The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/vm/vm_fault.c

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    1 /*-
    2  * Copyright (c) 1991, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  * Copyright (c) 1994 John S. Dyson
    5  * All rights reserved.
    6  * Copyright (c) 1994 David Greenman
    7  * All rights reserved.
    8  *
    9  *
   10  * This code is derived from software contributed to Berkeley by
   11  * The Mach Operating System project at Carnegie-Mellon University.
   12  *
   13  * Redistribution and use in source and binary forms, with or without
   14  * modification, are permitted provided that the following conditions
   15  * are met:
   16  * 1. Redistributions of source code must retain the above copyright
   17  *    notice, this list of conditions and the following disclaimer.
   18  * 2. Redistributions in binary form must reproduce the above copyright
   19  *    notice, this list of conditions and the following disclaimer in the
   20  *    documentation and/or other materials provided with the distribution.
   21  * 3. All advertising materials mentioning features or use of this software
   22  *    must display the following acknowledgement:
   23  *      This product includes software developed by the University of
   24  *      California, Berkeley and its contributors.
   25  * 4. Neither the name of the University nor the names of its contributors
   26  *    may be used to endorse or promote products derived from this software
   27  *    without specific prior written permission.
   28  *
   29  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   30  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   31  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   32  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   33  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   34  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   35  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   36  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   37  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   38  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   39  * SUCH DAMAGE.
   40  *
   41  *      from: @(#)vm_fault.c    8.4 (Berkeley) 1/12/94
   42  *
   43  *
   44  * Copyright (c) 1987, 1990 Carnegie-Mellon University.
   45  * All rights reserved.
   46  *
   47  * Authors: Avadis Tevanian, Jr., Michael Wayne Young
   48  *
   49  * Permission to use, copy, modify and distribute this software and
   50  * its documentation is hereby granted, provided that both the copyright
   51  * notice and this permission notice appear in all copies of the
   52  * software, derivative works or modified versions, and any portions
   53  * thereof, and that both notices appear in supporting documentation.
   54  *
   55  * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
   56  * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
   57  * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
   58  *
   59  * Carnegie Mellon requests users of this software to return to
   60  *
   61  *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
   62  *  School of Computer Science
   63  *  Carnegie Mellon University
   64  *  Pittsburgh PA 15213-3890
   65  *
   66  * any improvements or extensions that they make and grant Carnegie the
   67  * rights to redistribute these changes.
   68  */
   69 
   70 /*
   71  *      Page fault handling module.
   72  */
   73 
   74 #include <sys/cdefs.h>
   75 __FBSDID("$FreeBSD$");
   76 
   77 #include <sys/param.h>
   78 #include <sys/systm.h>
   79 #include <sys/kernel.h>
   80 #include <sys/lock.h>
   81 #include <sys/mutex.h>
   82 #include <sys/proc.h>
   83 #include <sys/resourcevar.h>
   84 #include <sys/sysctl.h>
   85 #include <sys/vmmeter.h>
   86 #include <sys/vnode.h>
   87 
   88 #include <vm/vm.h>
   89 #include <vm/vm_param.h>
   90 #include <vm/pmap.h>
   91 #include <vm/vm_map.h>
   92 #include <vm/vm_object.h>
   93 #include <vm/vm_page.h>
   94 #include <vm/vm_pageout.h>
   95 #include <vm/vm_kern.h>
   96 #include <vm/vm_pager.h>
   97 #include <vm/vnode_pager.h>
   98 #include <vm/vm_extern.h>
   99 
  100 #include <sys/mount.h>  /* XXX Temporary for VFS_LOCK_GIANT() */
  101 
  102 #define PFBAK 4
  103 #define PFFOR 4
  104 #define PAGEORDER_SIZE (PFBAK+PFFOR)
  105 
  106 static int prefault_pageorder[] = {
  107         -1 * PAGE_SIZE, 1 * PAGE_SIZE,
  108         -2 * PAGE_SIZE, 2 * PAGE_SIZE,
  109         -3 * PAGE_SIZE, 3 * PAGE_SIZE,
  110         -4 * PAGE_SIZE, 4 * PAGE_SIZE
  111 };
  112 
  113 static int vm_fault_additional_pages(vm_page_t, int, int, vm_page_t *, int *);
  114 static void vm_fault_prefault(pmap_t, vm_offset_t, vm_map_entry_t);
  115 
  116 #define VM_FAULT_READ_AHEAD 8
  117 #define VM_FAULT_READ_BEHIND 7
  118 #define VM_FAULT_READ (VM_FAULT_READ_AHEAD+VM_FAULT_READ_BEHIND+1)
  119 
  120 struct faultstate {
  121         vm_page_t m;
  122         vm_object_t object;
  123         vm_pindex_t pindex;
  124         vm_page_t first_m;
  125         vm_object_t     first_object;
  126         vm_pindex_t first_pindex;
  127         vm_map_t map;
  128         vm_map_entry_t entry;
  129         int lookup_still_valid;
  130         struct vnode *vp;
  131 };
  132 
  133 static inline void
  134 release_page(struct faultstate *fs)
  135 {
  136         vm_page_wakeup(fs->m);
  137         vm_page_lock_queues();
  138         vm_page_deactivate(fs->m);
  139         vm_page_unlock_queues();
  140         fs->m = NULL;
  141 }
  142 
  143 static inline void
  144 unlock_map(struct faultstate *fs)
  145 {
  146         if (fs->lookup_still_valid) {
  147                 vm_map_lookup_done(fs->map, fs->entry);
  148                 fs->lookup_still_valid = FALSE;
  149         }
  150 }
  151 
  152 static void
  153 unlock_and_deallocate(struct faultstate *fs)
  154 {
  155 
  156         vm_object_pip_wakeup(fs->object);
  157         VM_OBJECT_UNLOCK(fs->object);
  158         if (fs->object != fs->first_object) {
  159                 VM_OBJECT_LOCK(fs->first_object);
  160                 vm_page_lock_queues();
  161                 vm_page_free(fs->first_m);
  162                 vm_page_unlock_queues();
  163                 vm_object_pip_wakeup(fs->first_object);
  164                 VM_OBJECT_UNLOCK(fs->first_object);
  165                 fs->first_m = NULL;
  166         }
  167         vm_object_deallocate(fs->first_object);
  168         unlock_map(fs); 
  169         if (fs->vp != NULL) { 
  170                 int vfslocked;
  171 
  172                 vfslocked = VFS_LOCK_GIANT(fs->vp->v_mount);
  173                 vput(fs->vp);
  174                 fs->vp = NULL;
  175                 VFS_UNLOCK_GIANT(vfslocked);
  176         }
  177 }
  178 
  179 /*
  180  * TRYPAGER - used by vm_fault to calculate whether the pager for the
  181  *            current object *might* contain the page.
