Add -n option to print major and minor version, e.g. 5.99 on -current.

[netbsd-mini2440.git] / sys / uvm / uvm_loan.c

/*      $NetBSD: uvm_loan.c,v 1.72 2008/06/17 02:29:10 yamt Exp $       */

/*
 *
 * Copyright (c) 1997 Charles D. Cranor and Washington University.
 * All rights reserved.
 *
 * 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 Charles D. Cranor and
 *      Washington University.
 * 4. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 AUTHOR 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.
 *
 * from: Id: uvm_loan.c,v 1.1.6.4 1998/02/06 05:08:43 chs Exp
 */

/*
 * uvm_loan.c: page loanout handler
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: uvm_loan.c,v 1.72 2008/06/17 02:29:10 yamt Exp $");

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/malloc.h>
#include <sys/mman.h>

#include <uvm/uvm.h>

/*
 * "loaned" pages are pages which are (read-only, copy-on-write) loaned
 * from the VM system to other parts of the kernel.   this allows page
 * copying to be avoided (e.g. you can loan pages from objs/anons to
 * the mbuf system).
 *
 * there are 3 types of loans possible:
 *  O->K  uvm_object page to wired kernel page (e.g. mbuf data area)
 *  A->K  anon page to wired kernel page (e.g. mbuf data area)
 *  O->A  uvm_object to anon loan (e.g. vnode page to an anon)
 * note that it possible to have an O page loaned to both an A and K
 * at the same time.
 *
 * loans are tracked by pg->loan_count.  an O->A page will have both
 * a uvm_object and a vm_anon, but PQ_ANON will not be set.   this sort
 * of page is considered "owned" by the uvm_object (not the anon).
 *
 * each loan of a page to the kernel bumps the pg->wire_count.  the
 * kernel mappings for these pages will be read-only and wired.  since
 * the page will also be wired, it will not be a candidate for pageout,
 * and thus will never be pmap_page_protect()'d with VM_PROT_NONE.  a
 * write fault in the kernel to one of these pages will not cause
 * copy-on-write.  instead, the page fault is considered fatal.  this
 * is because the kernel mapping will have no way to look up the
 * object/anon which the page is owned by.  this is a good side-effect,
 * since a kernel write to a loaned page is an error.
 *
 * owners that want to free their pages and discover that they are
 * loaned out simply "disown" them (the page becomes an orphan).  these
 * pages should be freed when the last loan is dropped.   in some cases
 * an anon may "adopt" an orphaned page.
 *
 * locking: to read pg->loan_count either the owner or the page queues
 * must be locked.   to modify pg->loan_count, both the owner of the page
 * and the PQs must be locked.   pg->flags is (as always) locked by
 * the owner of the page.
 *
 * note that locking from the "loaned" side is tricky since the object
 * getting the loaned page has no reference to the page's owner and thus
 * the owner could "die" at any time.   in order to prevent the owner
 * from dying the page queues should be locked.   this forces us to sometimes
 * use "try" locking.
 *
 * loans are typically broken by the following events:
 *  1. user-level xwrite fault to a loaned page
 *  2. pageout of clean+inactive O->A loaned page
 *  3. owner frees page (e.g. pager flush)
 *
 * note that loaning a page causes all mappings of the page to become
 * read-only (via pmap_page_protect).   this could have an unexpected
 * effect on normal "wired" pages if one is not careful (XXX).
 */

/*
 * local prototypes
 */

static int      uvm_loananon(struct uvm_faultinfo *, void ***,
                             int, struct vm_anon *);
static int      uvm_loanuobj(struct uvm_faultinfo *, void ***,
                             int, vaddr_t);
static int      uvm_loanzero(struct uvm_faultinfo *, void ***, int);
static void     uvm_unloananon(struct vm_anon **, int);
static void     uvm_unloanpage(struct vm_page **, int);
static int      uvm_loanpage(struct vm_page **, int);


/*
 * inlines
 */

/*
 * uvm_loanentry: loan out pages in a map entry (helper fn for uvm_loan())
 *
 * => "ufi" is the result of a successful map lookup (meaning that
 *      on entry the map is locked by the caller)
 * => we may unlock and then relock the map if needed (for I/O)
 * => we put our output result in "output"
 * => we always return with the map unlocked
 * => possible return values:
 *      -1 == error, map is unlocked
 *       0 == map relock error (try again!), map is unlocked
 *      >0 == number of pages we loaned, map is unlocked
 *
 * NOTE: We can live with this being an inline, because it is only called
 * from one place.
 */

static inline int
uvm_loanentry(struct uvm_faultinfo *ufi, void ***output, int flags)
{
        vaddr_t curaddr = ufi->orig_rvaddr;
        vsize_t togo = ufi->size;
        struct vm_aref *aref = &ufi->entry->aref;
        struct uvm_object *uobj = ufi->entry->object.uvm_obj;
        struct vm_anon *anon;
        int rv, result = 0;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);

