate that a thread is part of the page-pushing
         * dance (e.g. an interrupt thread) and thus is entitled
         * to the same special dispensation we accord pageout.)
         */
        if (init_preserve == 0 || init_preserve >= throttlefree)
                pageout_reserve = throttlefree / 2;
        else
                pageout_reserve = init_preserve;

        /*
         * Maxpgio thresholds how much paging is acceptable.
         * This figures that 2/3 busy on an arm is all that is
         * tolerable for paging.  We assume one operation per disk rev.
         *
         * XXX - Does not account for multiple swap devices.
         */
        if (init_mpgio == 0)
                maxpgio = (DISKRPM * 2) / 3;
        else
                maxpgio = init_mpgio;

        /*
         * The clock scan rate varies between fastscan and slowscan
         * based on the amount of free memory available.  Fastscan
         * rate should be set based on the number pages that can be
         * scanned per sec using ~10% of processor time.  Since this
         * value depends on the processor, MMU, Mhz etc., it is
         * difficult to determine it in a generic manner for all
         * architectures.
         *
         * Instead of trying to determine the number of pages scanned
         * per sec for every processor, fastscan is set to be the smaller
         * of 1/2 of memory or MAXHANDSPREADPAGES and the sampling
         * time is limited to ~4% of processor time.
         *
         * Setting fastscan to be 1/2 of memory allows pageout to scan
         * all of memory in ~2 secs.  This implies that user pages not
         * accessed within 1 sec (assuming, handspreadpages == fastscan)
         * can be reclaimed when free memory is very low.  Stealing pages
         * not accessed within 1 sec seems reasonable and ensures that
         * active user processes don't thrash.
         *
         * Smaller values of fastscan result in scanning fewer pages
         * every second and consequently pageout may not be able to free
         * sufficient memory to maintain the minimum threshold.  Larger
         * values of fastscan result in scanning a lot more pages which
         * could lead to thrashing and higher CPU usage.
         *
         * Fastscan needs to be limited to a maximum value and should not
         * scale with memory to prevent pageout from consuming too much
         * time for scanning on slow CPU's and avoid thrashing, as a
         * result of scanning too many pages, on faster CPU's.
         * The value of 64 Meg was chosen for MAXHANDSPREADPAGES
         * (the upper bound for fastscan) based on the average number
         * of pages that can potentially be scanned in ~1 sec (using ~4%
         * of the CPU) on some of the following machines that currently
         * run Solaris 2.x:
         *
         *                      average memory scanned in ~1 sec
         *
         *      25 Mhz SS1+:            23 Meg
         *      LX:                     37 Meg
         *      50 Mhz SC2000:          68 Meg
         *
         *      40 Mhz 486:             26 Meg
         *      66 Mhz 486:             42 Meg
         *
         * When free memory falls just below lotsfree, the scan rate
         * goes from 0 to slowscan (i.e., pageout starts running).  This
         * transition needs to be smooth and is achieved by ensuring that
         * pageout scans a small number of pages to satisfy the transient
         * memory demand.  This is set to not exceed 100 pages/sec (25 per
         * wakeup) since scanning that many pages has no noticible impact
         * on system performance.
         *
         * In addition to setting fastscan and slowscan, pageout is
         * limited to using ~4% of the CPU.  This results in increasing
         * the time taken to scan all of memory, which in turn means that
         * user processes have a better opportunity of preventing their
         * pages from being stolen.  This has a positive effect on
         * interactive and overall system performance when memory demand
         * is high.
         *
         * Thus, the rate at which pages are scanned for replacement will
         * vary linearly between slowscan and the number of pages that
         * can be scanned using ~4% of processor time instead of varying
         * linearly between slowscan and fastscan.
         *
         * Also, the processor time used by pageout will vary from ~1%
         * at slowscan to ~4% at fastscan instead of varying between
         * ~1% at slowscan and ~10% at fastscan.
         *
         * The values chosen for the various VM parameters (fastscan,
         * handspreadpages, etc) are not universally true for all machines,
         * but appear to be a good rule of thumb for the machines we've
         * tested.  They have the following ranges:
         *
         *      cpu speed:      20 to 70 Mhz
         *      page size:      4K to 8K
         *      memory size:    16M to 5G
         *      page scan rate: 4000 - 17400 4K pages per sec
         *
         * The values need to be re-examined for machines which don't
         * fall into the various ranges (e.g., slower or faster CPUs,
         * smaller or larger pagesizes etc) shown above.
         *
         * On an MP machine, pageout is often unable to maintain the
         * minimum paging thresholds under heavy load.  This is due to
         * the fact that user processes running on other CPU's can be
         * dirtying memory at a much faster pace than pageout can find
         * pages to free.  The memory demands could be met by enabling
         * more than one CPU to run the clock algorithm in such a manner
         * that the various clock hands don't overlap.  This also makes
         * it more difficult to determine the values for fastscan, slowscan
         * and handspreadpages.
         *
         * The swapper is currently used to free up memory when pageout
         * is unable to meet memory demands by swapping out processes.
         * In addition to freeing up memory, swapping also reduces the
         * demand for memory by preventing user processes from running
         * and thereby consuming memory.
         */
        if (init_mfscan == 0) {
                if (pageout_new_spread != 0)
                        maxfastscan = pageout_new_spread;
                else
                        maxfastscan = MAXHANDSPREADPAGES;
        } else {
                maxfastscan = init_mfscan;
        }
        if (init_fscan == 0)
                fastscan = MIN(looppages / loopfraction, maxfastscan);
        else
                fastscan = init_fscan;
        if (fastscan > looppages / loopfraction)
                fastscan = looppages / loopfraction;

