man2/madvise.2

   1 .\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
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  24 .\"
  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
  29 .\"     MADV_UNMERGEABLE
  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
  34 .\"
  35 .TH MADVISE 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  36 .SH NAME
  37 madvise \- give advice about use of memory
  38 .SH SYNOPSIS
  39 .nf
  40 .B #include <sys/mman.h>
  41 .PP
  42 .BI "int madvise(void *" addr ", size_t " length ", int " advice );
  43 .fi
  44 .PP
  45 .RS -4
  46 Feature Test Macro Requirements for glibc (see
  47 .BR feature_test_macros (7)):
  48 .RE
  49 .PP
  50 .BR madvise ():
  51 .PD 0
  52 .RS 4
  53 .TP 4
  54 Since glibc 2.19:
  55 _DEFAULT_SOURCE
  56 .TP
  57 Up to and including glibc 2.19:
  58 _BSD_SOURCE
  59 .RE
  60 .PD
  61 .SH DESCRIPTION
  62 The
  63 .BR madvise ()
  64 system call is used to give advice or directions to the kernel
  65 about the address range beginning at address
  66 .I addr
  67 and with size
  68 .I length
  69 bytes
  70 In most cases,
  71 the goal of such advice is to improve system or application performance.
  72 .PP
  73 Initially, the system call supported a set of "conventional"
  74 .I advice
  75 values, which are also available on several other implementations.
  76 (Note, though, that
  77 .BR madvise ()
  78 is not specified in POSIX.)
  79 Subsequently, a number of Linux-specific
  80 .IR advice
  81 values have been added.
  82 .\"
  83 .\" ======================================================================
  84 .\"
  85 .SS Conventional advice values
  86 The
  87 .I advice
  88 values listed below
  89 allow an application to tell the kernel how it expects to use
  90 some mapped or shared memory areas, so that the kernel can choose
  91 appropriate read-ahead and caching techniques.
  92 These
  93 .I advice
  94 values do not influence the semantics of the application
  95 (except in the case of
  96 .BR MADV_DONTNEED ),
  97 but may influence its performance.
  98 All of the
  99 .I advice
 100 values listed here have analogs in the POSIX-specified
 101 .BR posix_madvise (3)
 102 function, and the values have the same meanings, with the exception of
 103 .BR MADV_DONTNEED .
 104 .PP
 105 The advice is indicated in the
 106 .I advice
 107 argument, which is one of the following:
 108 .TP
 109 .B MADV_NORMAL
 110 No special treatment.
 111 This is the default.
 112 .TP
 113 .B MADV_RANDOM
 114 Expect page references in random order.
 115 (Hence, read ahead may be less useful than normally.)
 116 .TP
 117 .B MADV_SEQUENTIAL
 118 Expect page references in sequential order.
 119 (Hence, pages in the given range can be aggressively read ahead,
 120 and may be freed soon after they are accessed.)
 121 .TP
 122 .B MADV_WILLNEED
 123 Expect access in the near future.
 124 (Hence, it might be a good idea to read some pages ahead.)
 125 .TP
 126 .B MADV_DONTNEED
 127 Do not expect access in the near future.
 128 (For the time being, the application is finished with the given range,
 129 so the kernel can free resources associated with it.)
 130 .IP
 131 After a successful
 132 .B MADV_DONTNEED
 133 operation,
 134 the semantics of memory access in the specified region are changed:
 135 subsequent accesses of pages in the range will succeed, but will result
 136 in either repopulating the memory contents from the
 137 up-to-date contents of the underlying mapped file
 138 (for shared file mappings, shared anonymous mappings,
 139 and shmem-based techniques such as System V shared memory segments)
 140 or zero-fill-on-demand pages for anonymous private mappings.
 141 .IP
 142 Note that, when applied to shared mappings,
 143 .BR MADV_DONTNEED
 144 might not lead to immediate freeing of the pages in the range.
 145 The kernel is free to delay freeing the pages until an appropriate moment.
 146 The resident set size (RSS) of the calling process will be immediately
 147 reduced however.
 148 .IP
 149 .B MADV_DONTNEED
 150 cannot be applied to locked pages, Huge TLB pages, or
 151 .BR VM_PFNMAP
 152 pages.
 153 (Pages marked with the kernel-internal
 154 .B VM_PFNMAP
 155 .\" http://lwn.net/Articles/162860/
 156 flag are special memory areas that are not managed
 157 by the virtual memory subsystem.
 158 Such pages are typically created by device drivers that
 159 map the pages into user space.)
 160 .\"
 161 .\" ======================================================================
 162 .\"
 163 .SS Linux-specific advice values
 164 The following Linux-specific
 165 .I advice
 166 values have no counterparts in the POSIX-specified
 167 .BR posix_madvise (3),
 168 and may or may not have counterparts in the
 169 .BR madvise ()
 170 interface available on other implementations.
 171 Note that some of these operations change the semantics of memory accesses.
 172 .TP
 173 .BR MADV_REMOVE " (since Linux 2.6.16)"
 174 .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
 175 Free up a given range of pages
 176 and its associated backing store.
 177 This is equivalent to punching a hole in the corresponding byte
 178 range of the backing store (see
 179 .BR fallocate (2)).
 180 Subsequent accesses in the specified address range will see
 181 bytes containing zero.