  182  *
  183  *            default objects are zero-fill, there is no real pager.
  184  */
  185 #define TRYPAGER        (fs.object->type != OBJT_DEFAULT && \
  186                         (((fault_flags & VM_FAULT_WIRE_MASK) == 0) || wired))
  187 
  188 /*
  189  *      vm_fault:
  190  *
  191  *      Handle a page fault occurring at the given address,
  192  *      requiring the given permissions, in the map specified.
  193  *      If successful, the page is inserted into the
  194  *      associated physical map.
  195  *
  196  *      NOTE: the given address should be truncated to the
  197  *      proper page address.
  198  *
  199  *      KERN_SUCCESS is returned if the page fault is handled; otherwise,
  200  *      a standard error specifying why the fault is fatal is returned.
  201  *
  202  *
  203  *      The map in question must be referenced, and remains so.
  204  *      Caller may hold no locks.
  205  */
  206 int
  207 vm_fault(vm_map_t map, vm_offset_t vaddr, vm_prot_t fault_type,
  208          int fault_flags)
  209 {
  210         vm_prot_t prot;
  211         int is_first_object_locked, result;
  212         boolean_t growstack, wired;
  213         int map_generation;
  214         vm_object_t next_object;
  215         vm_page_t marray[VM_FAULT_READ];
  216         int hardfault;
  217         int faultcount;
  218         struct faultstate fs;
  219 
  220         hardfault = 0;
  221         growstack = TRUE;
  222         PCPU_INC(cnt.v_vm_faults);
  223 
  224 RetryFault:;
  225 
  226         /*
  227          * Find the backing store object and offset into it to begin the
  228          * search.
  229          */
  230         fs.map = map;
  231         result = vm_map_lookup(&fs.map, vaddr, fault_type, &fs.entry,
  232             &fs.first_object, &fs.first_pindex, &prot, &wired);
  233         if (result != KERN_SUCCESS) {
  234                 if (result != KERN_PROTECTION_FAILURE ||
  235                     (fault_flags & VM_FAULT_WIRE_MASK) != VM_FAULT_USER_WIRE) {
  236                         if (growstack && result == KERN_INVALID_ADDRESS &&
  237                             map != kernel_map && curproc != NULL) {
  238                                 result = vm_map_growstack(curproc, vaddr);
  239                                 if (result != KERN_SUCCESS)
  240                                         return (KERN_FAILURE);
  241                                 growstack = FALSE;
  242                                 goto RetryFault;
  243                         }
  244                         return (result);
  245                 }
  246 
  247                 /*
  248                  * If we are user-wiring a r/w segment, and it is COW, then
  249                  * we need to do the COW operation.  Note that we don't COW
  250                  * currently RO sections now, because it is NOT desirable
  251                  * to COW .text.  We simply keep .text from ever being COW'ed
  252                  * and take the heat that one cannot debug wired .text sections.
  253                  */
  254                 result = vm_map_lookup(&fs.map, vaddr,
  255                         VM_PROT_READ|VM_PROT_WRITE|VM_PROT_OVERRIDE_WRITE,
  256                         &fs.entry, &fs.first_object, &fs.first_pindex, &prot, &wired);
  257                 if (result != KERN_SUCCESS)
  258                         return (result);
  259 
  260                 /*
  261                  * If we don't COW now, on a user wire, the user will never
  262                  * be able to write to the mapping.  If we don't make this
  263                  * restriction, the bookkeeping would be nearly impossible.
  264                  *
  265                  * XXX The following assignment modifies the map without
  266                  * holding a write lock on it.
  267                  */
  268                 if ((fs.entry->protection & VM_PROT_WRITE) == 0)
  269                         fs.entry->max_protection &= ~VM_PROT_WRITE;
  270         }
  271 
  272         map_generation = fs.map->timestamp;
  273 
  274         if (fs.entry->eflags & MAP_ENTRY_NOFAULT) {
  275                 panic("vm_fault: fault on nofault entry, addr: %lx",
  276                     (u_long)vaddr);
  277         }
  278 
  279         /*
  280          * Make a reference to this object to prevent its disposal while we
  281          * are messing with it.  Once we have the reference, the map is free
  282          * to be diddled.  Since objects reference their shadows (and copies),
  283          * they will stay around as well.
  284          *
  285          * Bump the paging-in-progress count to prevent size changes (e.g. 
  286          * truncation operations) during I/O.  This must be done after
  287          * obtaining the vnode lock in order to avoid possible deadlocks.
  288          *
  289          * XXX vnode_pager_lock() can block without releasing the map lock.
  290          */
  291         if (fs.first_object->flags & OBJ_NEEDGIANT)
  292                 mtx_lock(&Giant);
  293         VM_OBJECT_LOCK(fs.first_object);
  294         vm_object_reference_locked(fs.first_object);
  295         fs.vp = vnode_pager_lock(fs.first_object);
  296         KASSERT(fs.vp == NULL || !fs.map->system_map,
  297             ("vm_fault: vnode-backed object mapped by system map"));
  298         KASSERT((fs.first_object->flags & OBJ_NEEDGIANT) == 0 ||
  299             !fs.map->system_map,
  300             ("vm_fault: Object requiring giant mapped by system map"));
  301         if (fs.first_object->flags & OBJ_NEEDGIANT)
  302                 mtx_unlock(&Giant);
  303         vm_object_pip_add(fs.first_object, 1);
  304 
  305         fs.lookup_still_valid = TRUE;
  306 
  307         if (wired)
  308                 fault_type = prot;
  309 
  310         fs.first_m = NULL;
  311 
  312         /*
  313          * Search for the page at object/offset.
  314          */
  315         fs.object = fs.first_object;
  316         fs.pindex = fs.first_pindex;
  317         while (TRUE) {
  318                 /*
  319                  * If the object is dead, we stop here
  320                  */
  321                 if (fs.object->flags & OBJ_DEAD) {
  322                         unlock_and_deallocate(&fs);
  323                         return (KERN_PROTECTION_FAILURE);
  324                 }
  325 
  326                 /*
  327                  * See if page is resident
  328                  */
  329                 fs.m = vm_page_lookup(fs.object, fs.pindex);
  330                 if (fs.m != NULL) {
  331                         /* 
  332                          * check for page-based copy on write.
