FreeBSD/Linux Kernel Cross Reference
sys/uvm/uvm_aobj.c

     /*      $NetBSD: uvm_aobj.c,v 1.104 2008/10/18 03:46:22 rmind Exp $     */
     
     /*
      * Copyright (c) 1998 Chuck Silvers, 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_aobj.c,v 1.1.2.5 1998/02/06 05:14:38 chs Exp
     */
    /*
     * uvm_aobj.c: anonymous memory uvm_object pager
     *
     * author: Chuck Silvers <chuq@chuq.com>
     * started: Jan-1998
     *
     * - design mostly from Chuck Cranor
     */
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: uvm_aobj.c,v 1.104 2008/10/18 03:46:22 rmind Exp $");
    
    #include "opt_uvmhist.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/proc.h>
    #include <sys/kernel.h>
    #include <sys/kmem.h>
    #include <sys/pool.h>
    
    #include <uvm/uvm.h>
    
    /*
     * an aobj manages anonymous-memory backed uvm_objects.   in addition
     * to keeping the list of resident pages, it also keeps a list of
     * allocated swap blocks.  depending on the size of the aobj this list
     * of allocated swap blocks is either stored in an array (small objects)
     * or in a hash table (large objects).
     */
    
    /*
     * local structures
     */
    
    /*
     * for hash tables, we break the address space of the aobj into blocks
     * of UAO_SWHASH_CLUSTER_SIZE pages.   we require the cluster size to
     * be a power of two.
     */
    
    #define UAO_SWHASH_CLUSTER_SHIFT 4
    #define UAO_SWHASH_CLUSTER_SIZE (1 << UAO_SWHASH_CLUSTER_SHIFT)
    
    /* get the "tag" for this page index */
    #define UAO_SWHASH_ELT_TAG(PAGEIDX) \
            ((PAGEIDX) >> UAO_SWHASH_CLUSTER_SHIFT)
    
    #define UAO_SWHASH_ELT_PAGESLOT_IDX(PAGEIDX) \
            ((PAGEIDX) & (UAO_SWHASH_CLUSTER_SIZE - 1))
    
    /* given an ELT and a page index, find the swap slot */
    #define UAO_SWHASH_ELT_PAGESLOT(ELT, PAGEIDX) \
            ((ELT)->slots[UAO_SWHASH_ELT_PAGESLOT_IDX(PAGEIDX)])
    
    /* given an ELT, return its pageidx base */
    #define UAO_SWHASH_ELT_PAGEIDX_BASE(ELT) \
            ((ELT)->tag << UAO_SWHASH_CLUSTER_SHIFT)
    
    /*
     * the swhash hash function
     */
    
    #define UAO_SWHASH_HASH(AOBJ, PAGEIDX) \
           (&(AOBJ)->u_swhash[(((PAGEIDX) >> UAO_SWHASH_CLUSTER_SHIFT) \
                               & (AOBJ)->u_swhashmask)])
   
   /*
    * the swhash threshhold determines if we will use an array or a
    * hash table to store the list of allocated swap blocks.
    */
   
   #define UAO_SWHASH_THRESHOLD (UAO_SWHASH_CLUSTER_SIZE * 4)
   #define UAO_USES_SWHASH(AOBJ) \
           ((AOBJ)->u_pages > UAO_SWHASH_THRESHOLD)        /* use hash? */
   
   /*
    * the number of buckets in a swhash, with an upper bound
    */
   
   #define UAO_SWHASH_MAXBUCKETS 256
   #define UAO_SWHASH_BUCKETS(AOBJ) \
           (MIN((AOBJ)->u_pages >> UAO_SWHASH_CLUSTER_SHIFT, \
                UAO_SWHASH_MAXBUCKETS))
   
   /*
    * uao_swhash_elt: when a hash table is being used, this structure defines
    * the format of an entry in the bucket list.
    */
   
   struct uao_swhash_elt {
           LIST_ENTRY(uao_swhash_elt) list;        /* the hash list */
           voff_t tag;                             /* our 'tag' */
           int count;                              /* our number of active slots */
           int slots[UAO_SWHASH_CLUSTER_SIZE];     /* the slots */
   };
   
   /*
    * uao_swhash: the swap hash table structure
    */
   
   LIST_HEAD(uao_swhash, uao_swhash_elt);
   
   /*
    * uao_swhash_elt_pool: pool of uao_swhash_elt structures
    * NOTE: Pages for this pool must not come from a pageable kernel map!
    */
   static POOL_INIT(uao_swhash_elt_pool, sizeof(struct uao_swhash_elt), 0, 0, 0,
       "uaoeltpl", NULL, IPL_VM);
   
   static struct pool_cache uvm_aobj_cache;
   
   /*
    * uvm_aobj: the actual anon-backed uvm_object
    *
    * => the uvm_object is at the top of the structure, this allows
    *   (struct uvm_aobj *) == (struct uvm_object *)
    * => only one of u_swslots and u_swhash is used in any given aobj
    */
   
   struct uvm_aobj {
           struct uvm_object u_obj; /* has: lock, pgops, memq, #pages, #refs */
           pgoff_t u_pages;         /* number of pages in entire object */
           int u_flags;             /* the flags (see uvm_aobj.h) */
           int *u_swslots;          /* array of offset->swapslot mappings */
                                    /*
                                     * hashtable of offset->swapslot mappings
                                     * (u_swhash is an array of bucket heads)
                                     */
           struct uao_swhash *u_swhash;
           u_long u_swhashmask;            /* mask for hashtable */
           LIST_ENTRY(uvm_aobj) u_list;    /* global list of aobjs */
   };
   
   /*
    * local functions
    */
   
   static void     uao_free(struct uvm_aobj *);
   static int      uao_get(struct uvm_object *, voff_t, struct vm_page **,
                       int *, int, vm_prot_t, int, int);
   static int      uao_put(struct uvm_object *, voff_t, voff_t, int);
   
   #if defined(VMSWAP)
   static struct uao_swhash_elt *uao_find_swhash_elt
       (struct uvm_aobj *, int, bool);
   
   static bool uao_pagein(struct uvm_aobj *, int, int);
   static bool uao_pagein_page(struct uvm_aobj *, int);
   static void uao_dropswap_range1(struct uvm_aobj *, voff_t, voff_t);
   #endif /* defined(VMSWAP) */
   