        /*
         * lock us the rest of the way down (we unlock before return)
         */
        if (aref->ar_amap)
                amap_lock(aref->ar_amap);

        /*
         * loop until done
         */
        while (togo) {

                /*
                 * find the page we want.   check the anon layer first.
                 */

                if (aref->ar_amap) {
                        anon = amap_lookup(aref, curaddr - ufi->entry->start);
                } else {
                        anon = NULL;
                }

                /* locked: map, amap, uobj */
                if (anon) {
                        rv = uvm_loananon(ufi, output, flags, anon);
                } else if (uobj) {
                        rv = uvm_loanuobj(ufi, output, flags, curaddr);
                } else if (UVM_ET_ISCOPYONWRITE(ufi->entry)) {
                        rv = uvm_loanzero(ufi, output, flags);
                } else {
                        uvmfault_unlockall(ufi, aref->ar_amap, uobj, NULL);
                        rv = -1;
                }
                /* locked: if (rv > 0) => map, amap, uobj  [o.w. unlocked] */
                KASSERT(rv > 0 || aref->ar_amap == NULL ||
                    !mutex_owned(&aref->ar_amap->am_l));
                KASSERT(rv > 0 || uobj == NULL ||
                    !mutex_owned(&uobj->vmobjlock));

                /* total failure */
                if (rv < 0) {
                        UVMHIST_LOG(loanhist, "failure %d", rv, 0,0,0);
                        return (-1);
                }

                /* relock failed, need to do another lookup */
                if (rv == 0) {
                        UVMHIST_LOG(loanhist, "relock failure %d", result
                            ,0,0,0);
                        return (result);
                }

                /*
                 * got it... advance to next page
                 */

                result++;
                togo -= PAGE_SIZE;
                curaddr += PAGE_SIZE;
        }

        /*
         * unlock what we locked, unlock the maps and return
         */

        if (aref->ar_amap)
                amap_unlock(aref->ar_amap);
        uvmfault_unlockmaps(ufi, false);
        UVMHIST_LOG(loanhist, "done %d", result, 0,0,0);
        return (result);
}

/*
 * normal functions
 */

/*
 * uvm_loan: loan pages in a map out to anons or to the kernel
 *
 * => map should be unlocked
 * => start and len should be multiples of PAGE_SIZE
 * => result is either an array of anon's or vm_pages (depending on flags)
 * => flag values: UVM_LOAN_TOANON - loan to anons
 *                 UVM_LOAN_TOPAGE - loan to wired kernel page
 *    one and only one of these flags must be set!
 * => returns 0 (success), or an appropriate error number
 */

int
uvm_loan(struct vm_map *map, vaddr_t start, vsize_t len, void *v, int flags)
{
        struct uvm_faultinfo ufi;
        void **result, **output;
        int rv, error;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);

        /*
         * ensure that one and only one of the flags is set
         */

        KASSERT(((flags & UVM_LOAN_TOANON) == 0) ^
                ((flags & UVM_LOAN_TOPAGE) == 0));
        KASSERT((map->flags & VM_MAP_INTRSAFE) == 0);

        /*
         * "output" is a pointer to the current place to put the loaned page.
         */

        result = v;
        output = &result[0];    /* start at the beginning ... */

        /*
         * while we've got pages to do
         */

        while (len > 0) {

                /*
                 * fill in params for a call to uvmfault_lookup
                 */

                ufi.orig_map = map;
                ufi.orig_rvaddr = start;
                ufi.orig_size = len;

                /*
                 * do the lookup, the only time this will fail is if we hit on
                 * an unmapped region (an error)
                 */

                if (!uvmfault_lookup(&ufi, false)) {
                        error = ENOENT;
                        goto fail;
                }

                /*
                 * map now locked.  now do the loanout...
                 */

                rv = uvm_loanentry(&ufi, &output, flags);
                if (rv < 0) {
                        /* all unlocked due to error */
                        error = EINVAL;
                        goto fail;
                }