        /*
         * Set slow scan time to 1/10 the fast scan time, but
         * not to exceed maxslowscan.
         */
        if (init_sscan == 0)
                slowscan = MIN(fastscan / 10, maxslowscan);
        else
                slowscan = init_sscan;
        if (slowscan > fastscan / 2)
                slowscan = fastscan / 2;

        /*
         * Handspreadpages is distance (in pages) between front and back
         * pageout daemon hands.  The amount of time to reclaim a page
         * once pageout examines it increases with this distance and
         * decreases as the scan rate rises. It must be < the amount
         * of pageable memory.
         *
         * Since pageout is limited to ~4% of the CPU, setting handspreadpages
         * to be "fastscan" results in the front hand being a few secs
         * (varies based on the processor speed) ahead of the back hand
         * at fastscan rates.  This distance can be further reduced, if
         * necessary, by increasing the processor time used by pageout
         * to be more than ~4% and preferrably not more than ~10%.
         *
         * As a result, user processes have a much better chance of
         * referencing their pages before the back hand examines them.
         * This also significantly lowers the number of reclaims from
         * the freelist since pageout does not end up freeing pages which
         * may be referenced a sec later.
         */
        if (init_hspages == 0)
                handspreadpages = fastscan;
        else
                handspreadpages = init_hspages;

        /*
         * Make sure that back hand follows front hand by at least
         * 1/RATETOSCHEDPAGING seconds.  Without this test, it is possible
         * for the back hand to look at a page during the same wakeup of
         * the pageout daemon in which the front hand cleared its ref bit.
         */
        if (handspreadpages >= looppages)
                handspreadpages = looppages - 1;

        /*
         * If we have been called to recalculate the parameters,
         * set a flag to re-evaluate the clock hand pointers.
         */
        if (recalc)
                reset_hands = 1;
}

/*
 * Pageout scheduling.
 *
 * Schedpaging controls the rate at which the page out daemon runs by
 * setting the global variables nscan and desscan RATETOSCHEDPAGING
 * times a second.  Nscan records the number of pages pageout has examined
 * in its current pass; schedpaging resets this value to zero each time
 * it runs.  Desscan records the number of pages pageout should examine
 * in its next pass; schedpaging sets this value based on the amount of
 * currently available memory.
 */

#define RATETOSCHEDPAGING       4               /* hz that is */

static kmutex_t pageout_mutex;  /* held while pageout or schedpaging running */

/*
 * Pool of available async pageout putpage requests.
 */
static struct async_reqs *push_req;
static struct async_reqs *req_freelist; /* available req structs */
static struct async_reqs *push_list;    /* pending reqs */
static kmutex_t push_lock;              /* protects req pool */
static kcondvar_t push_cv;

static int async_list_size = 256;       /* number of async request structs */

static void pageout_scanner(void);