   /*
    * aobj_pager
    *
    * note that some functions (e.g. put) are handled elsewhere
    */
   
   const struct uvm_pagerops aobj_pager = {
           .pgo_reference = uao_reference,
           .pgo_detach = uao_detach,
           .pgo_get = uao_get,
           .pgo_put = uao_put,
   };
   
   /*
    * uao_list: global list of active aobjs, locked by uao_list_lock
    */
   
   static LIST_HEAD(aobjlist, uvm_aobj) uao_list;
   static kmutex_t uao_list_lock;
   
   /*
    * functions
    */
   
   /*
    * hash table/array related functions
    */
   
   #if defined(VMSWAP)
   
   /*
    * uao_find_swhash_elt: find (or create) a hash table entry for a page
    * offset.
    *
    * => the object should be locked by the caller
    */
   
   static struct uao_swhash_elt *
   uao_find_swhash_elt(struct uvm_aobj *aobj, int pageidx, bool create)
   {
           struct uao_swhash *swhash;
           struct uao_swhash_elt *elt;
           voff_t page_tag;
   
           swhash = UAO_SWHASH_HASH(aobj, pageidx);
           page_tag = UAO_SWHASH_ELT_TAG(pageidx);
   
           /*
            * now search the bucket for the requested tag
            */
   
           LIST_FOREACH(elt, swhash, list) {
                   if (elt->tag == page_tag) {
                           return elt;
                   }
           }
           if (!create) {
                   return NULL;
           }
   
           /*
            * allocate a new entry for the bucket and init/insert it in
            */
   
           elt = pool_get(&uao_swhash_elt_pool, PR_NOWAIT);
           if (elt == NULL) {
                   return NULL;
           }
           LIST_INSERT_HEAD(swhash, elt, list);
           elt->tag = page_tag;
           elt->count = 0;
           memset(elt->slots, 0, sizeof(elt->slots));
           return elt;
   }
   
   /*
    * uao_find_swslot: find the swap slot number for an aobj/pageidx
    *
    * => object must be locked by caller
    */
   
   int
   uao_find_swslot(struct uvm_object *uobj, int pageidx)
   {
           struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
           struct uao_swhash_elt *elt;
   
           /*
            * if noswap flag is set, then we never return a slot
            */
   
           if (aobj->u_flags & UAO_FLAG_NOSWAP)
                   return(0);
   
           /*
            * if hashing, look in hash table.
            */
   
           if (UAO_USES_SWHASH(aobj)) {
                   elt = uao_find_swhash_elt(aobj, pageidx, false);
                   if (elt)
                           return(UAO_SWHASH_ELT_PAGESLOT(elt, pageidx));
                   else
                           return(0);
           }
   
           /*
            * otherwise, look in the array
            */
   
           return(aobj->u_swslots[pageidx]);
   }
   
   /*
    * uao_set_swslot: set the swap slot for a page in an aobj.
    *
    * => setting a slot to zero frees the slot
    * => object must be locked by caller
    * => we return the old slot number, or -1 if we failed to allocate
    *    memory to record the new slot number
    */
   
   int
   uao_set_swslot(struct uvm_object *uobj, int pageidx, int slot)
   {
           struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
           struct uao_swhash_elt *elt;
           int oldslot;
           UVMHIST_FUNC("uao_set_swslot"); UVMHIST_CALLED(pdhist);
           UVMHIST_LOG(pdhist, "aobj %p pageidx %d slot %d",
               aobj, pageidx, slot, 0);
   
           /*
            * if noswap flag is set, then we can't set a non-zero slot.
            */
   
           if (aobj->u_flags & UAO_FLAG_NOSWAP) {
                   if (slot == 0)
                           return(0);
   
                   printf("uao_set_swslot: uobj = %p\n", uobj);
                   panic("uao_set_swslot: NOSWAP object");
           }
   
           /*
            * are we using a hash table?  if so, add it in the hash.
            */
   
           if (UAO_USES_SWHASH(aobj)) {
   
                   /*
                    * Avoid allocating an entry just to free it again if
                    * the page had not swap slot in the first place, and
                    * we are freeing.
                    */
   
                   elt = uao_find_swhash_elt(aobj, pageidx, slot != 0);
                   if (elt == NULL) {
                           return slot ? -1 : 0;
                   }
   
                   oldslot = UAO_SWHASH_ELT_PAGESLOT(elt, pageidx);
                   UAO_SWHASH_ELT_PAGESLOT(elt, pageidx) = slot;
   
                   /*
                    * now adjust the elt's reference counter and free it if we've
                    * dropped it to zero.
                    */
   
                   if (slot) {
                           if (oldslot == 0)
                                   elt->count++;
                   } else {
                           if (oldslot)
                                   elt->count--;
   
                           if (elt->count == 0) {
                                   LIST_REMOVE(elt, list);
                                   pool_put(&uao_swhash_elt_pool, elt);
                           }
                   }
           } else {
                   /* we are using an array */
                   oldslot = aobj->u_swslots[pageidx];
                   aobj->u_swslots[pageidx] = slot;
           }
           return (oldslot);
   }
   
   #endif /* defined(VMSWAP) */
   
   /*
    * end of hash/array functions
    */
   
   /*
    * uao_free: free all resources held by an aobj, and then free the aobj
    *
    * => the aobj should be dead
    */
   
   static void
   uao_free(struct uvm_aobj *aobj)
   {
           int swpgonlydelta = 0;
   
   
   #if defined(VMSWAP)
           uao_dropswap_range1(aobj, 0, 0);
   #endif /* defined(VMSWAP) */
   
           mutex_exit(&aobj->u_obj.vmobjlock);
   
   #if defined(VMSWAP)
           if (UAO_USES_SWHASH(aobj)) {
   
                   /*
                    * free the hash table itself.
                    */
   
                   hashdone(aobj->u_swhash, HASH_LIST, aobj->u_swhashmask);
           } else {
   
                   /*
                    * free the array itsself.
                    */
   
                   kmem_free(aobj->u_swslots, aobj->u_pages * sizeof(int));
           }
   #endif /* defined(VMSWAP) */
   