                /*
                 * done!  the map is unlocked.  advance, if possible.
                 *
                 * XXXCDC: could be recoded to hold the map lock with
                 *         smarter code (but it only happens on map entry
                 *         boundaries, so it isn't that bad).
                 */

                if (rv) {
                        rv <<= PAGE_SHIFT;
                        len -= rv;
                        start += rv;
                }
        }
        UVMHIST_LOG(loanhist, "success", 0,0,0,0);
        return 0;

fail:
        /*
         * failed to complete loans.  drop any loans and return failure code.
         * map is already unlocked.
         */

        if (output - result) {
                if (flags & UVM_LOAN_TOANON) {
                        uvm_unloananon((struct vm_anon **)result,
                            output - result);
                } else {
                        uvm_unloanpage((struct vm_page **)result,
                            output - result);
                }
        }
        UVMHIST_LOG(loanhist, "error %d", error,0,0,0);
        return (error);
}

/*
 * uvm_loananon: loan a page from an anon out
 *
 * => called with map, amap, uobj locked
 * => return value:
 *      -1 = fatal error, everything is unlocked, abort.
 *       0 = lookup in ufi went stale, everything unlocked, relookup and
 *              try again
 *       1 = got it, everything still locked
 */

int
uvm_loananon(struct uvm_faultinfo *ufi, void ***output, int flags,
    struct vm_anon *anon)
{
        struct vm_page *pg;
        int error;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);

        /*
         * if we are loaning to "another" anon then it is easy, we just
         * bump the reference count on the current anon and return a
         * pointer to it (it becomes copy-on-write shared).
         */

        if (flags & UVM_LOAN_TOANON) {
                mutex_enter(&anon->an_lock);
                pg = anon->an_page;
                if (pg && (pg->pqflags & PQ_ANON) != 0 && anon->an_ref == 1) {
                        if (pg->wire_count > 0) {
                                UVMHIST_LOG(loanhist, "->A wired %p", pg,0,0,0);
                                uvmfault_unlockall(ufi,
                                    ufi->entry->aref.ar_amap,
                                    ufi->entry->object.uvm_obj, anon);
                                return (-1);
                        }
                        pmap_page_protect(pg, VM_PROT_READ);
                }
                anon->an_ref++;
                **output = anon;
                (*output)++;
                mutex_exit(&anon->an_lock);
                UVMHIST_LOG(loanhist, "->A done", 0,0,0,0);
                return (1);
        }

        /*
         * we are loaning to a kernel-page.   we need to get the page
         * resident so we can wire it.   uvmfault_anonget will handle
         * this for us.
         */

        mutex_enter(&anon->an_lock);
        error = uvmfault_anonget(ufi, ufi->entry->aref.ar_amap, anon);

        /*
         * if we were unable to get the anon, then uvmfault_anonget has
         * unlocked everything and returned an error code.
         */

        if (error) {
                UVMHIST_LOG(loanhist, "error %d", error,0,0,0);

                /* need to refault (i.e. refresh our lookup) ? */
                if (error == ERESTART) {
                        return (0);
                }

                /* "try again"?   sleep a bit and retry ... */
                if (error == EAGAIN) {
                        tsleep(&lbolt, PVM, "loanagain", 0);
                        return (0);
                }

                /* otherwise flag it as an error */
                return (-1);
        }

        /*
         * we have the page and its owner locked: do the loan now.
         */

        pg = anon->an_page;
        mutex_enter(&uvm_pageqlock);
        if (pg->wire_count > 0) {
                mutex_exit(&uvm_pageqlock);
                UVMHIST_LOG(loanhist, "->K wired %p", pg,0,0,0);
                KASSERT(pg->uobject == NULL);
                uvmfault_unlockall(ufi, ufi->entry->aref.ar_amap,
                    NULL, anon);
                return (-1);
        }
        if (pg->loan_count == 0) {
                pmap_page_protect(pg, VM_PROT_READ);
        }
        pg->loan_count++;
        uvm_pageactivate(pg);
        mutex_exit(&uvm_pageqlock);
        **output = pg;
        (*output)++;

        /* unlock anon and return success */
        if (pg->uobject)
                mutex_exit(&pg->uobject->vmobjlock);
        mutex_exit(&anon->an_lock);
        UVMHIST_LOG(loanhist, "->K done", 0,0,0,0);
        return (1);
}

