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

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    1 /*-
    2  * Copyright (c) 1991, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  *
    5  * This code is derived from software contributed to Berkeley by
    6  * The Mach Operating System project at Carnegie-Mellon University.
    7  *
    8  * Redistribution and use in source and binary forms, with or without
    9  * modification, are permitted provided that the following conditions
   10  * are met:
   11  * 1. Redistributions of source code must retain the above copyright
   12  *    notice, this list of conditions and the following disclaimer.
   13  * 2. Redistributions in binary form must reproduce the above copyright
   14  *    notice, this list of conditions and the following disclaimer in the
   15  *    documentation and/or other materials provided with the distribution.
   16  * 4. Neither the name of the University nor the names of its contributors
   17  *    may be used to endorse or promote products derived from this software
   18  *    without specific prior written permission.
   19  *
   20  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   30  * SUCH DAMAGE.
   31  *
   32  *      from: @(#)vm_kern.c     8.3 (Berkeley) 1/12/94
   33  *
   34  *
   35  * Copyright (c) 1987, 1990 Carnegie-Mellon University.
   36  * All rights reserved.
   37  *
   38  * Authors: Avadis Tevanian, Jr., Michael Wayne Young
   39  *
   40  * Permission to use, copy, modify and distribute this software and
   41  * its documentation is hereby granted, provided that both the copyright
   42  * notice and this permission notice appear in all copies of the
   43  * software, derivative works or modified versions, and any portions
   44  * thereof, and that both notices appear in supporting documentation.
   45  *
   46  * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
   47  * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
   48  * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
   49  *
   50  * Carnegie Mellon requests users of this software to return to
   51  *
   52  *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
   53  *  School of Computer Science
   54  *  Carnegie Mellon University
   55  *  Pittsburgh PA 15213-3890
   56  *
   57  * any improvements or extensions that they make and grant Carnegie the
   58  * rights to redistribute these changes.
   59  */
   60 
   61 /*
   62  *      Kernel memory management.
   63  */
   64 
   65 #include <sys/cdefs.h>
   66 __FBSDID("$FreeBSD: releng/9.0/sys/vm/vm_kern.c 224746 2011-08-09 21:01:36Z kib $");
   67 
   68 #include <sys/param.h>
   69 #include <sys/systm.h>
   70 #include <sys/kernel.h>         /* for ticks and hz */
   71 #include <sys/eventhandler.h>
   72 #include <sys/lock.h>
   73 #include <sys/mutex.h>
   74 #include <sys/proc.h>
   75 #include <sys/malloc.h>
   76 #include <sys/sysctl.h>
   77 
   78 #include <vm/vm.h>
   79 #include <vm/vm_param.h>
   80 #include <vm/pmap.h>
   81 #include <vm/vm_map.h>
   82 #include <vm/vm_object.h>
   83 #include <vm/vm_page.h>
   84 #include <vm/vm_pageout.h>
   85 #include <vm/vm_extern.h>
   86 #include <vm/uma.h>
   87 
   88 vm_map_t kernel_map=0;
   89 vm_map_t kmem_map=0;
   90 vm_map_t exec_map=0;
   91 vm_map_t pipe_map;
   92 vm_map_t buffer_map=0;
   93 
   94 const void *zero_region;
   95 CTASSERT((ZERO_REGION_SIZE & PAGE_MASK) == 0);
   96 
   97 /*
   98  *      kmem_alloc_nofault:
   99  *
  100  *      Allocate a virtual address range with no underlying object and
  101  *      no initial mapping to physical memory.  Any mapping from this
  102  *      range to physical memory must be explicitly created prior to
  103  *      its use, typically with pmap_qenter().  Any attempt to create
  104  *      a mapping on demand through vm_fault() will result in a panic. 