           /*
            * finally free the aobj itself
            */
   
           UVM_OBJ_DESTROY(&aobj->u_obj);
           pool_cache_put(&uvm_aobj_cache, aobj);
   
           /*
            * adjust the counter of pages only in swap for all
            * the swap slots we've freed.
            */
   
           if (swpgonlydelta > 0) {
                   mutex_enter(&uvm_swap_data_lock);
                   KASSERT(uvmexp.swpgonly >= swpgonlydelta);
                   uvmexp.swpgonly -= swpgonlydelta;
                   mutex_exit(&uvm_swap_data_lock);
           }
   }
   
   /*
    * pager functions
    */
   
   /*
    * uao_create: create an aobj of the given size and return its uvm_object.
    *
    * => for normal use, flags are always zero
    * => for the kernel object, the flags are:
    *      UAO_FLAG_KERNOBJ - allocate the kernel object (can only happen once)
    *      UAO_FLAG_KERNSWAP - enable swapping of kernel object ("           ")
    */
   
   struct uvm_object *
   uao_create(vsize_t size, int flags)
   {
           static struct uvm_aobj kernel_object_store;
           static int kobj_alloced = 0;
           pgoff_t pages = round_page(size) >> PAGE_SHIFT;
           struct uvm_aobj *aobj;
           int refs;
   
           /*
            * malloc a new aobj unless we are asked for the kernel object
            */
   
           if (flags & UAO_FLAG_KERNOBJ) {
                   KASSERT(!kobj_alloced);
                   aobj = &kernel_object_store;
                   aobj->u_pages = pages;
                   aobj->u_flags = UAO_FLAG_NOSWAP;
                   refs = UVM_OBJ_KERN;
                   kobj_alloced = UAO_FLAG_KERNOBJ;
           } else if (flags & UAO_FLAG_KERNSWAP) {
                   KASSERT(kobj_alloced == UAO_FLAG_KERNOBJ);
                   aobj = &kernel_object_store;
                   kobj_alloced = UAO_FLAG_KERNSWAP;
                   refs = 0xdeadbeaf; /* XXX: gcc */
           } else {
                   aobj = pool_cache_get(&uvm_aobj_cache, PR_WAITOK);
                   aobj->u_pages = pages;
                   aobj->u_flags = 0;
                   refs = 1;
           }
   
           /*
            * allocate hash/array if necessary
            *
            * note: in the KERNSWAP case no need to worry about locking since
            * we are still booting we should be the only thread around.
            */
   
           if (flags == 0 || (flags & UAO_FLAG_KERNSWAP) != 0) {
   #if defined(VMSWAP)
                   const int kernswap = (flags & UAO_FLAG_KERNSWAP) != 0;
   
                   /* allocate hash table or array depending on object size */
                   if (UAO_USES_SWHASH(aobj)) {
                           aobj->u_swhash = hashinit(UAO_SWHASH_BUCKETS(aobj),
                               HASH_LIST, kernswap ? false : true,
                               &aobj->u_swhashmask);
                           if (aobj->u_swhash == NULL)
                                   panic("uao_create: hashinit swhash failed");
                   } else {
                           aobj->u_swslots = kmem_zalloc(pages * sizeof(int),
                               kernswap ? KM_NOSLEEP : KM_SLEEP);
                           if (aobj->u_swslots == NULL)
                                   panic("uao_create: malloc swslots failed");
                   }
   #endif /* defined(VMSWAP) */
   
                   if (flags) {
                           aobj->u_flags &= ~UAO_FLAG_NOSWAP; /* clear noswap */
                           return(&aobj->u_obj);
                   }
           }
   
           /*
            * init aobj fields
            */
   
           UVM_OBJ_INIT(&aobj->u_obj, &aobj_pager, refs);
   
           /*
            * now that aobj is ready, add it to the global list
            */
   
           mutex_enter(&uao_list_lock);
           LIST_INSERT_HEAD(&uao_list, aobj, u_list);
           mutex_exit(&uao_list_lock);
           return(&aobj->u_obj);
   }
   
   
   
   /*
    * uao_init: set up aobj pager subsystem
    *
    * => called at boot time from uvm_pager_init()
    */
   
   void
   uao_init(void)
   {
           static int uao_initialized;
   
           if (uao_initialized)
                   return;
           uao_initialized = true;
           LIST_INIT(&uao_list);
           mutex_init(&uao_list_lock, MUTEX_DEFAULT, IPL_NONE);
           pool_cache_bootstrap(&uvm_aobj_cache, sizeof(struct uvm_aobj), 0, 0,
               0, "aobj", NULL, IPL_NONE, NULL, NULL, NULL);
   }
   
   /*
    * uao_reference: add a ref to an aobj
    *
    * => aobj must be unlocked
    * => just lock it and call the locked version
    */
   
   void
   uao_reference(struct uvm_object *uobj)
   {
   
           /*
            * kernel_object already has plenty of references, leave it alone.
            */
   
           if (UVM_OBJ_IS_KERN_OBJECT(uobj))
                   return;
   
           mutex_enter(&uobj->vmobjlock);
           uao_reference_locked(uobj);
           mutex_exit(&uobj->vmobjlock);
   }
   
   /*
    * uao_reference_locked: add a ref to an aobj that is already locked
    *
    * => aobj must be locked
    * this needs to be separate from the normal routine
    * since sometimes we need to add a reference to an aobj when
    * it's already locked.
    */
   
   void
   uao_reference_locked(struct uvm_object *uobj)
   {
           UVMHIST_FUNC("uao_reference"); UVMHIST_CALLED(maphist);
   
           /*
            * kernel_object already has plenty of references, leave it alone.
            */
   
           if (UVM_OBJ_IS_KERN_OBJECT(uobj))
                   return;
   
           uobj->uo_refs++;
           UVMHIST_LOG(maphist, "<- done (uobj=0x%x, ref = %d)",
                       uobj, uobj->uo_refs,0,0);
   }
   
