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: stable/11/sys/vm/vm_kern.c 340660 2018-11-20 01:12:21Z markj $");
   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/proc.h>
   74 #include <sys/malloc.h>
   75 #include <sys/rwlock.h>
   76 #include <sys/sysctl.h>
   77 #include <sys/vmem.h>
   78 
   79 #include <vm/vm.h>
   80 #include <vm/vm_param.h>
   81 #include <vm/vm_kern.h>
   82 #include <vm/pmap.h>
   83 #include <vm/vm_map.h>
   84 #include <vm/vm_object.h>
   85 #include <vm/vm_page.h>
   86 #include <vm/vm_pageout.h>
   87 #include <vm/vm_phys.h>
   88 #include <vm/vm_radix.h>
   89 #include <vm/vm_extern.h>
   90 #include <vm/uma.h>
   91 
   92 vm_map_t kernel_map;
   93 vm_map_t exec_map;
   94 vm_map_t pipe_map;
   95 
   96 const void *zero_region;
   97 CTASSERT((ZERO_REGION_SIZE & PAGE_MASK) == 0);
   98 
   99 /* NB: Used by kernel debuggers. */
  100 const u_long vm_maxuser_address = VM_MAXUSER_ADDRESS;
  101 
  102 u_int exec_map_entry_size;
  103 u_int exec_map_entries;
  104 
  105 SYSCTL_ULONG(_vm, OID_AUTO, min_kernel_address, CTLFLAG_RD,
  106     SYSCTL_NULL_ULONG_PTR, VM_MIN_KERNEL_ADDRESS, "Min kernel address");
  107 
  108 SYSCTL_ULONG(_vm, OID_AUTO, max_kernel_address, CTLFLAG_RD,
  109 #if defined(__arm__) || defined(__sparc64__)
  110     &vm_max_kernel_address, 0,
  111 #else
  112     SYSCTL_NULL_ULONG_PTR, VM_MAX_KERNEL_ADDRESS,
  113 #endif
  114     "Max kernel address");
  115 
  116 /*
  117  *      kva_alloc:
  118  *
  119  *      Allocate a virtual address range with no underlying object and
  120  *      no initial mapping to physical memory.  Any mapping from this
  121  *      range to physical memory must be explicitly created prior to
  122  *      its use, typically with pmap_qenter().  Any attempt to create
  123  *      a mapping on demand through vm_fault() will result in a panic. 
  124  */
  125 vm_offset_t
  126 kva_alloc(vm_size_t size)
  127 {
  128         vm_offset_t addr;
  129 
  130         size = round_page(size);
  131         if (vmem_alloc(kernel_arena, size, M_BESTFIT | M_NOWAIT, &addr))
  132                 return (0);
  133 
  134         return (addr);
  135 }
  136 
  137 /*
  138  *      kva_free:
  139  *
  140  *      Release a region of kernel virtual memory allocated
  141  *      with kva_alloc, and return the physical pages
  142  *      associated with that region.
  143  *
  144  *      This routine may not block on kernel maps.
  145  */
  146 void
  147 kva_free(vm_offset_t addr, vm_size_t size)
  148 {
  149 
  150         size = round_page(size);
  151         vmem_free(kernel_arena, addr, size);
  152 }
  153 
  154 /*
  155  *      Allocates a region from the kernel address map and physical pages
  156  *      within the specified address range to the kernel object.  Creates a
  157  *      wired mapping from this region to these pages, and returns the
  158  *      region's starting virtual address.  The allocated pages are not
  159  *      necessarily physically contiguous.  If M_ZERO is specified through the
  160  *      given flags, then the pages are zeroed before they are mapped.