  105  */
  106 vm_offset_t
  107 kmem_alloc_nofault(map, size)
  108         vm_map_t map;
  109         vm_size_t size;
  110 {
  111         vm_offset_t addr;
  112         int result;
  113 
  114         size = round_page(size);
  115         addr = vm_map_min(map);
  116         result = vm_map_find(map, NULL, 0, &addr, size, VMFS_ANY_SPACE,
  117             VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
  118         if (result != KERN_SUCCESS) {
  119                 return (0);
  120         }
  121         return (addr);
  122 }
  123 
  124 /*
  125  *      kmem_alloc_nofault_space:
  126  *
  127  *      Allocate a virtual address range with no underlying object and
  128  *      no initial mapping to physical memory within the specified
  129  *      address space.  Any mapping from this range to physical memory
  130  *      must be explicitly created prior to its use, typically with
  131  *      pmap_qenter().  Any attempt to create a mapping on demand
  132  *      through vm_fault() will result in a panic. 
  133  */
  134 vm_offset_t
  135 kmem_alloc_nofault_space(map, size, find_space)
  136         vm_map_t map;
  137         vm_size_t size;
  138         int find_space;
  139 {
  140         vm_offset_t addr;
  141         int result;
  142 
  143         size = round_page(size);
  144         addr = vm_map_min(map);
  145         result = vm_map_find(map, NULL, 0, &addr, size, find_space,
  146             VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
  147         if (result != KERN_SUCCESS) {
  148                 return (0);
  149         }
  150         return (addr);
  151 }
  152 
  153 /*
  154  *      Allocate wired-down memory in the kernel's address map
  155  *      or a submap.
  156  */
  157 vm_offset_t
  158 kmem_alloc(map, size)
  159         vm_map_t map;
  160         vm_size_t size;
  161 {
  162         vm_offset_t addr;
  163         vm_offset_t offset;
  164         vm_offset_t i;
  165 
  166         size = round_page(size);
  167 
  168         /*
  169          * Use the kernel object for wired-down kernel pages. Assume that no
  170          * region of the kernel object is referenced more than once.
  171          */
  172 
  173         /*
  174          * Locate sufficient space in the map.  This will give us the final
  175          * virtual address for the new memory, and thus will tell us the
  176          * offset within the kernel map.
  177          */
  178         vm_map_lock(map);
  179         if (vm_map_findspace(map, vm_map_min(map), size, &addr)) {
  180                 vm_map_unlock(map);
  181                 return (0);
  182         }
  183         offset = addr - VM_MIN_KERNEL_ADDRESS;
  184         vm_object_reference(kernel_object);
  185         vm_map_insert(map, kernel_object, offset, addr, addr + size,
  186                 VM_PROT_ALL, VM_PROT_ALL, 0);
  187         vm_map_unlock(map);
  188 
  189         /*
  190          * Guarantee that there are pages already in this object before
  191          * calling vm_map_wire.  This is to prevent the following
  192          * scenario:
  193          *
  194          * 1) Threads have swapped out, so that there is a pager for the
  195          * kernel_object. 2) The kmsg zone is empty, and so we are
  196          * kmem_allocing a new page for it. 3) vm_map_wire calls vm_fault;
  197          * there is no page, but there is a pager, so we call
  198          * pager_data_request.  But the kmsg zone is empty, so we must
  199          * kmem_alloc. 4) goto 1 5) Even if the kmsg zone is not empty: when
  200          * we get the data back from the pager, it will be (very stale)
  201          * non-zero data.  kmem_alloc is defined to return zero-filled memory.
  202          *
  203          * We're intentionally not activating the pages we allocate to prevent a
  204          * race with page-out.  vm_map_wire will wire the pages.
  205          */
  206         VM_OBJECT_LOCK(kernel_object);
  207         for (i = 0; i < size; i += PAGE_SIZE) {
  208                 vm_page_t mem;
  209 
  210                 mem = vm_page_grab(kernel_object, OFF_TO_IDX(offset + i),
  211                     VM_ALLOC_NOBUSY | VM_ALLOC_ZERO | VM_ALLOC_RETRY);
  212                 mem->valid = VM_PAGE_BITS_ALL;
  213                 KASSERT((mem->oflags & VPO_UNMANAGED) != 0,
  214                     ("kmem_alloc: page %p is managed", mem));
  215         }
  216         VM_OBJECT_UNLOCK(kernel_object);
  217 
  218         /*
  219          * And finally, mark the data as non-pageable.