/*
 * uvm_loanpage: loan out pages to kernel (->K)
 *
 * => pages should be object-owned and the object should be locked.
 * => in the case of error, the object might be unlocked and relocked.
 * => caller should busy the pages beforehand.
 * => pages will be unbusied.
 * => fail with EBUSY if meet a wired page.
 */
static int
uvm_loanpage(struct vm_page **pgpp, int npages)
{
        int i;
        int error = 0;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);

        for (i = 0; i < npages; i++) {
                struct vm_page *pg = pgpp[i];

                KASSERT(pg->uobject != NULL);
                KASSERT(pg->uobject == pgpp[0]->uobject);
                KASSERT(!(pg->flags & (PG_RELEASED|PG_PAGEOUT)));
                KASSERT(mutex_owned(&pg->uobject->vmobjlock));
                KASSERT(pg->flags & PG_BUSY);

                mutex_enter(&uvm_pageqlock);
                if (pg->wire_count > 0) {
                        mutex_exit(&uvm_pageqlock);
                        UVMHIST_LOG(loanhist, "wired %p", pg,0,0,0);
                        error = EBUSY;
                        break;
                }
                if (pg->loan_count == 0) {
                        pmap_page_protect(pg, VM_PROT_READ);
                }
                pg->loan_count++;
                uvm_pageactivate(pg);
                mutex_exit(&uvm_pageqlock);
        }

        uvm_page_unbusy(pgpp, npages);

        if (error) {
                /*
                 * backout what we've done
                 */
                kmutex_t *slock = &pgpp[0]->uobject->vmobjlock;

                mutex_exit(slock);
                uvm_unloan(pgpp, i, UVM_LOAN_TOPAGE);
                mutex_enter(slock);
        }

        UVMHIST_LOG(loanhist, "done %d", error,0,0,0);
        return error;
}

/*
 * XXX UBC temp limit
 * number of pages to get at once.
 * should be <= MAX_READ_AHEAD in genfs_vnops.c
 */
#define UVM_LOAN_GET_CHUNK      16

/*
 * uvm_loanuobjpages: loan pages from a uobj out (O->K)
 *
 * => uobj shouldn't be locked.  (we'll lock it)
 * => fail with EBUSY if we meet a wired page.
 */
int
uvm_loanuobjpages(struct uvm_object *uobj, voff_t pgoff, int orignpages,
    struct vm_page **origpgpp)
{
        int ndone; /* # of pages loaned out */
        struct vm_page **pgpp;
        int error;
        int i;
        kmutex_t *slock;

        pgpp = origpgpp;
        for (ndone = 0; ndone < orignpages; ) {
                int npages;
                /* npendloan: # of pages busied but not loand out yet. */
                int npendloan = 0xdead; /* XXX gcc */
reget:
                npages = MIN(UVM_LOAN_GET_CHUNK, orignpages - ndone);
                mutex_enter(&uobj->vmobjlock);
                error = (*uobj->pgops->pgo_get)(uobj,
                    pgoff + (ndone << PAGE_SHIFT), pgpp, &npages, 0,
                    VM_PROT_READ, 0, PGO_SYNCIO);
                if (error == EAGAIN) {
                        tsleep(&lbolt, PVM, "loanuopg", 0);
                        continue;
                }
                if (error)
                        goto fail;

                KASSERT(npages > 0);

                /* loan and unbusy pages */
                slock = NULL;
                for (i = 0; i < npages; i++) {
                        kmutex_t *nextslock; /* slock for next page */
                        struct vm_page *pg = *pgpp;

                        /* XXX assuming that the page is owned by uobj */
                        KASSERT(pg->uobject != NULL);
                        nextslock = &pg->uobject->vmobjlock;

                        if (slock != nextslock) {
                                if (slock) {
                                        KASSERT(npendloan > 0);
                                        error = uvm_loanpage(pgpp - npendloan,
                                            npendloan);
                                        mutex_exit(slock);
                                        if (error)
                                                goto fail;
                                        ndone += npendloan;
                                        KASSERT(origpgpp + ndone == pgpp);
                                }
                                slock = nextslock;
                                npendloan = 0;
                                mutex_enter(slock);
                        }

                        if ((pg->flags & PG_RELEASED) != 0) {
                                /*
                                 * release pages and try again.
                                 */
                                mutex_exit(slock);
                                for (; i < npages; i++) {
                                        pg = pgpp[i];
                                        slock = &pg->uobject->vmobjlock;

                                        mutex_enter(slock);
                                        mutex_enter(&uvm_pageqlock);
                                        uvm_page_unbusy(&pg, 1);
                                        mutex_exit(&uvm_pageqlock);
                                        mutex_exit(slock);
                                }
                                goto reget;
                        }