   /*
    * uao_detach: drop a reference to an aobj
    *
    * => aobj must be unlocked
    * => just lock it and call the locked version
    */
   
   void
   uao_detach(struct uvm_object *uobj)
   {
   
           /*
            * detaching from kernel_object is a noop.
            */
   
           if (UVM_OBJ_IS_KERN_OBJECT(uobj))
                   return;
   
           mutex_enter(&uobj->vmobjlock);
           uao_detach_locked(uobj);
   }
   
   /*
    * uao_detach_locked: drop a reference to an aobj
    *
    * => aobj must be locked, and is unlocked (or freed) upon return.
    * this needs to be separate from the normal routine
    * since sometimes we need to detach from an aobj when
    * it's already locked.
    */
   
   void
   uao_detach_locked(struct uvm_object *uobj)
   {
           struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
           struct vm_page *pg;
           UVMHIST_FUNC("uao_detach"); UVMHIST_CALLED(maphist);
   
           /*
            * detaching from kernel_object is a noop.
            */
   
           if (UVM_OBJ_IS_KERN_OBJECT(uobj)) {
                   mutex_exit(&uobj->vmobjlock);
                   return;
           }
   
           UVMHIST_LOG(maphist,"  (uobj=0x%x)  ref=%d", uobj,uobj->uo_refs,0,0);
           uobj->uo_refs--;
           if (uobj->uo_refs) {
                   mutex_exit(&uobj->vmobjlock);
                   UVMHIST_LOG(maphist, "<- done (rc>0)", 0,0,0,0);
                   return;
           }
   
           /*
            * remove the aobj from the global list.
            */
   
           mutex_enter(&uao_list_lock);
           LIST_REMOVE(aobj, u_list);
           mutex_exit(&uao_list_lock);
   
           /*
            * free all the pages left in the aobj.  for each page,
            * when the page is no longer busy (and thus after any disk i/o that
            * it's involved in is complete), release any swap resources and
            * free the page itself.
            */
   
           mutex_enter(&uvm_pageqlock);
           while ((pg = TAILQ_FIRST(&uobj->memq)) != NULL) {
                   pmap_page_protect(pg, VM_PROT_NONE);
                   if (pg->flags & PG_BUSY) {
                           pg->flags |= PG_WANTED;
                           mutex_exit(&uvm_pageqlock);
                           UVM_UNLOCK_AND_WAIT(pg, &uobj->vmobjlock, false,
                               "uao_det", 0);
                           mutex_enter(&uobj->vmobjlock);
                           mutex_enter(&uvm_pageqlock);
                           continue;
                   }
                   uao_dropswap(&aobj->u_obj, pg->offset >> PAGE_SHIFT);
                   uvm_pagefree(pg);
           }
           mutex_exit(&uvm_pageqlock);
   
           /*
            * finally, free the aobj itself.
            */
   
           uao_free(aobj);
   }
   
   /*
    * uao_put: flush pages out of a uvm object
    *
    * => object should be locked by caller.  we may _unlock_ the object
    *      if (and only if) we need to clean a page (PGO_CLEANIT).
    *      XXXJRT Currently, however, we don't.  In the case of cleaning
    *      XXXJRT a page, we simply just deactivate it.  Should probably
    *      XXXJRT handle this better, in the future (although "flushing"
    *      XXXJRT anonymous memory isn't terribly important).
    * => if PGO_CLEANIT is not set, then we will neither unlock the object
    *      or block.
    * => if PGO_ALLPAGE is set, then all pages in the object are valid targets
    *      for flushing.
    * => NOTE: we rely on the fact that the object's memq is a TAILQ and
    *      that new pages are inserted on the tail end of the list.  thus,
    *      we can make a complete pass through the object in one go by starting
    *      at the head and working towards the tail (new pages are put in
    *      front of us).
    * => NOTE: we are allowed to lock the page queues, so the caller
    *      must not be holding the lock on them [e.g. pagedaemon had
    *      better not call us with the queues locked]
    * => we return 0 unless we encountered some sort of I/O error
    *      XXXJRT currently never happens, as we never directly initiate
    *      XXXJRT I/O
    *
    * note on page traversal:
    *      we can traverse the pages in an object either by going down the
    *      linked list in "uobj->memq", or we can go over the address range
    *      by page doing hash table lookups for each address.  depending
    *      on how many pages are in the object it may be cheaper to do one
    *      or the other.  we set "by_list" to true if we are using memq.
    *      if the cost of a hash lookup was equal to the cost of the list
    *      traversal we could compare the number of pages in the start->stop
    *      range to the total number of pages in the object.  however, it
    *      seems that a hash table lookup is more expensive than the linked
    *      list traversal, so we multiply the number of pages in the
    *      start->stop range by a penalty which we define below.
    */
   
   static int
   uao_put(struct uvm_object *uobj, voff_t start, voff_t stop, int flags)
   {
           struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
           struct vm_page *pg, *nextpg, curmp, endmp;
           bool by_list;
           voff_t curoff;
           UVMHIST_FUNC("uao_put"); UVMHIST_CALLED(maphist);
   
           KASSERT(mutex_owned(&uobj->vmobjlock));
   
           curoff = 0;
           if (flags & PGO_ALLPAGES) {
                   start = 0;
                   stop = aobj->u_pages << PAGE_SHIFT;
                   by_list = true;         /* always go by the list */
           } else {
                   start = trunc_page(start);
                   if (stop == 0) {
                           stop = aobj->u_pages << PAGE_SHIFT;
                   } else {
                           stop = round_page(stop);
                   }
                   if (stop > (aobj->u_pages << PAGE_SHIFT)) {
                           printf("uao_flush: strange, got an out of range "
                               "flush (fixed)\n");
                           stop = aobj->u_pages << PAGE_SHIFT;
                   }
                   by_list = (uobj->uo_npages <=
                       ((stop - start) >> PAGE_SHIFT) * UVM_PAGE_HASH_PENALTY);
           }
           UVMHIST_LOG(maphist,
               " flush start=0x%lx, stop=0x%x, by_list=%d, flags=0x%x",
               start, stop, by_list, flags);
   