/*
 * If a page is being shared more than "po_share" times
 * then leave it alone- don't page it out.
 */
#define MIN_PO_SHARE    (8)
#define MAX_PO_SHARE    ((MIN_PO_SHARE) << 24)
ulong_t po_share = MIN_PO_SHARE;

/*
 * Schedule rate for paging.
 * Rate is linear interpolation between
 * slowscan with lotsfree and fastscan when out of memory.
 */
static void
schedpaging(void *arg)
{
        spgcnt_t vavail;

        if (freemem < lotsfree + needfree + kmem_reapahead)
                kmem_reap();

        if (freemem < lotsfree + needfree)
                seg_preap();

        if (mutex_tryenter(&pageout_mutex)) {
                /* pageout() not running */
                nscan = 0;
                vavail = freemem - deficit;
                if (pageout_new_spread != 0)
                        vavail -= needfree;
                if (vavail < 0)
                        vavail = 0;
                if (vavail > lotsfree)
                        vavail = lotsfree;

                /*
                 * Fix for 1161438 (CRS SPR# 73922).  All variables
                 * in the original calculation for desscan were 32 bit signed
                 * ints.  As freemem approaches 0x0 on a system with 1 Gig or
                 * more of memory, the calculation can overflow.  When this
                 * happens, desscan becomes negative and pageout_scanner()
                 * stops paging out.
                 */
                if ((needfree) && (pageout_new_spread == 0)) {
                        /*
                         * If we've not yet collected enough samples to
                         * calculate a spread, use the old logic of kicking
                         * into high gear anytime needfree is non-zero.
                         */
                        desscan = fastscan / RATETOSCHEDPAGING;
                } else {
                        /*
                         * Once we've calculated a spread based on system
                         * memory and usage, just treat needfree as another
                         * form of deficit.
                         */
                        spgcnt_t faststmp, slowstmp, result;

                        slowstmp = slowscan * vavail;
                        faststmp = fastscan * (lotsfree - vavail);
                        result = (slowstmp + faststmp) /
                            nz(lotsfree) / RATETOSCHEDPAGING;
                        desscan = (pgcnt_t)result;
                }

                pageout_ticks = min_pageout_ticks + (lotsfree - vavail) *
                    (max_pageout_ticks - min_pageout_ticks) / nz(lotsfree);

                if (freemem < lotsfree + needfree ||
                    pageout_sample_cnt < pageout_sample_lim) {
                        TRACE_1(TR_FAC_VM, TR_PAGEOUT_CV_SIGNAL,
                            "pageout_cv_signal:freemem %ld", freemem);
                        cv_signal(&proc_pageout->p_cv);
                } else {
                        /*
                         * There are enough free pages, no need to
                         * kick the scanner thread.  And next time
                         * around, keep more of the `highly shared'
                         * pages.
                         */
                        cv_signal_pageout();
                        if (po_share > MIN_PO_SHARE) {
                                po_share >>= 1;
                        }
                }
                mutex_exit(&pageout_mutex);
        }

        /*
         * Signal threads waiting for available memory.
         * NOTE: usually we need to grab memavail_lock before cv_broadcast, but
         * in this case it is not needed - the waiters will be waken up during
         * the next invocation of this function.
         */
        if (kmem_avail() > 0)
                cv_broadcast(&memavail_cv);

        (void) timeout(schedpaging, arg, hz / RATETOSCHEDPAGING);
}

pgcnt_t         pushes;
ulong_t         push_list_size;         /* # of requests on pageout queue */