                        npendloan++;
                        pgpp++;
                        KASSERT(origpgpp + ndone + npendloan == pgpp);
                }
                KASSERT(slock != NULL);
                KASSERT(npendloan > 0);
                error = uvm_loanpage(pgpp - npendloan, npendloan);
                mutex_exit(slock);
                if (error)
                        goto fail;
                ndone += npendloan;
                KASSERT(origpgpp + ndone == pgpp);
        }

        return 0;

fail:
        uvm_unloan(origpgpp, ndone, UVM_LOAN_TOPAGE);

        return error;
}

/*
 * uvm_loanuobj: loan a page from a uobj out
 *
 * => called with map, amap, uobj locked
 * => return value:
 *      -1 = fatal error, everything is unlocked, abort.
 *       0 = lookup in ufi went stale, everything unlocked, relookup and
 *              try again
 *       1 = got it, everything still locked
 */

static int
uvm_loanuobj(struct uvm_faultinfo *ufi, void ***output, int flags, vaddr_t va)
{
        struct vm_amap *amap = ufi->entry->aref.ar_amap;
        struct uvm_object *uobj = ufi->entry->object.uvm_obj;
        struct vm_page *pg;
        struct vm_anon *anon;
        int error, npages;
        bool locked;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);

        /*
         * first we must make sure the page is resident.
         *
         * XXXCDC: duplicate code with uvm_fault().
         */

        mutex_enter(&uobj->vmobjlock);
        if (uobj->pgops->pgo_get) {     /* try locked pgo_get */
                npages = 1;
                pg = NULL;
                error = (*uobj->pgops->pgo_get)(uobj,
                    va - ufi->entry->start + ufi->entry->offset,
                    &pg, &npages, 0, VM_PROT_READ, MADV_NORMAL, PGO_LOCKED);
        } else {
                error = EIO;            /* must have pgo_get op */
        }

        /*
         * check the result of the locked pgo_get.  if there is a problem,
         * then we fail the loan.
         */

        if (error && error != EBUSY) {
                uvmfault_unlockall(ufi, amap, uobj, NULL);
                return (-1);
        }

        /*
         * if we need to unlock for I/O, do so now.
         */

        if (error == EBUSY) {
                uvmfault_unlockall(ufi, amap, NULL, NULL);

                /* locked: uobj */
                npages = 1;
                error = (*uobj->pgops->pgo_get)(uobj,
                    va - ufi->entry->start + ufi->entry->offset,
                    &pg, &npages, 0, VM_PROT_READ, MADV_NORMAL, PGO_SYNCIO);
                /* locked: <nothing> */

                if (error) {
                        if (error == EAGAIN) {
                                tsleep(&lbolt, PVM, "fltagain2", 0);
                                return (0);
                        }
                        return (-1);
                }

                /*
                 * pgo_get was a success.   attempt to relock everything.
                 */

                locked = uvmfault_relock(ufi);
                if (locked && amap)
                        amap_lock(amap);
                uobj = pg->uobject;
                mutex_enter(&uobj->vmobjlock);

                /*
                 * verify that the page has not be released and re-verify
                 * that amap slot is still free.   if there is a problem we
                 * drop our lock (thus force a lookup refresh/retry).
                 */

                if ((pg->flags & PG_RELEASED) != 0 ||
                    (locked && amap && amap_lookup(&ufi->entry->aref,
                    ufi->orig_rvaddr - ufi->entry->start))) {
                        if (locked)
                                uvmfault_unlockall(ufi, amap, NULL, NULL);
                        locked = false;
                }

                /*
                 * didn't get the lock?   release the page and retry.
                 */

                if (locked == false) {
                        if (pg->flags & PG_WANTED) {
                                wakeup(pg);
                        }
                        if (pg->flags & PG_RELEASED) {
                                mutex_enter(&uvm_pageqlock);
                                uvm_pagefree(pg);
                                mutex_exit(&uvm_pageqlock);
                                mutex_exit(&uobj->vmobjlock);
                                return (0);
                        }
                        mutex_enter(&uvm_pageqlock);
                        uvm_pageactivate(pg);
                        mutex_exit(&uvm_pageqlock);
                        pg->flags &= ~(PG_BUSY|PG_WANTED);
                        UVM_PAGE_OWN(pg, NULL);
                        mutex_exit(&uobj->vmobjlock);
                        return (0);
                }
        }

        KASSERT(uobj == pg->uobject);