  161  */
  162 vm_offset_t
  163 kmem_alloc_attr(vmem_t *vmem, vm_size_t size, int flags, vm_paddr_t low,
  164     vm_paddr_t high, vm_memattr_t memattr)
  165 {
  166         vm_object_t object = vmem == kmem_arena ? kmem_object : kernel_object;
  167         vm_offset_t addr, i, offset;
  168         vm_page_t m;
  169         int pflags, tries;
  170 
  171         size = round_page(size);
  172         if (vmem_alloc(vmem, size, M_BESTFIT | flags, &addr))
  173                 return (0);
  174         offset = addr - VM_MIN_KERNEL_ADDRESS;
  175         pflags = malloc2vm_flags(flags) | VM_ALLOC_NOBUSY | VM_ALLOC_WIRED;
  176         pflags &= ~(VM_ALLOC_NOWAIT | VM_ALLOC_WAITOK | VM_ALLOC_WAITFAIL);
  177         pflags |= VM_ALLOC_NOWAIT;
  178         VM_OBJECT_WLOCK(object);
  179         for (i = 0; i < size; i += PAGE_SIZE) {
  180                 tries = 0;
  181 retry:
  182                 m = vm_page_alloc_contig(object, atop(offset + i),
  183                     pflags, 1, low, high, PAGE_SIZE, 0, memattr);
  184                 if (m == NULL) {
  185                         VM_OBJECT_WUNLOCK(object);
  186                         if (tries < ((flags & M_NOWAIT) != 0 ? 1 : 3)) {
  187                                 if (!vm_page_reclaim_contig(pflags, 1,
  188                                     low, high, PAGE_SIZE, 0) &&
  189                                     (flags & M_WAITOK) != 0)
  190                                         VM_WAIT;
  191                                 VM_OBJECT_WLOCK(object);
  192                                 tries++;
  193                                 goto retry;
  194                         }
  195                         kmem_unback(object, addr, i);
  196                         vmem_free(vmem, addr, size);
  197                         return (0);
  198                 }
  199                 if ((flags & M_ZERO) && (m->flags & PG_ZERO) == 0)
  200                         pmap_zero_page(m);
  201                 m->valid = VM_PAGE_BITS_ALL;
  202                 pmap_enter(kernel_pmap, addr + i, m, VM_PROT_ALL,
  203                     VM_PROT_ALL | PMAP_ENTER_WIRED, 0);
  204         }
  205         VM_OBJECT_WUNLOCK(object);
  206         return (addr);
  207 }
  208 
  209 /*
  210  *      Allocates a region from the kernel address map and physically
  211  *      contiguous pages within the specified address range to the kernel
  212  *      object.  Creates a wired mapping from this region to these pages, and
  213  *      returns the region's starting virtual address.  If M_ZERO is specified
  214  *      through the given flags, then the pages are zeroed before they are
  215  *      mapped.
  216  */
  217 vm_offset_t
  218 kmem_alloc_contig(struct vmem *vmem, vm_size_t size, int flags, vm_paddr_t low,
  219     vm_paddr_t high, u_long alignment, vm_paddr_t boundary,
  220     vm_memattr_t memattr)
  221 {
  222         vm_object_t object = vmem == kmem_arena ? kmem_object : kernel_object;
  223         vm_offset_t addr, offset, tmp;
  224         vm_page_t end_m, m;
  225         u_long npages;
  226         int pflags, tries;
  227  
  228         size = round_page(size);
  229         if (vmem_alloc(vmem, size, flags | M_BESTFIT, &addr))
  230                 return (0);
  231         offset = addr - VM_MIN_KERNEL_ADDRESS;
  232         pflags = malloc2vm_flags(flags) | VM_ALLOC_NOBUSY | VM_ALLOC_WIRED;
  233         pflags &= ~(VM_ALLOC_NOWAIT | VM_ALLOC_WAITOK | VM_ALLOC_WAITFAIL);
  234         pflags |= VM_ALLOC_NOWAIT;
  235         npages = atop(size);
  236         VM_OBJECT_WLOCK(object);
  237         tries = 0;
  238 retry:
  239         m = vm_page_alloc_contig(object, atop(offset), pflags,
  240             npages, low, high, alignment, boundary, memattr);
  241         if (m == NULL) {
  242                 VM_OBJECT_WUNLOCK(object);
  243                 if (tries < ((flags & M_NOWAIT) != 0 ? 1 : 3)) {
  244                         if (!vm_page_reclaim_contig(pflags, npages, low, high,
  245                             alignment, boundary) && (flags & M_WAITOK) != 0)
  246                                 VM_WAIT;
  247                         VM_OBJECT_WLOCK(object);
  248                         tries++;
  249                         goto retry;
  250                 }
  251                 vmem_free(vmem, addr, size);
  252                 return (0);
  253         }
  254         end_m = m + npages;
  255         tmp = addr;
  256         for (; m < end_m; m++) {
  257                 if ((flags & M_ZERO) && (m->flags & PG_ZERO) == 0)
  258                         pmap_zero_page(m);
  259                 m->valid = VM_PAGE_BITS_ALL;
  260                 pmap_enter(kernel_pmap, tmp, m, VM_PROT_ALL,
  261                     VM_PROT_ALL | PMAP_ENTER_WIRED, 0);
  262                 tmp += PAGE_SIZE;
  263         }
  264         VM_OBJECT_WUNLOCK(object);
  265         return (addr);
  266 }
  267 
  268 /*
  269  *      kmem_suballoc:
  270  *
  271  *      Allocates a map to manage a subrange
  272  *      of the kernel virtual address space.