  333                          * We check fs.object == fs.first_object so
  334                          * as to ensure the legacy COW mechanism is
  335                          * used when the page in question is part of
  336                          * a shadow object.  Otherwise, vm_page_cowfault()
  337                          * removes the page from the backing object, 
  338                          * which is not what we want.
  339                          */
  340                         vm_page_lock_queues();
  341                         if ((fs.m->cow) && 
  342                             (fault_type & VM_PROT_WRITE) &&
  343                             (fs.object == fs.first_object)) {
  344                                 vm_page_cowfault(fs.m);
  345                                 vm_page_unlock_queues();
  346                                 unlock_and_deallocate(&fs);
  347                                 goto RetryFault;
  348                         }
  349 
  350                         /*
  351                          * Wait/Retry if the page is busy.  We have to do this
  352                          * if the page is busy via either VPO_BUSY or 
  353                          * vm_page_t->busy because the vm_pager may be using
  354                          * vm_page_t->busy for pageouts ( and even pageins if
  355                          * it is the vnode pager ), and we could end up trying
  356                          * to pagein and pageout the same page simultaneously.
  357                          *
  358                          * We can theoretically allow the busy case on a read
  359                          * fault if the page is marked valid, but since such
  360                          * pages are typically already pmap'd, putting that
  361                          * special case in might be more effort then it is 
  362                          * worth.  We cannot under any circumstances mess
  363                          * around with a vm_page_t->busy page except, perhaps,
  364                          * to pmap it.
  365                          */
  366                         if ((fs.m->oflags & VPO_BUSY) || fs.m->busy) {
  367                                 vm_page_unlock_queues();
  368                                 VM_OBJECT_UNLOCK(fs.object);
  369                                 if (fs.object != fs.first_object) {
  370                                         VM_OBJECT_LOCK(fs.first_object);
  371                                         vm_page_lock_queues();
  372                                         vm_page_free(fs.first_m);
  373                                         vm_page_unlock_queues();
  374                                         vm_object_pip_wakeup(fs.first_object);
  375                                         VM_OBJECT_UNLOCK(fs.first_object);
  376                                         fs.first_m = NULL;
  377                                 }
  378                                 unlock_map(&fs);
  379                                 if (fs.vp != NULL) {
  380                                         int vfslck;
  381 
  382                                         vfslck = VFS_LOCK_GIANT(fs.vp->v_mount);
  383                                         vput(fs.vp);
  384                                         fs.vp = NULL;
  385                                         VFS_UNLOCK_GIANT(vfslck);
  386                                 }
  387                                 VM_OBJECT_LOCK(fs.object);
  388                                 if (fs.m == vm_page_lookup(fs.object,
  389                                     fs.pindex)) {
  390                                         vm_page_sleep_if_busy(fs.m, TRUE,
  391                                             "vmpfw");
  392                                 }
  393                                 vm_object_pip_wakeup(fs.object);
  394                                 VM_OBJECT_UNLOCK(fs.object);
  395                                 PCPU_INC(cnt.v_intrans);
  396                                 vm_object_deallocate(fs.first_object);
  397                                 goto RetryFault;
  398                         }
  399                         vm_pageq_remove(fs.m);
  400                         vm_page_unlock_queues();
  401 
  402                         /*
  403                          * Mark page busy for other processes, and the 
  404                          * pagedaemon.  If it still isn't completely valid
  405                          * (readable), jump to readrest, else break-out ( we
  406                          * found the page ).
  407                          */
  408                         vm_page_busy(fs.m);
  409                         if (((fs.m->valid & VM_PAGE_BITS_ALL) != VM_PAGE_BITS_ALL) &&
  410                                 fs.m->object != kernel_object && fs.m->object != kmem_object) {
  411                                 goto readrest;
  412                         }
  413 
  414                         break;
  415                 }
  416 
  417                 /*
  418                  * Page is not resident, If this is the search termination
  419                  * or the pager might contain the page, allocate a new page.
  420                  */
  421                 if (TRYPAGER || fs.object == fs.first_object) {
  422                         if (fs.pindex >= fs.object->size) {
  423                                 unlock_and_deallocate(&fs);
  424                                 return (KERN_PROTECTION_FAILURE);
  425                         }
  426 
  427                         /*
  428                          * Allocate a new page for this object/offset pair.
  429                          */
  430                         fs.m = NULL;
  431                         if (!vm_page_count_severe()) {
  432                                 fs.m = vm_page_alloc(fs.object, fs.pindex,
  433                                     (fs.vp || fs.object->backing_object)? VM_ALLOC_NORMAL: VM_ALLOC_ZERO);
  434                         }
  435                         if (fs.m == NULL) {
  436                                 unlock_and_deallocate(&fs);
  437                                 VM_WAITPFAULT;
  438                                 goto RetryFault;
  439                         } else if ((fs.m->valid & VM_PAGE_BITS_ALL) == VM_PAGE_BITS_ALL)
  440                                 break;
  441                 }
  442 
  443 readrest:
  444                 /*
  445                  * We have found a valid page or we have allocated a new page.
  446                  * The page thus may not be valid or may not be entirely 
  447                  * valid.
  448                  *
  449                  * Attempt to fault-in the page if there is a chance that the
  450                  * pager has it, and potentially fault in additional pages
  451                  * at the same time.
  452                  */
  453                 if (TRYPAGER) {
  454                         int rv;
  455                         int reqpage = 0;
  456                         int ahead, behind;
  457                         u_char behavior = vm_map_entry_behavior(fs.entry);
  458 
  459                         if (behavior == MAP_ENTRY_BEHAV_RANDOM) {
  460                                 ahead = 0;
  461                                 behind = 0;
  462                         } else {
  463                                 behind = (vaddr - fs.entry->start) >> PAGE_SHIFT;
  464                                 if (behind > VM_FAULT_READ_BEHIND)
  465                                         behind = VM_FAULT_READ_BEHIND;
  466 
  467                                 ahead = ((fs.entry->end - vaddr) >> PAGE_SHIFT) - 1;
  468                                 if (ahead > VM_FAULT_READ_AHEAD)
  469                                         ahead = VM_FAULT_READ_AHEAD;
  470                         }
  471                         is_first_object_locked = FALSE;
  472                         if ((behavior == MAP_ENTRY_BEHAV_SEQUENTIAL ||
  473                              (behavior != MAP_ENTRY_BEHAV_RANDOM &&
  474                               fs.pindex >= fs.entry->lastr &&
  475                               fs.pindex < fs.entry->lastr + VM_FAULT_READ)) &&
  476                             (fs.first_object == fs.object ||
  477                              (is_first_object_locked = VM_OBJECT_TRYLOCK(fs.first_object))) &&
  478                             fs.first_object->type != OBJT_DEVICE) {
  479                                 vm_pindex_t firstpindex, tmppindex;
  480 
  481                                 if (fs.first_pindex < 2 * VM_FAULT_READ)
  482                                         firstpindex = 0;
  483                                 else
  484                                         firstpindex = fs.first_pindex - 2 * VM_FAULT_READ;
  485 
  486                                 vm_page_lock_queues();
  487                                 /*
  488                                  * note: partially valid pages cannot be 
  489                                  * included in the lookahead - NFS piecemeal
  490                                  * writes will barf on it badly.