#define FRONT   1
#define BACK    2

int dopageout = 1;      /* must be non-zero to turn page stealing on */

/*
 * The page out daemon, which runs as process 2.
 *
 * As long as there are at least lotsfree pages,
 * this process is not run.  When the number of free
 * pages stays in the range desfree to lotsfree,
 * this daemon runs through the pages in the loop
 * at a rate determined in schedpaging().  Pageout manages
 * two hands on the clock.  The front hand moves through
 * memory, clearing the reference bit,
 * and stealing pages from procs that are over maxrss.
 * The back hand travels a distance behind the front hand,
 * freeing the pages that have not been referenced in the time
 * since the front hand passed.  If modified, they are pushed to
 * swap before being freed.
 *
 * There are 2 threads that act on behalf of the pageout process.
 * One thread scans pages (pageout_scanner) and frees them up if
 * they don't require any fop_putpage operation. If a page must be
 * written back to its backing store, the request is put on a list
 * and the other (pageout) thread is signaled. The pageout thread
 * grabs fop_putpage requests from the list, and processes them.
 * Some filesystems may require resources for the fop_putpage
 * operations (like memory) and hence can block the pageout
 * thread, but the scanner thread can still operate. There is still
 * no guarantee that memory deadlocks cannot occur.
 *
 * For now, this thing is in very rough form.
 */
void
pageout()
{
        struct async_reqs *arg;
        pri_t pageout_pri;
        int i;
        pgcnt_t max_pushes;
        callb_cpr_t cprinfo;

        proc_pageout = ttoproc(curthread);
        proc_pageout->p_cstime = 0;
        proc_pageout->p_stime =  0;
        proc_pageout->p_cutime =  0;
        proc_pageout->p_utime = 0;
        bcopy("pageout", PTOU(curproc)->u_psargs, 8);
        bcopy("pageout", PTOU(curproc)->u_comm, 7);

        /*
         * Create pageout scanner thread
         */
        mutex_init(&pageout_mutex, NULL, MUTEX_DEFAULT, NULL);
        mutex_init(&push_lock, NULL, MUTEX_DEFAULT, NULL);

        /*
         * Allocate and initialize the async request structures
         * for pageout.
         */
        push_req = (struct async_reqs *)
            kmem_zalloc(async_list_size * sizeof (struct async_reqs), KM_SLEEP);

        req_freelist = push_req;
        for (i = 0; i < async_list_size - 1; i++)
                push_req[i].a_next = &push_req[i + 1];

        pageout_pri = curthread->t_pri;

        /* Create the pageout scanner thread. */
        (void) lwp_kernel_create(proc_pageout, pageout_scanner, NULL, TS_RUN,
            pageout_pri - 1);

        /*
         * kick off pageout scheduler.
         */
        schedpaging(NULL);

        /*
         * Limit pushes to avoid saturating pageout devices.
         */
        max_pushes = maxpgio / RATETOSCHEDPAGING;
        CALLB_CPR_INIT(&cprinfo, &push_lock, callb_generic_cpr, "pageout");

        for (;;) {
                mutex_enter(&push_lock);

                while ((arg = push_list) == NULL || pushes > max_pushes) {
                        CALLB_CPR_SAFE_BEGIN(&cprinfo);
                        cv_wait(&push_cv, &push_lock);
                        pushes = 0;
                        CALLB_CPR_SAFE_END(&cprinfo, &push_lock);
                }
                push_list = arg->a_next;
                arg->a_next = NULL;
                mutex_exit(&push_lock);

                if (fop_putpage(arg->a_vp, (offset_t)arg->a_off,
                    arg->a_len, arg->a_flags, arg->a_cred, NULL) == 0) {
                        pushes++;
                }

                /* vp held by checkpage() */
                VN_RELE(arg->a_vp);

                mutex_enter(&push_lock);
                arg->a_next = req_freelist;     /* back on freelist */
                req_freelist = arg;
                push_list_size--;
                mutex_exit(&push_lock);
        }
}

/*
 * Kernel thread that scans pages looking for ones to free
 */
static void
pageout_scanner(void)
{
        struct page *fronthand, *backhand;
        uint_t count;
        callb_cpr_t cprinfo;
        pgcnt_t nscan_limit;
        pgcnt_t pcount;

        CALLB_CPR_INIT(&cprinfo, &pageout_mutex, callb_generic_cpr, "poscan");
        mutex_enter(&pageout_mutex);

        /*
         * The restart case does not attempt to point the hands at roughly
         * the right point on the assumption that after one circuit things
         * will have settled down - and restarts shouldn't be that often.
         */

        /*
         * Set the two clock hands to be separated by a reasonable amount,
         * but no more than 360 degrees apart.
         */
        backhand = page_first();
        if (handspreadpages >= total_pages)
                fronthand = page_nextn(backhand, total_pages - 1);
        else
                fronthand = page_nextn(backhand, handspreadpages);

        min_pageout_ticks = MAX(1,
            ((hz * min_percent_cpu) / 100) / RATETOSCHEDPAGING);
        max_pageout_ticks = MAX(min_pageout_ticks,
            ((hz * max_percent_cpu) / 100) / RATETOSCHEDPAGING);

loop:
        cv_signal_pageout();