        /*
         * at this point we have the page we want ("pg") marked PG_BUSY for us
         * and we have all data structures locked.  do the loanout.  page can
         * not be PG_RELEASED (we caught this above).
         */

        if ((flags & UVM_LOAN_TOANON) == 0) {
                if (uvm_loanpage(&pg, 1)) {
                        uvmfault_unlockall(ufi, amap, uobj, NULL);
                        return (-1);
                }
                mutex_exit(&uobj->vmobjlock);
                **output = pg;
                (*output)++;
                return (1);
        }

        /*
         * must be a loan to an anon.   check to see if there is already
         * an anon associated with this page.  if so, then just return
         * a reference to this object.   the page should already be
         * mapped read-only because it is already on loan.
         */

        if (pg->uanon) {
                anon = pg->uanon;
                mutex_enter(&anon->an_lock);
                anon->an_ref++;
                mutex_exit(&anon->an_lock);
                if (pg->flags & PG_WANTED) {
                        wakeup(pg);
                }
                pg->flags &= ~(PG_WANTED|PG_BUSY);
                UVM_PAGE_OWN(pg, NULL);
                mutex_exit(&uobj->vmobjlock);
                **output = anon;
                (*output)++;
                return (1);
        }

        /*
         * need to allocate a new anon
         */

        anon = uvm_analloc();
        if (anon == NULL) {
                goto fail;
        }
        anon->an_page = pg;
        pg->uanon = anon;
        mutex_enter(&uvm_pageqlock);
        if (pg->wire_count > 0) {
                mutex_exit(&uvm_pageqlock);
                UVMHIST_LOG(loanhist, "wired %p", pg,0,0,0);
                pg->uanon = NULL;
                anon->an_page = NULL;
                anon->an_ref--;
                mutex_exit(&anon->an_lock);
                uvm_anfree(anon);
                goto fail;
        }
        if (pg->loan_count == 0) {
                pmap_page_protect(pg, VM_PROT_READ);
        }
        pg->loan_count++;
        uvm_pageactivate(pg);
        mutex_exit(&uvm_pageqlock);
        if (pg->flags & PG_WANTED) {
                wakeup(pg);
        }
        pg->flags &= ~(PG_WANTED|PG_BUSY);
        UVM_PAGE_OWN(pg, NULL);
        mutex_exit(&uobj->vmobjlock);
        mutex_exit(&anon->an_lock);
        **output = anon;
        (*output)++;
        return (1);

fail:
        UVMHIST_LOG(loanhist, "fail", 0,0,0,0);
        /*
         * unlock everything and bail out.
         */
        if (pg->flags & PG_WANTED) {
                wakeup(pg);
        }
        pg->flags &= ~(PG_WANTED|PG_BUSY);
        UVM_PAGE_OWN(pg, NULL);
        uvmfault_unlockall(ufi, amap, uobj, NULL);
        return (-1);
}

/*
 * uvm_loanzero: loan a zero-fill page out
 *
 * => called with map, amap, uobj locked
 * => return value:
 *      -1 = fatal error, everything is unlocked, abort.
 *       0 = lookup in ufi went stale, everything unlocked, relookup and
 *              try again
 *       1 = got it, everything still locked
 */

static struct uvm_object uvm_loanzero_object;

static int
uvm_loanzero(struct uvm_faultinfo *ufi, void ***output, int flags)
{
        struct vm_anon *anon;
        struct vm_page *pg;
        struct vm_amap *amap = ufi->entry->aref.ar_amap;

        UVMHIST_FUNC(__func__); UVMHIST_CALLED(loanhist);
again:
        mutex_enter(&uvm_loanzero_object.vmobjlock);

        /*
         * first, get ahold of our single zero page.
         */

        if (__predict_false((pg =
                             TAILQ_FIRST(&uvm_loanzero_object.memq)) == NULL)) {
                while ((pg = uvm_pagealloc(&uvm_loanzero_object, 0, NULL,
                                           UVM_PGA_ZERO)) == NULL) {
                        mutex_exit(&uvm_loanzero_object.vmobjlock);
                        uvmfault_unlockall(ufi, amap, NULL, NULL);
                        uvm_wait("loanzero");
                        if (!uvmfault_relock(ufi)) {
                                return (0);
                        }
                        if (amap) {
                                amap_lock(amap);
                        }
                        goto again;
                }

