1 .\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
26 .\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpages@gmail.com>
27 .\" Added notes on MADV_DONTNEED
28 .\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and
30 .\" 2010-06-15, Andi Kleen, Add documentation of MADV_HWPOISON.
31 .\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE.
32 .\" 2011-09-18, Doug Goldstein <cardoe@cardoe.com>
33 .\" Document MADV_HUGEPAGE and MADV_NOHUGEPAGE
35 .TH MADVISE 2 2017-09-15 "Linux" "Linux Programmer's Manual"
37 madvise \- give advice about use of memory
39 .B #include <sys/mman.h>
41 .BI "int madvise(void *" addr ", size_t " length ", int " advice );
44 Feature Test Macro Requirements for glibc (see
45 .BR feature_test_macros (7)):
55 Up to and including glibc 2.19:
62 system call is used to give advice or directions to the kernel
63 about the address range beginning at address
69 the goal of such advice is to improve system or application performance.
71 Initially, the system call supported a set of "conventional"
73 values, which are also available on several other implementations.
76 is not specified in POSIX.)
77 Subsequently, a number of Linux-specific
79 values have been added.
81 .\" ======================================================================
83 .SS Conventional advice values
87 allow an application to tell the kernel how it expects to use
88 some mapped or shared memory areas, so that the kernel can choose
89 appropriate read-ahead and caching techniques.
92 values do not influence the semantics of the application
93 (except in the case of
95 but may influence its performance.
98 values listed here have analogs in the POSIX-specified
100 function, and the values have the same meanings, with the exception of
103 The advice is indicated in the
105 argument, which is one of the following:
108 No special treatment.
112 Expect page references in random order.
113 (Hence, read ahead may be less useful than normally.)
116 Expect page references in sequential order.
117 (Hence, pages in the given range can be aggressively read ahead,
118 and may be freed soon after they are accessed.)
121 Expect access in the near future.
122 (Hence, it might be a good idea to read some pages ahead.)
125 Do not expect access in the near future.
126 (For the time being, the application is finished with the given range,
127 so the kernel can free resources associated with it.)
132 the semantics of memory access in the specified region are changed:
133 subsequent accesses of pages in the range will succeed, but will result
134 in either repopulating the memory contents from the
135 up-to-date contents of the underlying mapped file
136 (for shared file mappings, shared anonymous mappings,
137 and shmem-based techniques such as System V shared memory segments)
138 or zero-fill-on-demand pages for anonymous private mappings.
140 Note that, when applied to shared mappings,
142 might not lead to immediate freeing of the pages in the range.
143 The kernel is free to delay freeing the pages until an appropriate moment.
144 The resident set size (RSS) of the calling process will be immediately
148 cannot be applied to locked pages, Huge TLB pages, or
151 (Pages marked with the kernel-internal
153 .\" http://lwn.net/Articles/162860/
154 flag are special memory areas that are not managed
155 by the virtual memory subsystem.
156 Such pages are typically created by device drivers that
157 map the pages into user space.)
159 .\" ======================================================================
161 .SS Linux-specific advice values
162 The following Linux-specific
164 values have no counterparts in the POSIX-specified
165 .BR posix_madvise (3),
166 and may or may not have counterparts in the
168 interface available on other implementations.
169 Note that some of these operations change the semantics of memory accesses.
171 .BR MADV_REMOVE " (since Linux 2.6.16)"
172 .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
173 Free up a given range of pages
174 and its associated backing store.
175 This is equivalent to punching a hole in the corresponding byte
176 range of the backing store (see
178 Subsequent accesses in the specified address range will see
179 bytes containing zero.
180 .\" Databases want to use this feature to drop a section of their
181 .\" bufferpool (shared memory segments) - without writing back to
182 .\" disk/swap space. This feature is also useful for supporting
183 .\" hot-plug memory on UML.