           /*
            * Don't need to do any work here if we're not freeing
            * or deactivating pages.
            */
   
           if ((flags & (PGO_DEACTIVATE|PGO_FREE)) == 0) {
                   mutex_exit(&uobj->vmobjlock);
                   return 0;
           }
   
           /*
            * Initialize the marker pages.  See the comment in
            * genfs_putpages() also.
            */
   
           curmp.uobject = uobj;
           curmp.offset = (voff_t)-1;
           curmp.flags = PG_BUSY;
           endmp.uobject = uobj;
           endmp.offset = (voff_t)-1;
           endmp.flags = PG_BUSY;
   
           /*
            * now do it.  note: we must update nextpg in the body of loop or we
            * will get stuck.  we need to use nextpg if we'll traverse the list
            * because we may free "pg" before doing the next loop.
            */
   
           if (by_list) {
                   TAILQ_INSERT_TAIL(&uobj->memq, &endmp, listq.queue);
                   nextpg = TAILQ_FIRST(&uobj->memq);
                   uvm_lwp_hold(curlwp);
           } else {
                   curoff = start;
                   nextpg = NULL;  /* Quell compiler warning */
           }
   
           /* locked: uobj */
           for (;;) {
                   if (by_list) {
                           pg = nextpg;
                           if (pg == &endmp)
                                   break;
                           nextpg = TAILQ_NEXT(pg, listq.queue);
                           if (pg->offset < start || pg->offset >= stop)
                                   continue;
                   } else {
                           if (curoff < stop) {
                                   pg = uvm_pagelookup(uobj, curoff);
                                   curoff += PAGE_SIZE;
                           } else
                                   break;
                           if (pg == NULL)
                                   continue;
                   }
   
                   /*
                    * wait and try again if the page is busy.
                    */
   
                   if (pg->flags & PG_BUSY) {
                           if (by_list) {
                                   TAILQ_INSERT_BEFORE(pg, &curmp, listq.queue);
                           }
                           pg->flags |= PG_WANTED;
                           UVM_UNLOCK_AND_WAIT(pg, &uobj->vmobjlock, 0,
                               "uao_put", 0);
                           mutex_enter(&uobj->vmobjlock);
                           if (by_list) {
                                   nextpg = TAILQ_NEXT(&curmp, listq.queue);
                                   TAILQ_REMOVE(&uobj->memq, &curmp,
                                       listq.queue);
                           } else
                                   curoff -= PAGE_SIZE;
                           continue;
                   }
   
                   switch (flags & (PGO_CLEANIT|PGO_FREE|PGO_DEACTIVATE)) {
   
                   /*
                    * XXX In these first 3 cases, we always just
                    * XXX deactivate the page.  We may want to
                    * XXX handle the different cases more specifically
                    * XXX in the future.
                    */
   
                   case PGO_CLEANIT|PGO_FREE:
                   case PGO_CLEANIT|PGO_DEACTIVATE:
                   case PGO_DEACTIVATE:
    deactivate_it:
                           mutex_enter(&uvm_pageqlock);
                           /* skip the page if it's wired */
                           if (pg->wire_count == 0) {
                                   uvm_pagedeactivate(pg);
                           }
                           mutex_exit(&uvm_pageqlock);
                           break;
   
                   case PGO_FREE:
                           /*
                            * If there are multiple references to
                            * the object, just deactivate the page.
                            */
   
                           if (uobj->uo_refs > 1)
                                   goto deactivate_it;
   
                           /*
                            * free the swap slot and the page.
                            */
   
                           pmap_page_protect(pg, VM_PROT_NONE);
   
                           /*
                            * freeing swapslot here is not strictly necessary.
                            * however, leaving it here doesn't save much
                            * because we need to update swap accounting anyway.
                            */
   
                           uao_dropswap(uobj, pg->offset >> PAGE_SHIFT);
                           mutex_enter(&uvm_pageqlock);
                           uvm_pagefree(pg);
                           mutex_exit(&uvm_pageqlock);
                           break;
   
                   default:
                           panic("%s: impossible", __func__);
                   }
           }
           if (by_list) {
                   TAILQ_REMOVE(&uobj->memq, &endmp, listq.queue);
                   uvm_lwp_rele(curlwp);
           }
           mutex_exit(&uobj->vmobjlock);
           return 0;
   }
   
   /*
    * uao_get: fetch me a page
    *
    * we have three cases:
    * 1: page is resident     -> just return the page.
    * 2: page is zero-fill    -> allocate a new page and zero it.
    * 3: page is swapped out  -> fetch the page from swap.
    *
    * cases 1 and 2 can be handled with PGO_LOCKED, case 3 cannot.
    * so, if the "center" page hits case 3 (or any page, with PGO_ALLPAGES),
    * then we will need to return EBUSY.
    *
    * => prefer map unlocked (not required)
    * => object must be locked!  we will _unlock_ it before starting any I/O.
    * => flags: PGO_ALLPAGES: get all of the pages
    *           PGO_LOCKED: fault data structures are locked
    * => NOTE: offset is the offset of pps[0], _NOT_ pps[centeridx]
    * => NOTE: caller must check for released pages!!
    */
   
   static int
   uao_get(struct uvm_object *uobj, voff_t offset, struct vm_page **pps,
       int *npagesp, int centeridx, vm_prot_t access_type, int advice, int flags)
   {
   #if defined(VMSWAP)
           struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
   #endif /* defined(VMSWAP) */
           voff_t current_offset;
           struct vm_page *ptmp = NULL;    /* Quell compiler warning */
           int lcv, gotpages, maxpages, swslot, pageidx;
           bool done;
           UVMHIST_FUNC("uao_get"); UVMHIST_CALLED(pdhist);
   
           UVMHIST_LOG(pdhist, "aobj=%p offset=%d, flags=%d",
                       (struct uvm_aobj *)uobj, offset, flags,0);
   
           /*
            * get number of pages
            */
   
           maxpages = *npagesp;
   