                /* got a zero'd page. */
                pg->flags &= ~(PG_WANTED|PG_BUSY|PG_FAKE);
                pg->flags |= PG_RDONLY;
                mutex_enter(&uvm_pageqlock);
                uvm_pageactivate(pg);
                mutex_exit(&uvm_pageqlock);
                UVM_PAGE_OWN(pg, NULL);
        }

        if ((flags & UVM_LOAN_TOANON) == 0) {   /* loaning to kernel-page */
                mutex_enter(&uvm_pageqlock);
                pg->loan_count++;
                mutex_exit(&uvm_pageqlock);
                mutex_exit(&uvm_loanzero_object.vmobjlock);
                **output = pg;
                (*output)++;
                return (1);
        }

        /*
         * loaning to an anon.  check to see if there is already an anon
         * associated with this page.  if so, then just return a reference
         * to this object.
         */

        if (pg->uanon) {
                anon = pg->uanon;
                mutex_enter(&anon->an_lock);
                anon->an_ref++;
                mutex_exit(&anon->an_lock);
                mutex_exit(&uvm_loanzero_object.vmobjlock);
                **output = anon;
                (*output)++;
                return (1);
        }

        /*
         * need to allocate a new anon
         */

        anon = uvm_analloc();
        if (anon == NULL) {
                /* out of swap causes us to fail */
                mutex_exit(&uvm_loanzero_object.vmobjlock);
                uvmfault_unlockall(ufi, amap, NULL, NULL);
                return (-1);
        }
        anon->an_page = pg;
        pg->uanon = anon;
        mutex_enter(&uvm_pageqlock);
        pg->loan_count++;
        uvm_pageactivate(pg);
        mutex_exit(&uvm_pageqlock);
        mutex_exit(&anon->an_lock);
        mutex_exit(&uvm_loanzero_object.vmobjlock);
        **output = anon;
        (*output)++;
        return (1);
}


/*
 * uvm_unloananon: kill loans on anons (basically a normal ref drop)
 *
 * => we expect all our resources to be unlocked
 */

static void
uvm_unloananon(struct vm_anon **aloans, int nanons)
{
        struct vm_anon *anon;

        while (nanons-- > 0) {
                int refs;

                anon = *aloans++;
                mutex_enter(&anon->an_lock);
                refs = --anon->an_ref;
                mutex_exit(&anon->an_lock);

                if (refs == 0) {
                        uvm_anfree(anon);
                }
        }
}

/*
 * uvm_unloanpage: kill loans on pages loaned out to the kernel
 *
 * => we expect all our resources to be unlocked
 */

static void
uvm_unloanpage(struct vm_page **ploans, int npages)
{
        struct vm_page *pg;
        kmutex_t *slock;

        mutex_enter(&uvm_pageqlock);
        while (npages-- > 0) {
                pg = *ploans++;

                /*
                 * do a little dance to acquire the object or anon lock
                 * as appropriate.  we are locking in the wrong order,
                 * so we have to do a try-lock here.
                 */

                slock = NULL;
                while (pg->uobject != NULL || pg->uanon != NULL) {
                        if (pg->uobject != NULL) {
                                slock = &pg->uobject->vmobjlock;
                        } else {
                                slock = &pg->uanon->an_lock;
                        }
                        if (mutex_tryenter(slock)) {
                                break;
                        }
                        mutex_exit(&uvm_pageqlock);
                        /* XXX Better than yielding but inadequate. */
                        kpause("livelock", false, 1, NULL);
                        mutex_enter(&uvm_pageqlock);
                        slock = NULL;
                }

                /*
                 * drop our loan.  if page is owned by an anon but
                 * PQ_ANON is not set, the page was loaned to the anon
                 * from an object which dropped ownership, so resolve
                 * this by turning the anon's loan into real ownership
                 * (ie. decrement loan_count again and set PQ_ANON).
                 * after all this, if there are no loans left, put the
                 * page back a paging queue (if the page is owned by
                 * an anon) or free it (if the page is now unowned).
                 */

                KASSERT(pg->loan_count > 0);
                pg->loan_count--;
                if (pg->uobject == NULL && pg->uanon != NULL &&
                    (pg->pqflags & PQ_ANON) == 0) {
                        KASSERT(pg->loan_count > 0);
                        pg->loan_count--;
                        pg->pqflags |= PQ_ANON;
                }
                if (pg->loan_count == 0 && pg->uobject == NULL &&
                    pg->uanon == NULL) {
                        KASSERT((pg->flags & PG_BUSY) == 0);
                        uvm_pagefree(pg);
                }
                if (slock != NULL) {
                        mutex_exit(slock);
                }
        }
        mutex_exit(&uvm_pageqlock);
}