  491                                  */
  492                                 for (tmppindex = fs.first_pindex - 1;
  493                                         tmppindex >= firstpindex;
  494                                         --tmppindex) {
  495                                         vm_page_t mt;
  496 
  497                                         mt = vm_page_lookup(fs.first_object, tmppindex);
  498                                         if (mt == NULL || (mt->valid != VM_PAGE_BITS_ALL))
  499                                                 break;
  500                                         if (mt->busy ||
  501                                             (mt->oflags & VPO_BUSY) ||
  502                                             (mt->flags & (PG_FICTITIOUS | PG_UNMANAGED)) ||
  503                                                 mt->hold_count ||
  504                                                 mt->wire_count) 
  505                                                 continue;
  506                                         pmap_remove_all(mt);
  507                                         if (mt->dirty) {
  508                                                 vm_page_deactivate(mt);
  509                                         } else {
  510                                                 vm_page_cache(mt);
  511                                         }
  512                                 }
  513                                 vm_page_unlock_queues();
  514                                 ahead += behind;
  515                                 behind = 0;
  516                         }
  517                         if (is_first_object_locked)
  518                                 VM_OBJECT_UNLOCK(fs.first_object);
  519                         /*
  520                          * now we find out if any other pages should be paged
  521                          * in at this time this routine checks to see if the
  522                          * pages surrounding this fault reside in the same
  523                          * object as the page for this fault.  If they do,
  524                          * then they are faulted in also into the object.  The
  525                          * array "marray" returned contains an array of
  526                          * vm_page_t structs where one of them is the
  527                          * vm_page_t passed to the routine.  The reqpage
  528                          * return value is the index into the marray for the
  529                          * vm_page_t passed to the routine.
  530                          *
  531                          * fs.m plus the additional pages are VPO_BUSY'd.
  532                          *
  533                          * XXX vm_fault_additional_pages() can block
  534                          * without releasing the map lock.
  535                          */
  536                         faultcount = vm_fault_additional_pages(
  537                             fs.m, behind, ahead, marray, &reqpage);
  538 
  539                         /*
  540                          * update lastr imperfectly (we do not know how much
  541                          * getpages will actually read), but good enough.
  542                          *
  543                          * XXX The following assignment modifies the map
  544                          * without holding a write lock on it.
  545                          */
  546                         fs.entry->lastr = fs.pindex + faultcount - behind;
  547 
  548                         /*
  549                          * Call the pager to retrieve the data, if any, after
  550                          * releasing the lock on the map.  We hold a ref on
  551                          * fs.object and the pages are VPO_BUSY'd.
  552                          */
  553                         unlock_map(&fs);
  554 
  555                         rv = faultcount ?
  556                             vm_pager_get_pages(fs.object, marray, faultcount,
  557                                 reqpage) : VM_PAGER_FAIL;
  558 
  559                         if (rv == VM_PAGER_OK) {
  560                                 /*
  561                                  * Found the page. Leave it busy while we play
  562                                  * with it.
  563                                  */
  564 
  565                                 /*
  566                                  * Relookup in case pager changed page. Pager
  567                                  * is responsible for disposition of old page
  568                                  * if moved.
  569                                  */
  570                                 fs.m = vm_page_lookup(fs.object, fs.pindex);
  571                                 if (!fs.m) {
  572                                         unlock_and_deallocate(&fs);
  573                                         goto RetryFault;
  574                                 }
  575 
  576                                 hardfault++;
  577                                 break; /* break to PAGE HAS BEEN FOUND */
  578                         }
  579                         /*
  580                          * Remove the bogus page (which does not exist at this
  581                          * object/offset); before doing so, we must get back
  582                          * our object lock to preserve our invariant.
  583                          *
  584                          * Also wake up any other process that may want to bring
  585                          * in this page.
  586                          *
  587                          * If this is the top-level object, we must leave the
  588                          * busy page to prevent another process from rushing
  589                          * past us, and inserting the page in that object at
  590                          * the same time that we are.
  591                          */
  592                         if (rv == VM_PAGER_ERROR)
  593                                 printf("vm_fault: pager read error, pid %d (%s)\n",
  594                                     curproc->p_pid, curproc->p_comm);
  595                         /*
  596                          * Data outside the range of the pager or an I/O error
  597                          */
  598                         /*
  599                          * XXX - the check for kernel_map is a kludge to work
  600                          * around having the machine panic on a kernel space
  601                          * fault w/ I/O error.
  602                          */
  603                         if (((fs.map != kernel_map) && (rv == VM_PAGER_ERROR)) ||
  604                                 (rv == VM_PAGER_BAD)) {
  605                                 vm_page_lock_queues();
  606                                 vm_page_free(fs.m);
  607                                 vm_page_unlock_queues();
  608                                 fs.m = NULL;
  609                                 unlock_and_deallocate(&fs);
  610                                 return ((rv == VM_PAGER_ERROR) ? KERN_FAILURE : KERN_PROTECTION_FAILURE);
  611                         }
  612                         if (fs.object != fs.first_object) {
  613                                 vm_page_lock_queues();
  614                                 vm_page_free(fs.m);
  615                                 vm_page_unlock_queues();
  616                                 fs.m = NULL;
  617                                 /*
  618                                  * XXX - we cannot just fall out at this
  619                                  * point, m has been freed and is invalid!
  620                                  */
  621                         }
  622                 }
  623 
  624                 /*
  625                  * We get here if the object has default pager (or unwiring) 
  626                  * or the pager doesn't have the page.
  627                  */
  628                 if (fs.object == fs.first_object)
  629                         fs.first_m = fs.m;
  630 
  631                 /*
  632                  * Move on to the next object.  Lock the next object before
  633                  * unlocking the current one.