185 The specified address range must be mapped shared and writable.
186 This flag cannot be applied to locked pages, Huge TLB pages, or
190 In the initial implementation, only
195 .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
196 any filesystem which supports the
198 .BR FALLOC_FL_PUNCH_HOLE
201 Hugetlbfs fails with the error
203 and other filesystems fail with the error
206 .BR MADV_DONTFORK " (since Linux 2.6.16)"
207 .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
208 .\" See http://lwn.net/Articles/171941/
209 Do not make the pages in this range available to the child after a
211 This is useful to prevent copy-on-write semantics from changing
212 the physical location of a page if the parent writes to it after a
214 (Such page relocations cause problems for hardware that
216 .\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK
217 .\" Currently, copy-on-write may change the physical address of
218 .\" a page even if the user requested that the page is pinned in
219 .\" memory (either by mlock or by get_user_pages). This happens
220 .\" if the process forks meanwhile, and the parent writes to that
221 .\" page. As a result, the page is orphaned: in case of
222 .\" get_user_pages, the application will never see any data hardware
223 .\" DMA's into this page after the COW. In case of mlock'd memory,
224 .\" the parent is not getting the realtime/security benefits of mlock.
226 .\" In particular, this affects the Infiniband modules which do DMA from
227 .\" and into user pages all the time.
229 .\" This patch adds madvise options to control whether memory range is
230 .\" inherited across fork. Useful e.g. for when hardware is doing DMA
231 .\" from/into these pages. Could also be useful to an application
232 .\" wanting to speed up its forks by cutting large areas out of
235 .\" SEE ALSO: http://lwn.net/Articles/171941/
236 .\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006
238 .BR MADV_DOFORK " (since Linux 2.6.16)"
241 restoring the default behavior, whereby a mapping is inherited across
244 .BR MADV_HWPOISON " (since Linux 2.6.32)
245 .\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573
246 Poison the pages in the range specified by
250 and handle subsequent references to those pages
251 like a hardware memory corruption.
252 This operation is available only for privileged
253 .RB ( CAP_SYS_ADMIN )
255 This operation may result in the calling process receiving a
257 and the page being unmapped.
259 This feature is intended for testing of memory error-handling code;
260 it is available only if the kernel was configured with
261 .BR CONFIG_MEMORY_FAILURE .
263 .BR MADV_MERGEABLE " (since Linux 2.6.32)"
264 .\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5
265 Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
269 The kernel regularly scans those areas of user memory that have
270 been marked as mergeable,
271 looking for pages with identical content.
272 These are replaced by a single write-protected page (which is automatically
273 copied if a process later wants to update the content of the page).
274 KSM merges only private anonymous pages (see
277 The KSM feature is intended for applications that generate many
278 instances of the same data (e.g., virtualization systems such as KVM).
279 It can consume a lot of processing power; use with care.
280 See the Linux kernel source file
281 .I Documentation/admin-guide/mm/ksm.rst
288 operations are available only if the kernel was configured with
291 .BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
292 Undo the effect of an earlier
294 operation on the specified address range;
295 KSM unmerges whatever pages it had merged in the address range specified by
300 .BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)
301 .\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6
302 Soft offline the pages in the range specified by
306 The memory of each page in the specified range is preserved
307 (i.e., when next accessed, the same content will be visible,
308 but in a new physical page frame),
309 and the original page is offlined
310 (i.e., no longer used, and taken out of normal memory management).
313 operation is invisible to (i.e., does not change the semantics of)
316 This feature is intended for testing of memory error-handling code;
317 it is available only if the kernel was configured with
318 .BR CONFIG_MEMORY_FAILURE .
320 .BR MADV_HUGEPAGE " (since Linux 2.6.38)"
321 .\" commit 0af4e98b6b095c74588af04872f83d333c958c32
322 .\" http://lwn.net/Articles/358904/
323 .\" https://lwn.net/Articles/423584/
324 Enable Transparent Huge Pages (THP) for pages in the range specified by
328 Currently, Transparent Huge Pages work only with private anonymous pages (see
330 The kernel will regularly scan the areas marked as huge page candidates
331 to replace them with huge pages.
332 The kernel will also allocate huge pages directly when the region is
333 naturally aligned to the huge page size (see
334 .BR posix_memalign (2)).
336 This feature is primarily aimed at applications that use large mappings of
337 data and access large regions of that memory at a time (e.g., virtualization
338 systems such as QEMU).
339 It can very easily waste memory (e.g., a 2\ MB mapping that only ever accesses
340 1 byte will result in 2\ MB of wired memory instead of one 4\ KB page).