           /*
            * step 1: handled the case where fault data structures are locked.
            */
   
           if (flags & PGO_LOCKED) {
   
                   /*
                    * step 1a: get pages that are already resident.   only do
                    * this if the data structures are locked (i.e. the first
                    * time through).
                    */
   
                   done = true;    /* be optimistic */
                   gotpages = 0;   /* # of pages we got so far */
                   for (lcv = 0, current_offset = offset ; lcv < maxpages ;
                       lcv++, current_offset += PAGE_SIZE) {
                           /* do we care about this page?  if not, skip it */
                           if (pps[lcv] == PGO_DONTCARE)
                                   continue;
                           ptmp = uvm_pagelookup(uobj, current_offset);
   
                           /*
                            * if page is new, attempt to allocate the page,
                            * zero-fill'd.
                            */
   
                           if (ptmp == NULL && uao_find_swslot(&aobj->u_obj,
                               current_offset >> PAGE_SHIFT) == 0) {
                                   ptmp = uvm_pagealloc(uobj, current_offset,
                                       NULL, UVM_PGA_ZERO);
                                   if (ptmp) {
                                           /* new page */
                                           ptmp->flags &= ~(PG_FAKE);
                                           ptmp->pqflags |= PQ_AOBJ;
                                           goto gotpage;
                                   }
                           }
   
                           /*
                            * to be useful must get a non-busy page
                            */
   
                           if (ptmp == NULL || (ptmp->flags & PG_BUSY) != 0) {
                                   if (lcv == centeridx ||
                                       (flags & PGO_ALLPAGES) != 0)
                                           /* need to do a wait or I/O! */
                                           done = false;
                                           continue;
                           }
   
                          /*
                           * useful page: busy/lock it and plug it in our
                           * result array
                           */
  
                          /* caller must un-busy this page */
                          ptmp->flags |= PG_BUSY;
                          UVM_PAGE_OWN(ptmp, "uao_get1");
  gotpage:
                          pps[lcv] = ptmp;
                          gotpages++;
                  }
  
                  /*
                   * step 1b: now we've either done everything needed or we
                   * to unlock and do some waiting or I/O.
                   */
  
                  UVMHIST_LOG(pdhist, "<- done (done=%d)", done, 0,0,0);
                  *npagesp = gotpages;
                  if (done)
                          return 0;
                  else
                          return EBUSY;
          }
  
          /*
           * step 2: get non-resident or busy pages.
           * object is locked.   data structures are unlocked.
           */
  
          if ((flags & PGO_SYNCIO) == 0) {
                  goto done;
          }
  
          for (lcv = 0, current_offset = offset ; lcv < maxpages ;
              lcv++, current_offset += PAGE_SIZE) {
  
                  /*
                   * - skip over pages we've already gotten or don't want
                   * - skip over pages we don't _have_ to get
                   */
  
                  if (pps[lcv] != NULL ||
                      (lcv != centeridx && (flags & PGO_ALLPAGES) == 0))
                          continue;
  
                  pageidx = current_offset >> PAGE_SHIFT;
  
                  /*
                   * we have yet to locate the current page (pps[lcv]).   we
                   * first look for a page that is already at the current offset.
                   * if we find a page, we check to see if it is busy or
                   * released.  if that is the case, then we sleep on the page
                   * until it is no longer busy or released and repeat the lookup.
                   * if the page we found is neither busy nor released, then we
                   * busy it (so we own it) and plug it into pps[lcv].   this
                   * 'break's the following while loop and indicates we are
                   * ready to move on to the next page in the "lcv" loop above.
                   *
                   * if we exit the while loop with pps[lcv] still set to NULL,
                   * then it means that we allocated a new busy/fake/clean page
                   * ptmp in the object and we need to do I/O to fill in the data.
                   */
  
                  /* top of "pps" while loop */
                  while (pps[lcv] == NULL) {
                          /* look for a resident page */
                          ptmp = uvm_pagelookup(uobj, current_offset);
  
                          /* not resident?   allocate one now (if we can) */
                          if (ptmp == NULL) {
  
                                  ptmp = uvm_pagealloc(uobj, current_offset,
                                      NULL, 0);
  
                                  /* out of RAM? */
                                  if (ptmp == NULL) {
                                          mutex_exit(&uobj->vmobjlock);
                                          UVMHIST_LOG(pdhist,
                                              "sleeping, ptmp == NULL\n",0,0,0,0);
                                          uvm_wait("uao_getpage");
                                          mutex_enter(&uobj->vmobjlock);
                                          continue;
                                  }
  
                                  /*
                                   * safe with PQ's unlocked: because we just
                                   * alloc'd the page
                                   */
  
                                  ptmp->pqflags |= PQ_AOBJ;
  
                                  /*
                                   * got new page ready for I/O.  break pps while
                                   * loop.  pps[lcv] is still NULL.
                                   */
  
                                  break;
                          }
  
                          /* page is there, see if we need to wait on it */
                          if ((ptmp->flags & PG_BUSY) != 0) {
                                  ptmp->flags |= PG_WANTED;
                                  UVMHIST_LOG(pdhist,
                                      "sleeping, ptmp->flags 0x%x\n",
                                      ptmp->flags,0,0,0);
                                  UVM_UNLOCK_AND_WAIT(ptmp, &uobj->vmobjlock,
                                      false, "uao_get", 0);
                                  mutex_enter(&uobj->vmobjlock);
                                  continue;
                          }
  
                          /*
                           * if we get here then the page has become resident and
                           * unbusy between steps 1 and 2.  we busy it now (so we
                           * own it) and set pps[lcv] (so that we exit the while
                           * loop).
                           */
  
                          /* we own it, caller must un-busy */
                          ptmp->flags |= PG_BUSY;
                          UVM_PAGE_OWN(ptmp, "uao_get2");
                          pps[lcv] = ptmp;
                  }
  
                  /*
                   * if we own the valid page at the correct offset, pps[lcv] will
                   * point to it.   nothing more to do except go to the next page.
                   */
  
                  if (pps[lcv])
                          continue;                       /* next lcv */
  
                  /*
                   * we have a "fake/busy/clean" page that we just allocated.
                   * do the needed "i/o", either reading from swap or zeroing.
                   */
  
                  swslot = uao_find_swslot(&aobj->u_obj, pageidx);
  