  634                  */
  635                 fs.pindex += OFF_TO_IDX(fs.object->backing_object_offset);
  636                 next_object = fs.object->backing_object;
  637                 if (next_object == NULL) {
  638                         /*
  639                          * If there's no object left, fill the page in the top
  640                          * object with zeros.
  641                          */
  642                         if (fs.object != fs.first_object) {
  643                                 vm_object_pip_wakeup(fs.object);
  644                                 VM_OBJECT_UNLOCK(fs.object);
  645 
  646                                 fs.object = fs.first_object;
  647                                 fs.pindex = fs.first_pindex;
  648                                 fs.m = fs.first_m;
  649                                 VM_OBJECT_LOCK(fs.object);
  650                         }
  651                         fs.first_m = NULL;
  652 
  653                         /*
  654                          * Zero the page if necessary and mark it valid.
  655                          */
  656                         if ((fs.m->flags & PG_ZERO) == 0) {
  657                                 pmap_zero_page(fs.m);
  658                         } else {
  659                                 PCPU_INC(cnt.v_ozfod);
  660                         }
  661                         PCPU_INC(cnt.v_zfod);
  662                         fs.m->valid = VM_PAGE_BITS_ALL;
  663                         break;  /* break to PAGE HAS BEEN FOUND */
  664                 } else {
  665                         KASSERT(fs.object != next_object,
  666                             ("object loop %p", next_object));
  667                         VM_OBJECT_LOCK(next_object);
  668                         vm_object_pip_add(next_object, 1);
  669                         if (fs.object != fs.first_object)
  670                                 vm_object_pip_wakeup(fs.object);
  671                         VM_OBJECT_UNLOCK(fs.object);
  672                         fs.object = next_object;
  673                 }
  674         }
  675 
  676         KASSERT((fs.m->oflags & VPO_BUSY) != 0,
  677             ("vm_fault: not busy after main loop"));
  678 
  679         /*
  680          * PAGE HAS BEEN FOUND. [Loop invariant still holds -- the object lock
  681          * is held.]
  682          */
  683 
  684         /*
  685          * If the page is being written, but isn't already owned by the
  686          * top-level object, we have to copy it into a new page owned by the
  687          * top-level object.
  688          */
  689         if (fs.object != fs.first_object) {
  690                 /*
  691                  * We only really need to copy if we want to write it.
  692                  */
  693                 if (fault_type & VM_PROT_WRITE) {
  694                         /*
  695                          * This allows pages to be virtually copied from a 
  696                          * backing_object into the first_object, where the 
  697                          * backing object has no other refs to it, and cannot
  698                          * gain any more refs.  Instead of a bcopy, we just 
  699                          * move the page from the backing object to the 
  700                          * first object.  Note that we must mark the page 
  701                          * dirty in the first object so that it will go out 
  702                          * to swap when needed.
  703                          */
  704                         is_first_object_locked = FALSE;
  705                         if (
  706                                 /*
  707                                  * Only one shadow object
  708                                  */
  709                                 (fs.object->shadow_count == 1) &&
  710                                 /*
  711                                  * No COW refs, except us
  712                                  */
  713                                 (fs.object->ref_count == 1) &&
  714                                 /*
  715                                  * No one else can look this object up
  716                                  */
  717                                 (fs.object->handle == NULL) &&
  718                                 /*
  719                                  * No other ways to look the object up
  720                                  */
  721                                 ((fs.object->type == OBJT_DEFAULT) ||
  722                                  (fs.object->type == OBJT_SWAP)) &&
  723                             (is_first_object_locked = VM_OBJECT_TRYLOCK(fs.first_object)) &&
  724                                 /*
  725                                  * We don't chase down the shadow chain
  726                                  */
  727                             fs.object == fs.first_object->backing_object) {
  728                                 vm_page_lock_queues();
  729                                 /*
  730                                  * get rid of the unnecessary page
  731                                  */
  732                                 vm_page_free(fs.first_m);
  733                                 /*
  734                                  * grab the page and put it into the 
  735                                  * process'es object.  The page is 
  736                                  * automatically made dirty.
  737                                  */
  738                                 vm_page_rename(fs.m, fs.first_object, fs.first_pindex);
  739                                 vm_page_unlock_queues();
  740                                 vm_page_busy(fs.m);
  741                                 fs.first_m = fs.m;
  742                                 fs.m = NULL;
  743                                 PCPU_INC(cnt.v_cow_optim);
  744                         } else {
  745                                 /*
  746                                  * Oh, well, lets copy it.
  747                                  */
  748                                 pmap_copy_page(fs.m, fs.first_m);
  749                                 fs.first_m->valid = VM_PAGE_BITS_ALL;
  750                         }
  751                         if (fs.m) {
  752                                 /*
  753                                  * We no longer need the old page or object.
  754                                  */
  755                                 release_page(&fs);
  756                         }
  757                         /*
  758                          * fs.object != fs.first_object due to above 
  759                          * conditional
  760                          */
  761                         vm_object_pip_wakeup(fs.object);
  762                         VM_OBJECT_UNLOCK(fs.object);
  763                         /*
  764                          * Only use the new page below...
  765                          */
  766                         fs.object = fs.first_object;
  767                         fs.pindex = fs.first_pindex;
  768                         fs.m = fs.first_m;
  769                         if (!is_first_object_locked)
  770                                 VM_OBJECT_LOCK(fs.object);
  771                         PCPU_INC(cnt.v_cow_faults);
  772                 } else {
  773                         prot &= ~VM_PROT_WRITE;
  774                 }
  775         }
  776 
  777         /*
  778          * We must verify that the maps have not changed since our last
  779          * lookup.
  780          */
  781         if (!fs.lookup_still_valid) {
  782                 vm_object_t retry_object;
  783                 vm_pindex_t retry_pindex;
  784                 vm_prot_t retry_prot;
  785 
  786                 if (!vm_map_trylock_read(fs.map)) {
  787                         release_page(&fs);
  788                         unlock_and_deallocate(&fs);
  789                         goto RetryFault;
  790                 }
  791                 fs.lookup_still_valid = TRUE;
  792                 if (fs.map->timestamp != map_generation) {
  793                         result = vm_map_lookup_locked(&fs.map, vaddr, fault_type,
  794                             &fs.entry, &retry_object, &retry_pindex, &retry_prot, &wired);
  795 
  796                         /*
  797                          * If we don't need the page any longer, put it on the inactive
  798                          * list (the easiest thing to do here).  If no one needs it,
  799                          * pageout will grab it eventually.