  220          */
  221         (void) vm_map_wire(map, addr, addr + size,
  222             VM_MAP_WIRE_SYSTEM|VM_MAP_WIRE_NOHOLES);
  223 
  224         return (addr);
  225 }
  226 
  227 /*
  228  *      kmem_free:
  229  *
  230  *      Release a region of kernel virtual memory allocated
  231  *      with kmem_alloc, and return the physical pages
  232  *      associated with that region.
  233  *
  234  *      This routine may not block on kernel maps.
  235  */
  236 void
  237 kmem_free(map, addr, size)
  238         vm_map_t map;
  239         vm_offset_t addr;
  240         vm_size_t size;
  241 {
  242 
  243         (void) vm_map_remove(map, trunc_page(addr), round_page(addr + size));
  244 }
  245 
  246 /*
  247  *      kmem_suballoc:
  248  *
  249  *      Allocates a map to manage a subrange
  250  *      of the kernel virtual address space.
  251  *
  252  *      Arguments are as follows:
  253  *
  254  *      parent          Map to take range from
  255  *      min, max        Returned endpoints of map
  256  *      size            Size of range to find
  257  *      superpage_align Request that min is superpage aligned
  258  */
  259 vm_map_t
  260 kmem_suballoc(vm_map_t parent, vm_offset_t *min, vm_offset_t *max,
  261     vm_size_t size, boolean_t superpage_align)
  262 {
  263         int ret;
  264         vm_map_t result;
  265 
  266         size = round_page(size);
  267 
  268         *min = vm_map_min(parent);
  269         ret = vm_map_find(parent, NULL, 0, min, size, superpage_align ?
  270             VMFS_ALIGNED_SPACE : VMFS_ANY_SPACE, VM_PROT_ALL, VM_PROT_ALL,
  271             MAP_ACC_NO_CHARGE);
  272         if (ret != KERN_SUCCESS)
  273                 panic("kmem_suballoc: bad status return of %d", ret);
  274         *max = *min + size;
  275         result = vm_map_create(vm_map_pmap(parent), *min, *max);
  276         if (result == NULL)
  277                 panic("kmem_suballoc: cannot create submap");
  278         if (vm_map_submap(parent, *min, *max, result) != KERN_SUCCESS)
  279                 panic("kmem_suballoc: unable to change range to submap");
  280         return (result);
  281 }
  282 
  283 /*
  284  *      kmem_malloc:
  285  *
  286  *      Allocate wired-down memory in the kernel's address map for the higher
  287  *      level kernel memory allocator (kern/kern_malloc.c).  We cannot use
  288  *      kmem_alloc() because we may need to allocate memory at interrupt
  289  *      level where we cannot block (canwait == FALSE).
  290  *
  291  *      This routine has its own private kernel submap (kmem_map) and object
  292  *      (kmem_object).  This, combined with the fact that only malloc uses
  293  *      this routine, ensures that we will never block in map or object waits.
  294  *
  295  *      We don't worry about expanding the map (adding entries) since entries
  296  *      for wired maps are statically allocated.
  297  *
  298  *      `map' is ONLY allowed to be kmem_map or one of the mbuf submaps to
  299  *      which we never free.
  300  */
  301 vm_offset_t
  302 kmem_malloc(map, size, flags)
  303         vm_map_t map;
  304         vm_size_t size;
  305         int flags;
  306 {
  307         vm_offset_t addr;
  308         int i, rv;
  309 
  310         size = round_page(size);
  311         addr = vm_map_min(map);
  312 
  313         /*
  314          * Locate sufficient space in the map.  This will give us the final
  315          * virtual address for the new memory, and thus will tell us the
  316          * offset within the kernel map.