        CALLB_CPR_SAFE_BEGIN(&cprinfo);
        cv_wait(&proc_pageout->p_cv, &pageout_mutex);
        CALLB_CPR_SAFE_END(&cprinfo, &pageout_mutex);

        if (!dopageout)
                goto loop;

        if (reset_hands) {
                reset_hands = 0;

                backhand = page_first();
                if (handspreadpages >= total_pages)
                        fronthand = page_nextn(backhand, total_pages - 1);
                else
                        fronthand = page_nextn(backhand, handspreadpages);
        }

        CPU_STATS_ADDQ(CPU, vm, pgrrun, 1);
        count = 0;

        TRACE_4(TR_FAC_VM, TR_PAGEOUT_START,
            "pageout_start:freemem %ld lotsfree %ld nscan %ld desscan %ld",
            freemem, lotsfree, nscan, desscan);

        pcount = 0;
        if (pageout_sample_cnt < pageout_sample_lim) {
                nscan_limit = total_pages;
        } else {
                nscan_limit = desscan;
        }
        pageout_lbolt = ddi_get_lbolt();
        sample_start = gethrtime();

        /*
         * Scan the appropriate number of pages for a single duty cycle.
         * However, stop scanning as soon as there is enough free memory.
         * For a short while, we will be sampling the performance of the
         * scanner and need to keep running just to get sample data, in
         * which case we keep going and don't pay attention to whether
         * or not there is enough free memory.
         */

        while (nscan < nscan_limit && (freemem < lotsfree + needfree ||
            pageout_sample_cnt < pageout_sample_lim)) {
                int rvfront, rvback;

                /*
                 * Check to see if we have exceeded our %CPU budget
                 * for this wakeup, but not on every single page visited,
                 * just every once in a while.
                 */
                if ((pcount & PAGES_POLL_MASK) == PAGES_POLL_MASK) {
                        pageout_cycle_ticks = ddi_get_lbolt() - pageout_lbolt;
                        if (pageout_cycle_ticks >= pageout_ticks) {
                                ++pageout_timeouts;
                                break;
                        }
                }

                /*
                 * If checkpage manages to add a page to the free list,
                 * we give ourselves another couple of trips around the loop.
                 */
                if ((rvfront = checkpage(fronthand, FRONT)) == 1)
                        count = 0;
                if ((rvback = checkpage(backhand, BACK)) == 1)
                        count = 0;

                ++pcount;

                /*
                 * protected by pageout_mutex instead of cpu_stat_lock
                 */
                CPU_STATS_ADDQ(CPU, vm, scan, 1);

                /*
                 * Don't include ineligible pages in the number scanned.
                 */
                if (rvfront != -1 || rvback != -1)
                        nscan++;

                backhand = page_next(backhand);

                /*
                 * backhand update and wraparound check are done separately
                 * because lint barks when it finds an empty "if" body
                 */

                if ((fronthand = page_next(fronthand)) == page_first()) {
                        TRACE_2(TR_FAC_VM, TR_PAGEOUT_HAND_WRAP,
                            "pageout_hand_wrap:freemem %ld whichhand %d",
                            freemem, FRONT);

                        /*
                         * protected by pageout_mutex instead of cpu_stat_lock
                         */
                        CPU_STATS_ADDQ(CPU, vm, rev, 1);
                        if (++count > 1) {
                                /*
                                 * Extremely unlikely, but it happens.
                                 * We went around the loop at least once
                                 * and didn't get far enough.
                                 * If we are still skipping `highly shared'
                                 * pages, skip fewer of them.  Otherwise,
                                 * give up till the next clock tick.
                                 */
                                if (po_share < MAX_PO_SHARE) {
                                        po_share <<= 1;
                                } else {
                                        /*
                                         * Really a "goto loop", but
                                         * if someone is TRACing,  at least
                                         * make records to show where we
                                         * are.
                                         */
                                        break;
                                }
                        }
                }
        }

        sample_end = gethrtime();