  800                          */
  801                         if (result != KERN_SUCCESS) {
  802                                 release_page(&fs);
  803                                 unlock_and_deallocate(&fs);
  804 
  805                                 /*
  806                                  * If retry of map lookup would have blocked then
  807                                  * retry fault from start.
  808                                  */
  809                                 if (result == KERN_FAILURE)
  810                                         goto RetryFault;
  811                                 return (result);
  812                         }
  813                         if ((retry_object != fs.first_object) ||
  814                             (retry_pindex != fs.first_pindex)) {
  815                                 release_page(&fs);
  816                                 unlock_and_deallocate(&fs);
  817                                 goto RetryFault;
  818                         }
  819 
  820                         /*
  821                          * Check whether the protection has changed or the object has
  822                          * been copied while we left the map unlocked. Changing from
  823                          * read to write permission is OK - we leave the page
  824                          * write-protected, and catch the write fault. Changing from
  825                          * write to read permission means that we can't mark the page
  826                          * write-enabled after all.
  827                          */
  828                         prot &= retry_prot;
  829                 }
  830         }
  831         if (prot & VM_PROT_WRITE) {
  832                 vm_object_set_writeable_dirty(fs.object);
  833 
  834                 /*
  835                  * If the fault is a write, we know that this page is being
  836                  * written NOW so dirty it explicitly to save on 
  837                  * pmap_is_modified() calls later.
  838                  *
  839                  * If this is a NOSYNC mmap we do not want to set VPO_NOSYNC
  840                  * if the page is already dirty to prevent data written with
  841                  * the expectation of being synced from not being synced.
  842                  * Likewise if this entry does not request NOSYNC then make
  843                  * sure the page isn't marked NOSYNC.  Applications sharing
  844                  * data should use the same flags to avoid ping ponging.
  845                  *
  846                  * Also tell the backing pager, if any, that it should remove
  847                  * any swap backing since the page is now dirty.
  848                  */
  849                 if (fs.entry->eflags & MAP_ENTRY_NOSYNC) {
  850                         if (fs.m->dirty == 0)
  851                                 fs.m->oflags |= VPO_NOSYNC;
  852                 } else {
  853                         fs.m->oflags &= ~VPO_NOSYNC;
  854                 }
  855                 if (fault_flags & VM_FAULT_DIRTY) {
  856                         vm_page_dirty(fs.m);
  857                         vm_pager_page_unswapped(fs.m);
  858                 }
  859         }
  860 
  861         /*
  862          * Page had better still be busy
  863          */
  864         KASSERT(fs.m->oflags & VPO_BUSY,
  865                 ("vm_fault: page %p not busy!", fs.m));
  866         /*
  867          * Sanity check: page must be completely valid or it is not fit to
  868          * map into user space.  vm_pager_get_pages() ensures this.
  869          */
  870         if (fs.m->valid != VM_PAGE_BITS_ALL) {
  871                 vm_page_zero_invalid(fs.m, TRUE);
  872                 printf("Warning: page %p partially invalid on fault\n", fs.m);
  873         }
  874         VM_OBJECT_UNLOCK(fs.object);
  875 
  876         /*
  877          * Put this page into the physical map.  We had to do the unlock above
  878          * because pmap_enter() may sleep.  We don't put the page
  879          * back on the active queue until later so that the pageout daemon
  880          * won't find it (yet).
  881          */
  882         pmap_enter(fs.map->pmap, vaddr, fs.m, prot, wired);
  883         if (((fault_flags & VM_FAULT_WIRE_MASK) == 0) && (wired == 0)) {
  884                 vm_fault_prefault(fs.map->pmap, vaddr, fs.entry);
  885         }
  886         VM_OBJECT_LOCK(fs.object);
  887         vm_page_lock_queues();
  888         vm_page_flag_set(fs.m, PG_REFERENCED);
  889 
  890         /*
  891          * If the page is not wired down, then put it where the pageout daemon
  892          * can find it.
  893          */
  894         if (fault_flags & VM_FAULT_WIRE_MASK) {
  895                 if (wired)
  896                         vm_page_wire(fs.m);
  897                 else
  898                         vm_page_unwire(fs.m, 1);
  899         } else {
  900                 vm_page_activate(fs.m);
  901         }
  902         vm_page_unlock_queues();
  903         vm_page_wakeup(fs.m);
  904 
  905         /*
  906          * Unlock everything, and return
  907          */
  908         unlock_and_deallocate(&fs);
  909         if (hardfault)
  910                 curthread->td_ru.ru_majflt++;
  911         else
  912                 curthread->td_ru.ru_minflt++;
  913 
  914         return (KERN_SUCCESS);
  915 }
  916 
  917 /*
  918  * vm_fault_prefault provides a quick way of clustering
  919  * pagefaults into a processes address space.  It is a "cousin"
  920  * of vm_map_pmap_enter, except it runs at page fault time instead
  921  * of mmap time.
  922  */
  923 static void
  924 vm_fault_prefault(pmap_t pmap, vm_offset_t addra, vm_map_entry_t entry)
  925 {
  926         int i;
  927         vm_offset_t addr, starta;
  928         vm_pindex_t pindex;
  929         vm_page_t m;
  930         vm_object_t object;
  931 
  932         if (pmap != vmspace_pmap(curthread->td_proc->p_vmspace))
  933                 return;
  934 
  935         object = entry->object.vm_object;
  936 
  937         starta = addra - PFBAK * PAGE_SIZE;
  938         if (starta < entry->start) {
  939                 starta = entry->start;
  940         } else if (starta > addra) {
  941                 starta = 0;
  942         }
  943 
  944         for (i = 0; i < PAGEORDER_SIZE; i++) {
  945                 vm_object_t backing_object, lobject;
  946 
  947                 addr = addra + prefault_pageorder[i];
  948                 if (addr > addra + (PFFOR * PAGE_SIZE))
  949                         addr = 0;
  950 
  951                 if (addr < starta || addr >= entry->end)
  952                         continue;
  953 
  954                 if (!pmap_is_prefaultable(pmap, addr))
  955                         continue;
  956 
  957                 pindex = ((addr - entry->start) + entry->offset) >> PAGE_SHIFT;
  958                 lobject = object;
  959                 VM_OBJECT_LOCK(lobject);
  960                 while ((m = vm_page_lookup(lobject, pindex)) == NULL &&
  961                     lobject->type == OBJT_DEFAULT &&
  962                     (backing_object = lobject->backing_object) != NULL) {
  963                         if (lobject->backing_object_offset & PAGE_MASK)
  964                                 break;
  965                         pindex += lobject->backing_object_offset >> PAGE_SHIFT;
  966                         VM_OBJECT_LOCK(backing_object);
  967                         VM_OBJECT_UNLOCK(lobject);
  968                         lobject = backing_object;
  969                 }
  970                 /*
  971                  * give-up when a page is not in memory
  972                  */
  973                 if (m == NULL) {
  974                         VM_OBJECT_UNLOCK(lobject);
  975                         break;
  976                 }
  977                 if (((m->valid & VM_PAGE_BITS_ALL) == VM_PAGE_BITS_ALL) &&
  978                         (m->busy == 0) &&
  979                     (m->flags & PG_FICTITIOUS) == 0) {
  980 
  981                         vm_page_lock_queues();
  982                         pmap_enter_quick(pmap, addr, m, entry->protection);
  983                         vm_page_unlock_queues();
  984                 }
  985                 VM_OBJECT_UNLOCK(lobject);
  986         }
  987 }
  988 
  989 /*
  990  *      vm_fault_quick:
  991  *
  992  *      Ensure that the requested virtual address, which may be in userland,
  993  *      is valid.  Fault-in the page if necessary.  Return -1 on failure.