/*
 * uvm_unloan: kill loans on pages or anons.
 */

void
uvm_unloan(void *v, int npages, int flags)
{
        if (flags & UVM_LOAN_TOANON) {
                uvm_unloananon(v, npages);
        } else {
                uvm_unloanpage(v, npages);
        }
}

/*
 * Minimal pager for uvm_loanzero_object.  We need to provide a "put"
 * method, because the page can end up on a paging queue, and the
 * page daemon will want to call pgo_put when it encounters the page
 * on the inactive list.
 */

static int
ulz_put(struct uvm_object *uobj, voff_t start, voff_t stop, int flags)
{
        struct vm_page *pg;

        KDASSERT(uobj == &uvm_loanzero_object);

        /*
         * Don't need to do any work here if we're not freeing pages.
         */

        if ((flags & PGO_FREE) == 0) {
                mutex_exit(&uobj->vmobjlock);
                return 0;
        }

        /*
         * we don't actually want to ever free the uvm_loanzero_page, so
         * just reactivate or dequeue it.
         */

        pg = TAILQ_FIRST(&uobj->memq);
        KASSERT(pg != NULL);
        KASSERT(TAILQ_NEXT(pg, listq.queue) == NULL);

        mutex_enter(&uvm_pageqlock);
        if (pg->uanon)
                uvm_pageactivate(pg);
        else
                uvm_pagedequeue(pg);
        mutex_exit(&uvm_pageqlock);

        mutex_exit(&uobj->vmobjlock);
        return 0;
}

static const struct uvm_pagerops ulz_pager = {
        .pgo_put = ulz_put,
};

/*
 * uvm_loan_init(): initialize the uvm_loan() facility.
 */

void
uvm_loan_init(void)
{

        UVM_OBJ_INIT(&uvm_loanzero_object, &ulz_pager, 0);

        UVMHIST_INIT(loanhist, 300);
}

/*
 * uvm_loanbreak: break loan on a uobj page
 *
 * => called with uobj locked
 * => the page should be busy
 * => return value:
 *      newly allocated page if succeeded
 */
struct vm_page *
uvm_loanbreak(struct vm_page *uobjpage)
{
        struct vm_page *pg;
#ifdef DIAGNOSTIC
        struct uvm_object *uobj = uobjpage->uobject;
#endif

        KASSERT(uobj != NULL);
        KASSERT(mutex_owned(&uobj->vmobjlock));
        KASSERT(uobjpage->flags & PG_BUSY);

        /* alloc new un-owned page */
        pg = uvm_pagealloc(NULL, 0, NULL, 0);
        if (pg == NULL)
                return NULL;

        /*
         * copy the data from the old page to the new
         * one and clear the fake flags on the new page (keep it busy).
         * force a reload of the old page by clearing it from all
         * pmaps.
         * transfer dirtiness of the old page to the new page.
         * then lock the page queues to rename the pages.
         */

        uvm_pagecopy(uobjpage, pg);     /* old -> new */
        pg->flags &= ~PG_FAKE;
        pmap_page_protect(uobjpage, VM_PROT_NONE);
        if ((uobjpage->flags & PG_CLEAN) != 0 && !pmap_clear_modify(uobjpage)) {
                pmap_clear_modify(pg);
                pg->flags |= PG_CLEAN;
        } else {
                /* uvm_pagecopy marked it dirty */
                KASSERT((pg->flags & PG_CLEAN) == 0);
                /* a object with a dirty page should be dirty. */
                KASSERT(!UVM_OBJ_IS_CLEAN(uobj));
        }
        if (uobjpage->flags & PG_WANTED)
                wakeup(uobjpage);
        /* uobj still locked */
        uobjpage->flags &= ~(PG_WANTED|PG_BUSY);
        UVM_PAGE_OWN(uobjpage, NULL);

        mutex_enter(&uvm_pageqlock);

        /*
         * replace uobjpage with new page.
         */

        uvm_pagereplace(uobjpage, pg);

        /*
         * if the page is no longer referenced by
         * an anon (i.e. we are breaking an O->K
         * loan), then remove it from any pageq's.
         */
        if (uobjpage->uanon == NULL)
                uvm_pagedequeue(uobjpage);

        /*
         * at this point we have absolutely no
         * control over uobjpage
         */

        /* install new page */
        uvm_pageactivate(pg);
        mutex_exit(&uvm_pageqlock);

        /*
         * done!  loan is broken and "pg" is
         * PG_BUSY.   it can now replace uobjpage.
         */

        return pg;
}