341 See the Linux kernel source file
342 .I Documentation/admin-guide/mm/transhuge.rst
349 operations are available only if the kernel was configured with
350 .BR CONFIG_TRANSPARENT_HUGEPAGE .
352 .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
353 Ensures that memory in the address range specified by
357 will not be collapsed into huge pages.
359 .BR MADV_DONTDUMP " (since Linux 3.4)"
360 .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
361 .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
362 Exclude from a core dump those pages in the range specified by
366 This is useful in applications that have large areas of memory
367 that are known not to be useful in a core dump.
370 takes precedence over the bit mask that is set via the
371 .I /proc/[pid]/coredump_filter
375 .BR MADV_DODUMP " (since Linux 3.4)"
376 Undo the effect of an earlier
379 .BR MADV_FREE " (since Linux 4.5)"
380 The application no longer requires the pages in the range specified by
384 The kernel can thus free these pages,
385 but the freeing could be delayed until memory pressure occurs.
386 For each of the pages that has been marked to be freed
387 but has not yet been freed,
388 the free operation will be canceled if the caller writes into the page.
391 operation, any stale data (i.e., dirty, unwritten pages) will be lost
392 when the kernel frees the pages.
393 However, subsequent writes to pages in the range will succeed
394 and then kernel cannot free those dirtied pages,
395 so that the caller can always see just written data.
396 If there is no subsequent write,
397 the kernel can free the pages at any time.
398 Once pages in the range have been freed, the caller will
399 see zero-fill-on-demand pages upon subsequent page references.
404 can be applied only to private anonymous pages (see
406 On a swapless system, freeing pages in a given range happens instantly,
407 regardless of memory pressure.
409 .BR MADV_WIPEONFORK " (since Linux 4.14)"
410 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
411 Present the child process with zero-filled memory in this range after a
413 This is useful in forking servers in order to ensure
414 that sensitive per-process data
415 (for example, PRNG seeds, cryptographic secrets, and so on)
416 is not handed to child processes.
420 operation can be applied only to private anonymous pages (see
423 Within the child created by
427 setting remains in place on the specified address range.
428 This setting is cleared during
431 .BR MADV_KEEPONFORK " (since Linux 4.14)"
432 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
433 Undo the effect of an earlier
434 .BR MADV_WIPEONFORK .
439 On error, it returns \-1 and
441 is set appropriately.
448 but the specified address range is not a shared writable mapping.
451 A kernel resource was temporarily unavailable.
454 The map exists, but the area maps something that isn't a file.
458 is not page-aligned or
474 and the specified address range includes locked, Huge TLB pages, or
483 .BR MADV_UNMERGEABLE ,
484 but the kernel was not configured with
493 but the specified address range includes file, Huge TLB,
502 Paging in this area would exceed the process's
503 maximum resident set size.
508 Not enough memory: paging in failed.
511 Addresses in the specified range are not currently
512 mapped, or are outside the address space of the process.
518 but the caller does not have the
523 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
524 support for this system call is optional,
525 depending on the setting of the
526 .B CONFIG_ADVISE_SYSCALLS
527 configuration option.
530 is not specified by any standards.
531 Versions of this system call, implementing a wide variety of
533 values, exist on many other implementations.
534 Other implementations typically implement at least the flags listed
536 .IR "Conventional advice flags" ,
537 albeit with some variation in semantics.
539 POSIX.1-2001 describes
540 .BR posix_madvise (3)
542 .BR POSIX_MADV_NORMAL ,
543 .BR POSIX_MADV_RANDOM ,
544 .BR POSIX_MADV_SEQUENTIAL ,
545 .BR POSIX_MADV_WILLNEED ,
547 .BR POSIX_MADV_DONTNEED ,
548 and so on, with behavior close to the similarly named flags listed above.
551 The Linux implementation requires that the address
553 be page-aligned, and allows
556 If there are some parts of the specified address range
557 that are not mapped, the Linux version of
559 ignores them and applies the call to the rest (but returns
561 from the system call, as it should).
565 .\" function first appeared in 4.4BSD.
574 .BR posix_madvise (3),