  273  *
  274  *      Arguments are as follows:
  275  *
  276  *      parent          Map to take range from
  277  *      min, max        Returned endpoints of map
  278  *      size            Size of range to find
  279  *      superpage_align Request that min is superpage aligned
  280  */
  281 vm_map_t
  282 kmem_suballoc(vm_map_t parent, vm_offset_t *min, vm_offset_t *max,
  283     vm_size_t size, boolean_t superpage_align)
  284 {
  285         int ret;
  286         vm_map_t result;
  287 
  288         size = round_page(size);
  289 
  290         *min = vm_map_min(parent);
  291         ret = vm_map_find(parent, NULL, 0, min, size, 0, superpage_align ?
  292             VMFS_SUPER_SPACE : VMFS_ANY_SPACE, VM_PROT_ALL, VM_PROT_ALL,
  293             MAP_ACC_NO_CHARGE);
  294         if (ret != KERN_SUCCESS)
  295                 panic("kmem_suballoc: bad status return of %d", ret);
  296         *max = *min + size;
  297         result = vm_map_create(vm_map_pmap(parent), *min, *max);
  298         if (result == NULL)
  299                 panic("kmem_suballoc: cannot create submap");
  300         if (vm_map_submap(parent, *min, *max, result) != KERN_SUCCESS)
  301                 panic("kmem_suballoc: unable to change range to submap");
  302         return (result);
  303 }
  304 
  305 /*
  306  *      kmem_malloc:
  307  *
  308  *      Allocate wired-down pages in the kernel's address space.
  309  */
  310 vm_offset_t
  311 kmem_malloc(struct vmem *vmem, vm_size_t size, int flags)
  312 {
  313         vm_offset_t addr;
  314         int rv;
  315 
  316         size = round_page(size);
  317         if (vmem_alloc(vmem, size, flags | M_BESTFIT, &addr))
  318                 return (0);
  319 
  320         rv = kmem_back((vmem == kmem_arena) ? kmem_object : kernel_object,
  321             addr, size, flags);
  322         if (rv != KERN_SUCCESS) {
  323                 vmem_free(vmem, addr, size);
  324                 return (0);
  325         }
  326         return (addr);
  327 }
  328 
  329 /*
  330  *      kmem_back:
  331  *
  332  *      Allocate physical pages for the specified virtual address range.
  333  */
  334 int
  335 kmem_back(vm_object_t object, vm_offset_t addr, vm_size_t size, int flags)
  336 {
  337         vm_offset_t offset, i;
  338         vm_page_t m, mpred;
  339         int pflags;
  340 
  341         KASSERT(object == kmem_object || object == kernel_object,
  342             ("kmem_back: only supports kernel objects."));
  343 
  344         offset = addr - VM_MIN_KERNEL_ADDRESS;
  345         pflags = malloc2vm_flags(flags) | VM_ALLOC_NOBUSY | VM_ALLOC_WIRED;
  346         pflags &= ~(VM_ALLOC_NOWAIT | VM_ALLOC_WAITOK | VM_ALLOC_WAITFAIL);
  347         if (flags & M_WAITOK)
  348                 pflags |= VM_ALLOC_WAITFAIL;
  349 
  350         i = 0;
  351         VM_OBJECT_WLOCK(object);
  352 retry:
  353         mpred = vm_radix_lookup_le(&object->rtree, atop(offset + i));
  354         for (; i < size; i += PAGE_SIZE, mpred = m) {
  355                 m = vm_page_alloc_after(object, atop(offset + i), pflags,
  356                     mpred);
  357 
  358                 /*
  359                  * Ran out of space, free everything up and return. Don't need
  360                  * to lock page queues here as we know that the pages we got
  361                  * aren't on any queues.
  362                  */
  363                 if (m == NULL) {
  364                         if ((flags & M_NOWAIT) == 0)
  365                                 goto retry;
  366                         VM_OBJECT_WUNLOCK(object);
  367                         kmem_unback(object, addr, i);
  368                         return (KERN_NO_SPACE);
  369                 }
  370                 if (flags & M_ZERO && (m->flags & PG_ZERO) == 0)
  371                         pmap_zero_page(m);
  372                 KASSERT((m->oflags & VPO_UNMANAGED) != 0,
  373                     ("kmem_malloc: page %p is managed", m));
  374                 m->valid = VM_PAGE_BITS_ALL;
  375                 pmap_enter(kernel_pmap, addr + i, m, VM_PROT_ALL,
  376                     VM_PROT_ALL | PMAP_ENTER_WIRED, 0);
  377         }
  378         VM_OBJECT_WUNLOCK(object);
  379 
  380         return (KERN_SUCCESS);
  381 }
  382 
  383 /*
  384  *      kmem_unback:
  385  *
  386  *      Unmap and free the physical pages underlying the specified virtual
  387  *      address range.