  994  */
  995 int
  996 vm_fault_quick(caddr_t v, int prot)
  997 {
  998         int r;
  999 
 1000         if (prot & VM_PROT_WRITE)
 1001                 r = subyte(v, fubyte(v));
 1002         else
 1003                 r = fubyte(v);
 1004         return(r);
 1005 }
 1006 
 1007 /*
 1008  *      vm_fault_wire:
 1009  *
 1010  *      Wire down a range of virtual addresses in a map.
 1011  */
 1012 int
 1013 vm_fault_wire(vm_map_t map, vm_offset_t start, vm_offset_t end,
 1014     boolean_t user_wire, boolean_t fictitious)
 1015 {
 1016         vm_offset_t va;
 1017         int rv;
 1018 
 1019         /*
 1020          * We simulate a fault to get the page and enter it in the physical
 1021          * map.  For user wiring, we only ask for read access on currently
 1022          * read-only sections.
 1023          */
 1024         for (va = start; va < end; va += PAGE_SIZE) {
 1025                 rv = vm_fault(map, va,
 1026                     user_wire ? VM_PROT_READ : VM_PROT_READ | VM_PROT_WRITE,
 1027                     user_wire ? VM_FAULT_USER_WIRE : VM_FAULT_CHANGE_WIRING);
 1028                 if (rv) {
 1029                         if (va != start)
 1030                                 vm_fault_unwire(map, start, va, fictitious);
 1031                         return (rv);
 1032                 }
 1033         }
 1034         return (KERN_SUCCESS);
 1035 }
 1036 
 1037 /*
 1038  *      vm_fault_unwire:
 1039  *
 1040  *      Unwire a range of virtual addresses in a map.
 1041  */
 1042 void
 1043 vm_fault_unwire(vm_map_t map, vm_offset_t start, vm_offset_t end,
 1044     boolean_t fictitious)
 1045 {
 1046         vm_paddr_t pa;
 1047         vm_offset_t va;
 1048         pmap_t pmap;
 1049 
 1050         pmap = vm_map_pmap(map);
 1051 
 1052         /*
 1053          * Since the pages are wired down, we must be able to get their
 1054          * mappings from the physical map system.
 1055          */
 1056         for (va = start; va < end; va += PAGE_SIZE) {
 1057                 pa = pmap_extract(pmap, va);
 1058                 if (pa != 0) {
 1059                         pmap_change_wiring(pmap, va, FALSE);
 1060                         if (!fictitious) {
 1061                                 vm_page_lock_queues();
 1062                                 vm_page_unwire(PHYS_TO_VM_PAGE(pa), 1);
 1063                                 vm_page_unlock_queues();
 1064                         }
 1065                 }
 1066         }
 1067 }
 1068 
 1069 /*
 1070  *      Routine:
 1071  *              vm_fault_copy_entry
 1072  *      Function:
 1073  *              Copy all of the pages from a wired-down map entry to another.
 1074  *
 1075  *      In/out conditions:
 1076  *              The source and destination maps must be locked for write.
 1077  *              The source map entry must be wired down (or be a sharing map
 1078  *              entry corresponding to a main map entry that is wired down).
 1079  */
 1080 void
 1081 vm_fault_copy_entry(dst_map, src_map, dst_entry, src_entry)
 1082         vm_map_t dst_map;
 1083         vm_map_t src_map;
 1084         vm_map_entry_t dst_entry;
 1085         vm_map_entry_t src_entry;
 1086 {
 1087         vm_object_t backing_object, dst_object, object;
 1088         vm_object_t src_object;
 1089         vm_ooffset_t dst_offset;
 1090         vm_ooffset_t src_offset;
 1091         vm_pindex_t pindex;
 1092         vm_prot_t prot;
 1093         vm_offset_t vaddr;
 1094         vm_page_t dst_m;
 1095         vm_page_t src_m;
 1096 
 1097 #ifdef  lint
 1098         src_map++;
 1099 #endif  /* lint */
 1100 
 1101         src_object = src_entry->object.vm_object;
 1102         src_offset = src_entry->offset;
 1103 
 1104         /*
 1105          * Create the top-level object for the destination entry. (Doesn't
 1106          * actually shadow anything - we copy the pages directly.)
 1107          */
 1108         dst_object = vm_object_allocate(OBJT_DEFAULT,
 1109             OFF_TO_IDX(dst_entry->end - dst_entry->start));
 1110 
 1111         VM_OBJECT_LOCK(dst_object);
 1112         dst_entry->object.vm_object = dst_object;
 1113         dst_entry->offset = 0;
 1114 
 1115         prot = dst_entry->max_protection;
 1116 
 1117         /*
 1118          * Loop through all of the pages in the entry's range, copying each
 1119          * one from the source object (it should be there) to the destination
 1120          * object.
 1121          */
 1122         for (vaddr = dst_entry->start, dst_offset = 0;
 1123             vaddr < dst_entry->end;
 1124             vaddr += PAGE_SIZE, dst_offset += PAGE_SIZE) {
 1125 
 1126                 /*
 1127                  * Allocate a page in the destination object
 1128                  */
 1129                 do {
 1130                         dst_m = vm_page_alloc(dst_object,
 1131                                 OFF_TO_IDX(dst_offset), VM_ALLOC_NORMAL);
 1132                         if (dst_m == NULL) {
 1133                                 VM_OBJECT_UNLOCK(dst_object);
 1134                                 VM_WAIT;
 1135                                 VM_OBJECT_LOCK(dst_object);
 1136                         }
 1137                 } while (dst_m == NULL);
 1138 
 1139                 /*
 1140                  * Find the page in the source object, and copy it in.