                  /*
                   * just zero the page if there's nothing in swap.
                   */
  
                  if (swslot == 0) {
  
                          /*
                           * page hasn't existed before, just zero it.
                           */
  
                          uvm_pagezero(ptmp);
                  } else {
  #if defined(VMSWAP)
                          int error;
  
                          UVMHIST_LOG(pdhist, "pagein from swslot %d",
                               swslot, 0,0,0);
  
                          /*
                           * page in the swapped-out page.
                           * unlock object for i/o, relock when done.
                           */
  
                          mutex_exit(&uobj->vmobjlock);
                          error = uvm_swap_get(ptmp, swslot, PGO_SYNCIO);
                          mutex_enter(&uobj->vmobjlock);
  
                          /*
                           * I/O done.  check for errors.
                           */
  
                          if (error != 0) {
                                  UVMHIST_LOG(pdhist, "<- done (error=%d)",
                                      error,0,0,0);
                                  if (ptmp->flags & PG_WANTED)
                                          wakeup(ptmp);
  
                                  /*
                                   * remove the swap slot from the aobj
                                   * and mark the aobj as having no real slot.
                                   * don't free the swap slot, thus preventing
                                   * it from being used again.
                                   */
  
                                  swslot = uao_set_swslot(&aobj->u_obj, pageidx,
                                                          SWSLOT_BAD);
                                  if (swslot > 0) {
                                          uvm_swap_markbad(swslot, 1);
                                  }
  
                                  mutex_enter(&uvm_pageqlock);
                                  uvm_pagefree(ptmp);
                                  mutex_exit(&uvm_pageqlock);
                                  mutex_exit(&uobj->vmobjlock);
                                  return error;
                          }
  #else /* defined(VMSWAP) */
                          panic("%s: pagein", __func__);
  #endif /* defined(VMSWAP) */
                  }
  
                  if ((access_type & VM_PROT_WRITE) == 0) {
                          ptmp->flags |= PG_CLEAN;
                          pmap_clear_modify(ptmp);
                  }
  
                  /*
                   * we got the page!   clear the fake flag (indicates valid
                   * data now in page) and plug into our result array.   note
                   * that page is still busy.
                   *
                   * it is the callers job to:
                   * => check if the page is released
                   * => unbusy the page
                   * => activate the page
                   */
  
                  ptmp->flags &= ~PG_FAKE;
                  pps[lcv] = ptmp;
          }
  
          /*
           * finally, unlock object and return.
           */
  
  done:
          mutex_exit(&uobj->vmobjlock);
          UVMHIST_LOG(pdhist, "<- done (OK)",0,0,0,0);
          return 0;
  }
  
  #if defined(VMSWAP)
  
  /*
   * uao_dropswap:  release any swap resources from this aobj page.
   *
   * => aobj must be locked or have a reference count of 0.
   */
  
  void
  uao_dropswap(struct uvm_object *uobj, int pageidx)
  {
          int slot;
  
          slot = uao_set_swslot(uobj, pageidx, 0);
          if (slot) {
                  uvm_swap_free(slot, 1);
          }
  }
  
  /*
   * page in every page in every aobj that is paged-out to a range of swslots.
   *
   * => nothing should be locked.
   * => returns true if pagein was aborted due to lack of memory.
   */
  
  bool
  uao_swap_off(int startslot, int endslot)
  {
          struct uvm_aobj *aobj, *nextaobj;
          bool rv;
  
          /*
           * walk the list of all aobjs.
           */
  
  restart:
          mutex_enter(&uao_list_lock);
          for (aobj = LIST_FIRST(&uao_list);
               aobj != NULL;
               aobj = nextaobj) {
  
                  /*
                   * try to get the object lock, start all over if we fail.
                   * most of the time we'll get the aobj lock,
                   * so this should be a rare case.
                   */
  
                  if (!mutex_tryenter(&aobj->u_obj.vmobjlock)) {
                          mutex_exit(&uao_list_lock);
                          /* XXX Better than yielding but inadequate. */
                          kpause("livelock", false, 1, NULL);
                          goto restart;
                  }
  
                  /*
                   * add a ref to the aobj so it doesn't disappear
                   * while we're working.
                   */
  
                  uao_reference_locked(&aobj->u_obj);
  
                  /*
                   * now it's safe to unlock the uao list.
                   */
  
                  mutex_exit(&uao_list_lock);
  
                  /*
                   * page in any pages in the swslot range.
                   * if there's an error, abort and return the error.
                   */
  
                  rv = uao_pagein(aobj, startslot, endslot);
                  if (rv) {
                          uao_detach_locked(&aobj->u_obj);
                          return rv;
                  }
  
                  /*
                   * we're done with this aobj.
                   * relock the list and drop our ref on the aobj.
                   */
  
                  mutex_enter(&uao_list_lock);
                  nextaobj = LIST_NEXT(aobj, u_list);
                  uao_detach_locked(&aobj->u_obj);
          }
  
          /*
           * done with traversal, unlock the list
           */
          mutex_exit(&uao_list_lock);
          return false;
  }
  
  
  /*
   * page in any pages from aobj in the given range.
   *
   * => aobj must be locked and is returned locked.
   * => returns true if pagein was aborted due to lack of memory.
   */
  static bool
  uao_pagein(struct uvm_aobj *aobj, int startslot, int endslot)
  {
          bool rv;
  
          if (UAO_USES_SWHASH(aobj)) {
                  struct uao_swhash_elt *elt;
                  int buck;
  
  restart:
                  for (buck = aobj->u_swhashmask; buck >= 0; buck--) {
                          for (elt = LIST_FIRST(&aobj->u_swhash[buck]);
                               elt != NULL;
                               elt = LIST_NEXT(elt, list)) {
                                  int i;
  
                                  for (i = 0; i < UAO_SWHASH_CLUSTER_SIZE; i++) {
                                          int slot = elt->slots[i];
  
                                          /*
                                           * if the slot isn't in range, skip it.
                                           */
  
                                          if (slot < startslot ||
                                              slot >= endslot) {
                                                  continue;
                                          }
  
                                          /*
                                           * process the page,
                                           * the start over on this object
                                           * since the swhash elt
                                           * may have been freed.
                                           */
  