  388  *
  389  *      A physical page must exist within the specified object at each index
  390  *      that is being unmapped.
  391  */
  392 void
  393 kmem_unback(vm_object_t object, vm_offset_t addr, vm_size_t size)
  394 {
  395         vm_page_t m, next;
  396         vm_offset_t end, offset;
  397 
  398         KASSERT(object == kmem_object || object == kernel_object,
  399             ("kmem_unback: only supports kernel objects."));
  400 
  401         pmap_remove(kernel_pmap, addr, addr + size);
  402         offset = addr - VM_MIN_KERNEL_ADDRESS;
  403         end = offset + size;
  404         VM_OBJECT_WLOCK(object);
  405         for (m = vm_page_lookup(object, atop(offset)); offset < end;
  406             offset += PAGE_SIZE, m = next) {
  407                 next = vm_page_next(m);
  408                 vm_page_unwire(m, PQ_NONE);
  409                 vm_page_free(m);
  410         }
  411         VM_OBJECT_WUNLOCK(object);
  412 }
  413 
  414 /*
  415  *      kmem_free:
  416  *
  417  *      Free memory allocated with kmem_malloc.  The size must match the
  418  *      original allocation.
  419  */
  420 void
  421 kmem_free(struct vmem *vmem, vm_offset_t addr, vm_size_t size)
  422 {
  423 
  424         size = round_page(size);
  425         kmem_unback((vmem == kmem_arena) ? kmem_object : kernel_object,
  426             addr, size);
  427         vmem_free(vmem, addr, size);
  428 }
  429 
  430 /*
  431  *      kmap_alloc_wait:
  432  *
  433  *      Allocates pageable memory from a sub-map of the kernel.  If the submap
  434  *      has no room, the caller sleeps waiting for more memory in the submap.
  435  *
  436  *      This routine may block.
  437  */
  438 vm_offset_t
  439 kmap_alloc_wait(vm_map_t map, vm_size_t size)
  440 {
  441         vm_offset_t addr;
  442 
  443         size = round_page(size);
  444         if (!swap_reserve(size))
  445                 return (0);
  446 
  447         for (;;) {
  448                 /*
  449                  * To make this work for more than one map, use the map's lock
  450                  * to lock out sleepers/wakers.
  451                  */
  452                 vm_map_lock(map);
  453                 if (vm_map_findspace(map, vm_map_min(map), size, &addr) == 0)
  454                         break;
  455                 /* no space now; see if we can ever get space */
  456                 if (vm_map_max(map) - vm_map_min(map) < size) {
  457                         vm_map_unlock(map);
  458                         swap_release(size);
  459                         return (0);
  460                 }
  461                 map->needs_wakeup = TRUE;
  462                 vm_map_unlock_and_wait(map, 0);
  463         }
  464         vm_map_insert(map, NULL, 0, addr, addr + size, VM_PROT_RW, VM_PROT_RW,
  465             MAP_ACC_CHARGED);
  466         vm_map_unlock(map);
  467         return (addr);
  468 }
  469 
  470 /*
  471  *      kmap_free_wakeup:
  472  *
  473  *      Returns memory to a submap of the kernel, and wakes up any processes
  474  *      waiting for memory in that map.
  475  */
  476 void
  477 kmap_free_wakeup(vm_map_t map, vm_offset_t addr, vm_size_t size)
  478 {
  479 
  480         vm_map_lock(map);
  481         (void) vm_map_delete(map, trunc_page(addr), round_page(addr + size));
  482         if (map->needs_wakeup) {
  483                 map->needs_wakeup = FALSE;
  484                 vm_map_wakeup(map);
  485         }
  486         vm_map_unlock(map);
  487 }
  488 
  489 void
  490 kmem_init_zero_region(void)
  491 {
  492         vm_offset_t addr, i;
  493         vm_page_t m;
  494 
  495         /*
  496          * Map a single physical page of zeros to a larger virtual range.
  497          * This requires less looping in places that want large amounts of
  498          * zeros, while not using much more physical resources.