 1141                  * (Because the source is wired down, the page will be in
 1142                  * memory.)
 1143                  */
 1144                 VM_OBJECT_LOCK(src_object);
 1145                 object = src_object;
 1146                 pindex = 0;
 1147                 while ((src_m = vm_page_lookup(object, pindex +
 1148                     OFF_TO_IDX(dst_offset + src_offset))) == NULL &&
 1149                     (src_entry->protection & VM_PROT_WRITE) == 0 &&
 1150                     (backing_object = object->backing_object) != NULL) {
 1151                         /*
 1152                          * Allow fallback to backing objects if we are reading.
 1153                          */
 1154                         VM_OBJECT_LOCK(backing_object);
 1155                         pindex += OFF_TO_IDX(object->backing_object_offset);
 1156                         VM_OBJECT_UNLOCK(object);
 1157                         object = backing_object;
 1158                 }
 1159                 if (src_m == NULL)
 1160                         panic("vm_fault_copy_wired: page missing");
 1161                 pmap_copy_page(src_m, dst_m);
 1162                 VM_OBJECT_UNLOCK(object);
 1163                 dst_m->valid = VM_PAGE_BITS_ALL;
 1164                 VM_OBJECT_UNLOCK(dst_object);
 1165 
 1166                 /*
 1167                  * Enter it in the pmap...
 1168                  */
 1169                 pmap_enter(dst_map->pmap, vaddr, dst_m, prot, FALSE);
 1170 
 1171                 /*
 1172                  * Mark it no longer busy, and put it on the active list.
 1173                  */
 1174                 VM_OBJECT_LOCK(dst_object);
 1175                 vm_page_lock_queues();
 1176                 vm_page_activate(dst_m);
 1177                 vm_page_unlock_queues();
 1178                 vm_page_wakeup(dst_m);
 1179         }
 1180         VM_OBJECT_UNLOCK(dst_object);
 1181 }
 1182 
 1183 
 1184 /*
 1185  * This routine checks around the requested page for other pages that
 1186  * might be able to be faulted in.  This routine brackets the viable
 1187  * pages for the pages to be paged in.
 1188  *
 1189  * Inputs:
 1190  *      m, rbehind, rahead
 1191  *
 1192  * Outputs:
 1193  *  marray (array of vm_page_t), reqpage (index of requested page)
 1194  *
 1195  * Return value:
 1196  *  number of pages in marray
 1197  *
 1198  * This routine can't block.
 1199  */
 1200 static int
 1201 vm_fault_additional_pages(m, rbehind, rahead, marray, reqpage)
 1202         vm_page_t m;
 1203         int rbehind;
 1204         int rahead;
 1205         vm_page_t *marray;
 1206         int *reqpage;
 1207 {
 1208         int i,j;
 1209         vm_object_t object;
 1210         vm_pindex_t pindex, startpindex, endpindex, tpindex;
 1211         vm_page_t rtm;
 1212         int cbehind, cahead;
 1213 
 1214         VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED);
 1215 
 1216         object = m->object;
 1217         pindex = m->pindex;
 1218         cbehind = cahead = 0;
 1219 
 1220         /*
 1221          * if the requested page is not available, then give up now
 1222          */
 1223         if (!vm_pager_has_page(object, pindex, &cbehind, &cahead)) {
 1224                 return 0;
 1225         }
 1226 
 1227         if ((cbehind == 0) && (cahead == 0)) {
 1228                 *reqpage = 0;
 1229                 marray[0] = m;
 1230                 return 1;
 1231         }
 1232 
 1233         if (rahead > cahead) {
 1234                 rahead = cahead;
 1235         }
 1236 
 1237         if (rbehind > cbehind) {
 1238                 rbehind = cbehind;
 1239         }
 1240 
 1241         /*
 1242          * scan backward for the read behind pages -- in memory 
 1243          */
 1244         if (pindex > 0) {
 1245                 if (rbehind > pindex) {
 1246                         rbehind = pindex;
 1247                         startpindex = 0;
 1248                 } else {
 1249                         startpindex = pindex - rbehind;
 1250                 }
 1251 
 1252                 if ((rtm = TAILQ_PREV(m, pglist, listq)) != NULL &&
 1253                     rtm->pindex >= startpindex)
 1254                         startpindex = rtm->pindex + 1;
 1255 
 1256                 /* tpindex is unsigned; beware of numeric underflow. */
 1257                 for (i = 0, tpindex = pindex - 1; tpindex >= startpindex &&
 1258                     tpindex < pindex; i++, tpindex--) {
 1259 
 1260                         rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
 1261                             VM_ALLOC_IFNOTCACHED);
 1262                         if (rtm == NULL) {
 1263                                 /*
 1264                                  * Shift the allocated pages to the
 1265                                  * beginning of the array.
 1266                                  */
 1267                                 for (j = 0; j < i; j++) {
 1268                                         marray[j] = marray[j + tpindex + 1 -
 1269                                             startpindex];
 1270                                 }
 1271                                 break;
 1272                         }
 1273 
 1274                         marray[tpindex - startpindex] = rtm;
 1275                 }
 1276         } else {
 1277                 startpindex = 0;
 1278                 i = 0;
 1279         }
 1280 
 1281         marray[i] = m;
 1282         /* page offset of the required page */
 1283         *reqpage = i;
 1284 
 1285         tpindex = pindex + 1;
 1286         i++;
 1287 
 1288         /*
 1289          * scan forward for the read ahead pages
 1290          */
 1291         endpindex = tpindex + rahead;
 1292         if ((rtm = TAILQ_NEXT(m, listq)) != NULL && rtm->pindex < endpindex)
 1293                 endpindex = rtm->pindex;
 1294         if (endpindex > object->size)
 1295                 endpindex = object->size;
 1296 
 1297         for (; tpindex < endpindex; i++, tpindex++) {
 1298 
 1299                 rtm = vm_page_alloc(object, tpindex, VM_ALLOC_NORMAL |
 1300                     VM_ALLOC_IFNOTCACHED);
 1301                 if (rtm == NULL) {
 1302                         break;
 1303                 }
 1304 
 1305                 marray[i] = rtm;
 1306         }
 1307 
 1308         /* return number of pages */
 1309         return i;
 1310 }

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