  317          */
  318         vm_map_lock(map);
  319         if (vm_map_findspace(map, vm_map_min(map), size, &addr)) {
  320                 vm_map_unlock(map);
  321                 if ((flags & M_NOWAIT) == 0) {
  322                         for (i = 0; i < 8; i++) {
  323                                 EVENTHANDLER_INVOKE(vm_lowmem, 0);
  324                                 uma_reclaim();
  325                                 vm_map_lock(map);
  326                                 if (vm_map_findspace(map, vm_map_min(map),
  327                                     size, &addr) == 0) {
  328                                         break;
  329                                 }
  330                                 vm_map_unlock(map);
  331                                 tsleep(&i, 0, "nokva", (hz / 4) * (i + 1));
  332                         }
  333                         if (i == 8) {
  334                                 panic("kmem_malloc(%ld): kmem_map too small: %ld total allocated",
  335                                     (long)size, (long)map->size);
  336                         }
  337                 } else {
  338                         return (0);
  339                 }
  340         }
  341 
  342         rv = kmem_back(map, addr, size, flags);
  343         vm_map_unlock(map);
  344         return (rv == KERN_SUCCESS ? addr : 0);
  345 }
  346 
  347 /*
  348  *      kmem_back:
  349  *
  350  *      Allocate physical pages for the specified virtual address range.
  351  */
  352 int
  353 kmem_back(vm_map_t map, vm_offset_t addr, vm_size_t size, int flags)
  354 {
  355         vm_offset_t offset, i;
  356         vm_map_entry_t entry;
  357         vm_page_t m;
  358         int pflags;
  359         boolean_t found;
  360 
  361         KASSERT(vm_map_locked(map), ("kmem_back: map %p is not locked", map));
  362         offset = addr - VM_MIN_KERNEL_ADDRESS;
  363         vm_object_reference(kmem_object);
  364         vm_map_insert(map, kmem_object, offset, addr, addr + size,
  365             VM_PROT_ALL, VM_PROT_ALL, 0);
  366 
  367         /*
  368          * Assert: vm_map_insert() will never be able to extend the
  369          * previous entry so vm_map_lookup_entry() will find a new
  370          * entry exactly corresponding to this address range and it
  371          * will have wired_count == 0.
  372          */
  373         found = vm_map_lookup_entry(map, addr, &entry);
  374         KASSERT(found && entry->start == addr && entry->end == addr + size &&
  375             entry->wired_count == 0 && (entry->eflags & MAP_ENTRY_IN_TRANSITION)
  376             == 0, ("kmem_back: entry not found or misaligned"));
  377 
  378         if ((flags & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
  379                 pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
  380         else
  381                 pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
  382 
  383         if (flags & M_ZERO)
  384                 pflags |= VM_ALLOC_ZERO;
  385 
  386         VM_OBJECT_LOCK(kmem_object);
  387         for (i = 0; i < size; i += PAGE_SIZE) {
  388 retry:
  389                 m = vm_page_alloc(kmem_object, OFF_TO_IDX(offset + i), pflags);
  390 
  391                 /*
  392                  * Ran out of space, free everything up and return. Don't need
  393                  * to lock page queues here as we know that the pages we got
  394                  * aren't on any queues.
  395                  */
  396                 if (m == NULL) {
  397                         if ((flags & M_NOWAIT) == 0) {
  398                                 VM_OBJECT_UNLOCK(kmem_object);
  399                                 entry->eflags |= MAP_ENTRY_IN_TRANSITION;
  400                                 vm_map_unlock(map);
  401                                 VM_WAIT;
  402                                 vm_map_lock(map);
  403                                 KASSERT(
  404 (entry->eflags & (MAP_ENTRY_IN_TRANSITION | MAP_ENTRY_NEEDS_WAKEUP)) ==
  405                                     MAP_ENTRY_IN_TRANSITION,
  406                                     ("kmem_back: volatile entry"));
  407                                 entry->eflags &= ~MAP_ENTRY_IN_TRANSITION;
  408                                 VM_OBJECT_LOCK(kmem_object);
  409                                 goto retry;
  410                         }
  411                         /* 
  412                          * Free the pages before removing the map entry.