  499          */
  500         addr = kva_alloc(ZERO_REGION_SIZE);
  501         m = vm_page_alloc(NULL, 0, VM_ALLOC_NORMAL |
  502             VM_ALLOC_NOOBJ | VM_ALLOC_WIRED | VM_ALLOC_ZERO);
  503         if ((m->flags & PG_ZERO) == 0)
  504                 pmap_zero_page(m);
  505         for (i = 0; i < ZERO_REGION_SIZE; i += PAGE_SIZE)
  506                 pmap_qenter(addr + i, &m, 1);
  507         pmap_protect(kernel_pmap, addr, addr + ZERO_REGION_SIZE, VM_PROT_READ);
  508 
  509         zero_region = (const void *)addr;
  510 }
  511 
  512 /*
  513  *      kmem_init:
  514  *
  515  *      Create the kernel map; insert a mapping covering kernel text, 
  516  *      data, bss, and all space allocated thus far (`boostrap' data).  The 
  517  *      new map will thus map the range between VM_MIN_KERNEL_ADDRESS and 
  518  *      `start' as allocated, and the range between `start' and `end' as free.
  519  */
  520 void
  521 kmem_init(vm_offset_t start, vm_offset_t end)
  522 {
  523         vm_map_t m;
  524 
  525         m = vm_map_create(kernel_pmap, VM_MIN_KERNEL_ADDRESS, end);
  526         m->system_map = 1;
  527         vm_map_lock(m);
  528         /* N.B.: cannot use kgdb to debug, starting with this assignment ... */
  529         kernel_map = m;
  530         (void) vm_map_insert(m, NULL, (vm_ooffset_t) 0,
  531 #ifdef __amd64__
  532             KERNBASE,
  533 #else                
  534             VM_MIN_KERNEL_ADDRESS,
  535 #endif
  536             start, VM_PROT_ALL, VM_PROT_ALL, MAP_NOFAULT);
  537         /* ... and ending with the completion of the above `insert' */
  538         vm_map_unlock(m);
  539 }
  540 
  541 /*
  542  *      kmem_bootstrap_free:
  543  *
  544  *      Free pages backing preloaded data (e.g., kernel modules) to the
  545  *      system.  Currently only supported on platforms that create a
  546  *      vm_phys segment for preloaded data.
  547  */
  548 void
  549 kmem_bootstrap_free(vm_offset_t start, vm_size_t size)
  550 {
  551 #if defined(__i386__) || defined(__amd64__)
  552         struct vm_domain *vmd;
  553         vm_offset_t end, va;
  554         vm_paddr_t pa;
  555         vm_page_t m;
  556 
  557         end = trunc_page(start + size);
  558         start = round_page(start);
  559 
  560         for (va = start; va < end; va += PAGE_SIZE) {
  561                 pa = pmap_kextract(va);
  562                 m = PHYS_TO_VM_PAGE(pa);
  563 
  564                 vmd = vm_phys_domain(m);
  565                 mtx_lock(&vm_page_queue_free_mtx);
  566                 vm_phys_free_pages(m, 0);
  567                 vmd->vmd_page_count++;
  568                 vm_phys_freecnt_adj(m, 1);
  569                 mtx_unlock(&vm_page_queue_free_mtx);
  570 
  571                 vm_cnt.v_page_count++;
  572         }
  573         pmap_remove(kernel_pmap, start, end);
  574         (void)vmem_add(kernel_arena, start, end - start, M_WAITOK);
  575 #endif
  576 }
  577 
  578 #ifdef DIAGNOSTIC
  579 /*
  580  * Allow userspace to directly trigger the VM drain routine for testing
  581  * purposes.
  582  */
  583 static int
  584 debug_vm_lowmem(SYSCTL_HANDLER_ARGS)
  585 {
  586         int error, i;
  587 
  588         i = 0;
  589         error = sysctl_handle_int(oidp, &i, 0, req);
  590         if (error)
  591                 return (error);
  592         if ((i & ~(VM_LOW_KMEM | VM_LOW_PAGES)) != 0)
  593                 return (EINVAL);
  594         if (i != 0)
  595                 EVENTHANDLER_INVOKE(vm_lowmem, i);
  596         return (0);
  597 }
  598 
  599 SYSCTL_PROC(_debug, OID_AUTO, vm_lowmem, CTLTYPE_INT | CTLFLAG_RW, 0, 0,
  600     debug_vm_lowmem, "I", "set to trigger vm_lowmem event with given flags");
  601 #endif

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