        TRACE_5(TR_FAC_VM, TR_PAGEOUT_END,
            "pageout_end:freemem %ld lots %ld nscan %ld des %ld count %u",
            freemem, lotsfree, nscan, desscan, count);

        if (pageout_sample_cnt < pageout_sample_lim) {
                pageout_sample_pages += pcount;
                pageout_sample_etime += sample_end - sample_start;
                ++pageout_sample_cnt;
        }
        if (pageout_sample_cnt >= pageout_sample_lim &&
            pageout_new_spread == 0) {
                pageout_rate = (hrrate_t)pageout_sample_pages *
                    (hrrate_t)(NANOSEC) / pageout_sample_etime;
                pageout_new_spread = pageout_rate / 10;
                setupclock(1);
        }

        goto loop;
}

/*
 * Look at the page at hand.  If it is locked (e.g., for physical i/o),
 * system (u., page table) or free, then leave it alone.  Otherwise,
 * if we are running the front hand, turn off the page's reference bit.
 * If the proc is over maxrss, we take it.  If running the back hand,
 * check whether the page has been reclaimed.  If not, free the page,
 * pushing it to disk first if necessary.
 *
 * Return values:
 *      -1 if the page is not a candidate at all,
 *       0 if not freed, or
 *       1 if we freed it.
 */
static int
checkpage(struct page *pp, int whichhand)
{
        int ppattr;
        int isfs = 0;
        int isexec = 0;
        int pagesync_flag;

        /*
         * Skip pages:
         *      - associated with the kernel vnode since
         *          they are always "exclusively" locked.
         *      - that are free
         *      - that are shared more than po_share'd times
         *      - its already locked
         *
         * NOTE:  These optimizations assume that reads are atomic.
         */

        if (PP_ISKAS(pp) || PAGE_LOCKED(pp) || PP_ISFREE(pp) ||
            pp->p_lckcnt != 0 || pp->p_cowcnt != 0 ||
            hat_page_checkshare(pp, po_share)) {
                return (-1);
        }

        if (!page_trylock(pp, SE_EXCL)) {
                /*
                 * Skip the page if we can't acquire the "exclusive" lock.
                 */
                return (-1);
        } else if (PP_ISFREE(pp)) {
                /*
                 * It became free between the above check and our actually
                 * locking the page.  Oh, well there will be other pages.
                 */
                page_unlock(pp);
                return (-1);
        }

        /*
         * Reject pages that cannot be freed. The page_struct_lock
         * need not be acquired to examine these
         * fields since the page has an "exclusive" lock.
         */
        if (pp->p_lckcnt != 0 || pp->p_cowcnt != 0) {
                page_unlock(pp);
                return (-1);
        }

        /*
         * Maintain statistics for what we are freeing
         */

        if (pp->p_vnode != NULL) {
                if (pp->p_vnode->v_flag & VVMEXEC)
                        isexec = 1;

                if (!IS_SWAPFSVP(pp->p_vnode))
                        isfs = 1;
        }

        /*
         * Turn off REF and MOD bits with the front hand.
         * The back hand examines the REF bit and always considers
         * SHARED pages as referenced.
         */
        if (whichhand == FRONT)
                pagesync_flag = HAT_SYNC_ZERORM;
        else
                pagesync_flag = HAT_SYNC_DONTZERO | HAT_SYNC_STOPON_REF |
                    HAT_SYNC_STOPON_SHARED;

        ppattr = hat_pagesync(pp, pagesync_flag);

recheck:
        /*
         * If page is referenced; make unreferenced but reclaimable.
         * If this page is not referenced, then it must be reclaimable
         * and we can add it to the free list.
         */
        if (ppattr & P_REF) {
                TRACE_2(TR_FAC_VM, TR_PAGEOUT_ISREF,
                    "pageout_isref:pp %p whichhand %d", pp, whichhand);
                if (whichhand == FRONT) {
                        /*
                         * Checking of rss or madvise flags needed here...
                         *
                         * If not "well-behaved", fall through into the code
                         * for not referenced.
                         */
                        hat_clrref(pp);
                }
                /*
                 * Somebody referenced the page since the front
                 * hand went by, so it's not a candidate for
                 * freeing up.
                 */
                page_unlock(pp);
                return (0);
        }