  413                          * They are already marked busy.  Calling
  414                          * vm_map_delete before the pages has been freed or
  415                          * unbusied will cause a deadlock.
  416                          */
  417                         while (i != 0) {
  418                                 i -= PAGE_SIZE;
  419                                 m = vm_page_lookup(kmem_object,
  420                                                    OFF_TO_IDX(offset + i));
  421                                 vm_page_unwire(m, 0);
  422                                 vm_page_free(m);
  423                         }
  424                         VM_OBJECT_UNLOCK(kmem_object);
  425                         vm_map_delete(map, addr, addr + size);
  426                         return (KERN_NO_SPACE);
  427                 }
  428                 if (flags & M_ZERO && (m->flags & PG_ZERO) == 0)
  429                         pmap_zero_page(m);
  430                 m->valid = VM_PAGE_BITS_ALL;
  431                 KASSERT((m->oflags & VPO_UNMANAGED) != 0,
  432                     ("kmem_malloc: page %p is managed", m));
  433         }
  434         VM_OBJECT_UNLOCK(kmem_object);
  435 
  436         /*
  437          * Mark map entry as non-pageable.  Repeat the assert.
  438          */
  439         KASSERT(entry->start == addr && entry->end == addr + size &&
  440             entry->wired_count == 0,
  441             ("kmem_back: entry not found or misaligned after allocation"));
  442         entry->wired_count = 1;
  443 
  444         /*
  445          * At this point, the kmem_object must be unlocked because
  446          * vm_map_simplify_entry() calls vm_object_deallocate(), which
  447          * locks the kmem_object.
  448          */
  449         vm_map_simplify_entry(map, entry);
  450 
  451         /*
  452          * Loop thru pages, entering them in the pmap.
  453          */
  454         VM_OBJECT_LOCK(kmem_object);
  455         for (i = 0; i < size; i += PAGE_SIZE) {
  456                 m = vm_page_lookup(kmem_object, OFF_TO_IDX(offset + i));
  457                 /*
  458                  * Because this is kernel_pmap, this call will not block.
  459                  */
  460                 pmap_enter(kernel_pmap, addr + i, VM_PROT_ALL, m, VM_PROT_ALL,
  461                     TRUE);
  462                 vm_page_wakeup(m);
  463         }
  464         VM_OBJECT_UNLOCK(kmem_object);
  465 
  466         return (KERN_SUCCESS);
  467 }
  468 
  469 /*
  470  *      kmem_alloc_wait:
  471  *
  472  *      Allocates pageable memory from a sub-map of the kernel.  If the submap
  473  *      has no room, the caller sleeps waiting for more memory in the submap.
  474  *
  475  *      This routine may block.
  476  */
  477 vm_offset_t
  478 kmem_alloc_wait(map, size)
  479         vm_map_t map;
  480         vm_size_t size;
  481 {
  482         vm_offset_t addr;
  483 
  484         size = round_page(size);
  485         if (!swap_reserve(size))
  486                 return (0);
  487 
  488         for (;;) {
  489                 /*
  490                  * To make this work for more than one map, use the map's lock
  491                  * to lock out sleepers/wakers.
  492                  */
  493                 vm_map_lock(map);
  494                 if (vm_map_findspace(map, vm_map_min(map), size, &addr) == 0)
  495                         break;
  496                 /* no space now; see if we can ever get space */
  497                 if (vm_map_max(map) - vm_map_min(map) < size) {
  498                         vm_map_unlock(map);
  499                         swap_release(size);
  500                         return (0);
  501                 }
  502                 map->needs_wakeup = TRUE;
  503                 vm_map_unlock_and_wait(map, 0);
  504         }
  505         vm_map_insert(map, NULL, 0, addr, addr + size, VM_PROT_ALL,
  506             VM_PROT_ALL, MAP_ACC_CHARGED);
  507         vm_map_unlock(map);
  508         return (addr);
  509 }
  510 
  511 /*
  512  *      kmem_free_wakeup:
  513  *
  514  *      Returns memory to a submap of the kernel, and wakes up any processes
  515  *      waiting for memory in that map.