                                          rv = uao_pagein_page(aobj,
                                            UAO_SWHASH_ELT_PAGEIDX_BASE(elt) + i);
                                          if (rv) {
                                                  return rv;
                                          }
                                          goto restart;
                                  }
                          }
                  }
          } else {
                  int i;
  
                  for (i = 0; i < aobj->u_pages; i++) {
                          int slot = aobj->u_swslots[i];
  
                          /*
                           * if the slot isn't in range, skip it
                           */
  
                          if (slot < startslot || slot >= endslot) {
                                  continue;
                          }
  
                          /*
                           * process the page.
                           */
  
                          rv = uao_pagein_page(aobj, i);
                          if (rv) {
                                  return rv;
                          }
                  }
          }
  
          return false;
  }
  
  /*
   * page in a page from an aobj.  used for swap_off.
   * returns true if pagein was aborted due to lack of memory.
   *
   * => aobj must be locked and is returned locked.
   */
  
  static bool
  uao_pagein_page(struct uvm_aobj *aobj, int pageidx)
  {
          struct vm_page *pg;
          int rv, npages;
  
          pg = NULL;
          npages = 1;
          /* locked: aobj */
          rv = uao_get(&aobj->u_obj, pageidx << PAGE_SHIFT,
              &pg, &npages, 0, VM_PROT_READ|VM_PROT_WRITE, 0, PGO_SYNCIO);
          /* unlocked: aobj */
  
          /*
           * relock and finish up.
           */
  
          mutex_enter(&aobj->u_obj.vmobjlock);
          switch (rv) {
          case 0:
                  break;
  
          case EIO:
          case ERESTART:
  
                  /*
                   * nothing more to do on errors.
                   * ERESTART can only mean that the anon was freed,
                   * so again there's nothing to do.
                   */
  
                  return false;
  
          default:
                  return true;
          }
  
          /*
           * ok, we've got the page now.
           * mark it as dirty, clear its swslot and un-busy it.
           */
          uao_dropswap(&aobj->u_obj, pageidx);
  
          /*
           * make sure it's on a page queue.
           */
          mutex_enter(&uvm_pageqlock);
          if (pg->wire_count == 0)
                  uvm_pageenqueue(pg);
          mutex_exit(&uvm_pageqlock);
  
          if (pg->flags & PG_WANTED) {
                  wakeup(pg);
          }
          pg->flags &= ~(PG_WANTED|PG_BUSY|PG_CLEAN|PG_FAKE);
          UVM_PAGE_OWN(pg, NULL);
  
          return false;
  }
  
  /*
   * uao_dropswap_range: drop swapslots in the range.
   *
   * => aobj must be locked and is returned locked.
   * => start is inclusive.  end is exclusive.
   */
  
  void
  uao_dropswap_range(struct uvm_object *uobj, voff_t start, voff_t end)
  {
          struct uvm_aobj *aobj = (struct uvm_aobj *)uobj;
  
          KASSERT(mutex_owned(&uobj->vmobjlock));
  
          uao_dropswap_range1(aobj, start, end);
  }
  
  static void
  uao_dropswap_range1(struct uvm_aobj *aobj, voff_t start, voff_t end)
  {
          int swpgonlydelta = 0;
  
          if (end == 0) {
                  end = INT64_MAX;
          }
  
          if (UAO_USES_SWHASH(aobj)) {
                  int i, hashbuckets = aobj->u_swhashmask + 1;
                  voff_t taghi;
                  voff_t taglo;
  
                  taglo = UAO_SWHASH_ELT_TAG(start);
                  taghi = UAO_SWHASH_ELT_TAG(end);
  
                  for (i = 0; i < hashbuckets; i++) {
                          struct uao_swhash_elt *elt, *next;
  
                          for (elt = LIST_FIRST(&aobj->u_swhash[i]);
                               elt != NULL;
                               elt = next) {
                                  int startidx, endidx;
                                  int j;
  
                                  next = LIST_NEXT(elt, list);
  
                                  if (elt->tag < taglo || taghi < elt->tag) {
                                          continue;
                                  }
  
                                  if (elt->tag == taglo) {
                                          startidx =
                                              UAO_SWHASH_ELT_PAGESLOT_IDX(start);
                                  } else {
                                          startidx = 0;
                                  }
  
                                  if (elt->tag == taghi) {
                                          endidx =
                                              UAO_SWHASH_ELT_PAGESLOT_IDX(end);
                                  } else {
                                          endidx = UAO_SWHASH_CLUSTER_SIZE;
                                  }
  
                                  for (j = startidx; j < endidx; j++) {
                                          int slot = elt->slots[j];
  
                                          KASSERT(uvm_pagelookup(&aobj->u_obj,
                                              (UAO_SWHASH_ELT_PAGEIDX_BASE(elt)
                                              + j) << PAGE_SHIFT) == NULL);
                                          if (slot > 0) {
                                                  uvm_swap_free(slot, 1);
                                                  swpgonlydelta++;
                                                  KASSERT(elt->count > 0);
                                                  elt->slots[j] = 0;
                                                  elt->count--;
                                          }
                                  }
  
                                  if (elt->count == 0) {
                                          LIST_REMOVE(elt, list);
                                          pool_put(&uao_swhash_elt_pool, elt);
                                  }
                          }
                  }
          } else {
                  int i;
  
                  if (aobj->u_pages < end) {
                          end = aobj->u_pages;
                  }
                  for (i = start; i < end; i++) {
                          int slot = aobj->u_swslots[i];
  
                          if (slot > 0) {
                                  uvm_swap_free(slot, 1);
                                  swpgonlydelta++;
                          }
                  }
          }
  
          /*
           * adjust the counter of pages only in swap for all
           * the swap slots we've freed.
           */
  
          if (swpgonlydelta > 0) {
                  mutex_enter(&uvm_swap_data_lock);
                  KASSERT(uvmexp.swpgonly >= swpgonlydelta);
                  uvmexp.swpgonly -= swpgonlydelta;
                  mutex_exit(&uvm_swap_data_lock);
          }
  }
  
  #endif /* defined(VMSWAP) */

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