        VM_STAT_ADD(pageoutvmstats.checkpage[0]);

        /*
         * If large page, attempt to demote it. If successfully demoted,
         * retry the checkpage.
         */
        if (pp->p_szc != 0) {
                if (!page_try_demote_pages(pp)) {
                        VM_STAT_ADD(pageoutvmstats.checkpage[1]);
                        page_unlock(pp);
                        return (-1);
                }
                ASSERT(pp->p_szc == 0);
                VM_STAT_ADD(pageoutvmstats.checkpage[2]);
                /*
                 * since page_try_demote_pages() could have unloaded some
                 * mappings it makes sense to reload ppattr.
                 */
                ppattr = hat_page_getattr(pp, P_MOD | P_REF);
        }

        /*
         * If the page is currently dirty, we have to arrange
         * to have it cleaned before it can be freed.
         *
         * XXX - ASSERT(pp->p_vnode != NULL);
         */
        if ((ppattr & P_MOD) && pp->p_vnode) {
                struct vnode *vp = pp->p_vnode;
                uoff_t offset = pp->p_offset;

                /*
                 * XXX - Test for process being swapped out or about to exit?
                 * [Can't get back to process(es) using the page.]
                 */

                /*
                 * Hold the vnode before releasing the page lock to
                 * prevent it from being freed and re-used by some
                 * other thread.
                 */
                VN_HOLD(vp);
                page_unlock(pp);

                /*
                 * Queue i/o request for the pageout thread.
                 */
                if (!queue_io_request(vp, offset)) {
                        VN_RELE(vp);
                        return (0);
                }
                return (1);
        }

        /*
         * Now we unload all the translations,
         * and put the page back on to the free list.
         * If the page was used (referenced or modified) after
         * the pagesync but before it was unloaded we catch it
         * and handle the page properly.
         */
        TRACE_2(TR_FAC_VM, TR_PAGEOUT_FREE,
            "pageout_free:pp %p whichhand %d", pp, whichhand);
        (void) hat_pageunload(pp, HAT_FORCE_PGUNLOAD);
        ppattr = hat_page_getattr(pp, P_MOD | P_REF);
        if ((ppattr & P_REF) || ((ppattr & P_MOD) && pp->p_vnode))
                goto recheck;

        VN_DISPOSE(pp, B_FREE, 0, kcred);

        CPU_STATS_ADD_K(vm, dfree, 1);

        if (isfs) {
                if (isexec) {
                        CPU_STATS_ADD_K(vm, execfree, 1);
                } else {
                        CPU_STATS_ADD_K(vm, fsfree, 1);
                }
        } else {
                CPU_STATS_ADD_K(vm, anonfree, 1);
        }

        return (1);             /* freed a page! */
}

/*
 * Queue async i/o request from pageout_scanner and segment swapout
 * routines on one common list.  This ensures that pageout devices (swap)
 * are not saturated by pageout_scanner or swapout requests.
 * The pageout thread empties this list by initiating i/o operations.
 */
int
queue_io_request(vnode_t *vp, uoff_t off)
{
        struct async_reqs *arg;

        /*
         * If we cannot allocate an async request struct,
         * skip this page.
         */
        mutex_enter(&push_lock);
        if ((arg = req_freelist) == NULL) {
                mutex_exit(&push_lock);
                return (0);
        }
        req_freelist = arg->a_next;             /* adjust freelist */
        push_list_size++;

        arg->a_vp = vp;
        arg->a_off = off;
        arg->a_len = PAGESIZE;
        arg->a_flags = B_ASYNC | B_FREE;
        arg->a_cred = kcred;            /* always held */

        /*
         * Add to list of pending write requests.
         */
        arg->a_next = push_list;
        push_list = arg;

        if (req_freelist == NULL) {
                /*
                 * No free async requests left. The lock is held so we
                 * might as well signal the pusher thread now.
                 */
                cv_signal(&push_cv);
        }
        mutex_exit(&push_lock);
        return (1);
}

/*
 * Wakeup pageout to initiate i/o if push_list is not empty.
 */
void
cv_signal_pageout()
{
        if (push_list != NULL) {
                mutex_enter(&push_lock);
                cv_signal(&push_cv);
                mutex_exit(&push_lock);
        }
}