  516  */
  517 void
  518 kmem_free_wakeup(map, addr, size)
  519         vm_map_t map;
  520         vm_offset_t addr;
  521         vm_size_t size;
  522 {
  523 
  524         vm_map_lock(map);
  525         (void) vm_map_delete(map, trunc_page(addr), round_page(addr + size));
  526         if (map->needs_wakeup) {
  527                 map->needs_wakeup = FALSE;
  528                 vm_map_wakeup(map);
  529         }
  530         vm_map_unlock(map);
  531 }
  532 
  533 static void
  534 kmem_init_zero_region(void)
  535 {
  536         vm_offset_t addr, i;
  537         vm_page_t m;
  538         int error;
  539 
  540         /*
  541          * Map a single physical page of zeros to a larger virtual range.
  542          * This requires less looping in places that want large amounts of
  543          * zeros, while not using much more physical resources.
  544          */
  545         addr = kmem_alloc_nofault(kernel_map, ZERO_REGION_SIZE);
  546         m = vm_page_alloc(NULL, OFF_TO_IDX(addr - VM_MIN_KERNEL_ADDRESS),
  547             VM_ALLOC_NOOBJ | VM_ALLOC_WIRED | VM_ALLOC_ZERO);
  548         if ((m->flags & PG_ZERO) == 0)
  549                 pmap_zero_page(m);
  550         for (i = 0; i < ZERO_REGION_SIZE; i += PAGE_SIZE)
  551                 pmap_qenter(addr + i, &m, 1);
  552         error = vm_map_protect(kernel_map, addr, addr + ZERO_REGION_SIZE,
  553             VM_PROT_READ, TRUE);
  554         KASSERT(error == 0, ("error=%d", error));
  555 
  556         zero_region = (const void *)addr;
  557 }
  558 
  559 /*
  560  *      kmem_init:
  561  *
  562  *      Create the kernel map; insert a mapping covering kernel text, 
  563  *      data, bss, and all space allocated thus far (`boostrap' data).  The 
  564  *      new map will thus map the range between VM_MIN_KERNEL_ADDRESS and 
  565  *      `start' as allocated, and the range between `start' and `end' as free.
  566  */
  567 void
  568 kmem_init(start, end)
  569         vm_offset_t start, end;
  570 {
  571         vm_map_t m;
  572 
  573         m = vm_map_create(kernel_pmap, VM_MIN_KERNEL_ADDRESS, end);
  574         m->system_map = 1;
  575         vm_map_lock(m);
  576         /* N.B.: cannot use kgdb to debug, starting with this assignment ... */
  577         kernel_map = m;
  578         (void) vm_map_insert(m, NULL, (vm_ooffset_t) 0,
  579 #ifdef __amd64__
  580             KERNBASE,
  581 #else                
  582             VM_MIN_KERNEL_ADDRESS,
  583 #endif
  584             start, VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
  585         /* ... and ending with the completion of the above `insert' */
  586         vm_map_unlock(m);
  587 
  588         kmem_init_zero_region();
  589 }
  590 
  591 #ifdef DIAGNOSTIC
  592 /*
  593  * Allow userspace to directly trigger the VM drain routine for testing
  594  * purposes.
  595  */
  596 static int
  597 debug_vm_lowmem(SYSCTL_HANDLER_ARGS)
  598 {
  599         int error, i;
  600 
  601         i = 0;
  602         error = sysctl_handle_int(oidp, &i, 0, req);
  603         if (error)
  604                 return (error);
  605         if (i)   
  606                 EVENTHANDLER_INVOKE(vm_lowmem, 0);
  607         return (0);
  608 }
  609 
  610 SYSCTL_PROC(_debug, OID_AUTO, vm_lowmem, CTLTYPE_INT | CTLFLAG_RW, 0, 0,
  611     debug_vm_lowmem, "I", "set to trigger vm_lowmem event");
  612 #endif

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