 182 .\" Databases want to use this feature to drop a section of their
 183 .\" bufferpool (shared memory segments) - without writing back to
 184 .\" disk/swap space.  This feature is also useful for supporting
 185 .\" hot-plug memory on UML.
 186 .IP
 187 The specified address range must be mapped shared and writable.
 188 This flag cannot be applied to locked pages, Huge TLB pages, or
 189 .BR VM_PFNMAP
 190 pages.
 191 .IP
 192 In the initial implementation, only
 193 .BR tmpfs (5)
 194 was supported
 195 .BR MADV_REMOVE ;
 196 but since Linux 3.5,
 197 .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
 198 any filesystem which supports the
 199 .BR fallocate (2)
 200 .BR FALLOC_FL_PUNCH_HOLE
 201 mode also supports
 202 .BR MADV_REMOVE .
 203 Hugetlbfs fails with the error
 204 .BR EINVAL
 205 and other filesystems fail with the error
 206 .BR EOPNOTSUPP .
 207 .TP
 208 .BR MADV_DONTFORK " (since Linux 2.6.16)"
 209 .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
 210 .\" See http://lwn.net/Articles/171941/
 211 Do not make the pages in this range available to the child after a
 212 .BR fork (2).
 213 This is useful to prevent copy-on-write semantics from changing
 214 the physical location of a page if the parent writes to it after a
 215 .BR fork (2).
 216 (Such page relocations cause problems for hardware that
 217 DMAs into the page.)
 218 .\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK
 219 .\" Currently, copy-on-write may change the physical address of
 220 .\" a page even if the user requested that the page is pinned in
 221 .\" memory (either by mlock or by get_user_pages).  This happens
 222 .\" if the process forks meanwhile, and the parent writes to that
 223 .\" page.  As a result, the page is orphaned: in case of
 224 .\" get_user_pages, the application will never see any data hardware
 225 .\" DMA's into this page after the COW.  In case of mlock'd memory,
 226 .\" the parent is not getting the realtime/security benefits of mlock.
 227 .\"
 228 .\" In particular, this affects the Infiniband modules which do DMA from
 229 .\" and into user pages all the time.
 230 .\"
 231 .\" This patch adds madvise options to control whether memory range is
 232 .\" inherited across fork. Useful e.g. for when hardware is doing DMA
 233 .\" from/into these pages.  Could also be useful to an application
 234 .\" wanting to speed up its forks by cutting large areas out of
 235 .\" consideration.
 236 .\"
 237 .\" SEE ALSO: http://lwn.net/Articles/171941/
 238 .\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006
 239 .TP
 240 .BR MADV_DOFORK " (since Linux 2.6.16)"
 241 Undo the effect of
 242 .BR MADV_DONTFORK ,
 243 restoring the default behavior, whereby a mapping is inherited across
 244 .BR fork (2).
 245 .TP
 246 .BR MADV_HWPOISON " (since Linux 2.6.32)"
 247 .\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573
 248 Poison the pages in the range specified by
 249 .I addr
 250 and
 251 .IR length
 252 and handle subsequent references to those pages
 253 like a hardware memory corruption.
 254 This operation is available only for privileged
 255 .RB ( CAP_SYS_ADMIN )
 256 processes.
 257 This operation may result in the calling process receiving a
 258 .B SIGBUS
 259 and the page being unmapped.
 260 .IP
 261 This feature is intended for testing of memory error-handling code;
 262 it is available only if the kernel was configured with
 263 .BR CONFIG_MEMORY_FAILURE .
 264 .TP
 265 .BR MADV_MERGEABLE " (since Linux 2.6.32)"
 266 .\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5
 267 Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
 268 .I addr
 269 and
 270 .IR length .
 271 The kernel regularly scans those areas of user memory that have
 272 been marked as mergeable,
 273 looking for pages with identical content.
 274 These are replaced by a single write-protected page (which is automatically
 275 copied if a process later wants to update the content of the page).
 276 KSM merges only private anonymous pages (see
 277 .BR mmap (2)).
 278 .IP
 279 The KSM feature is intended for applications that generate many
 280 instances of the same data (e.g., virtualization systems such as KVM).
 281 It can consume a lot of processing power; use with care.
 282 See the Linux kernel source file
 283 .I Documentation/admin-guide/mm/ksm.rst
 284 for more details.
 285 .IP
 286 The
 287 .BR MADV_MERGEABLE
 288 and
 289 .BR MADV_UNMERGEABLE
 290 operations are available only if the kernel was configured with
 291 .BR CONFIG_KSM .
 292 .TP
 293 .BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
 294 Undo the effect of an earlier
 295 .BR MADV_MERGEABLE
 296 operation on the specified address range;
 297 KSM unmerges whatever pages it had merged in the address range specified by
 298 .IR addr
 299 and
 300 .IR length .
 301 .TP
 302 .BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)"
 303 .\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6
 304 Soft offline the pages in the range specified by
 305 .I addr
 306 and
 307 .IR length .
 308 The memory of each page in the specified range is preserved
 309 (i.e., when next accessed, the same content will be visible,
 310 but in a new physical page frame),
 311 and the original page is offlined
 312 (i.e., no longer used, and taken out of normal memory management).
 313 The effect of the
 314 .B MADV_SOFT_OFFLINE
 315 operation is invisible to (i.e., does not change the semantics of)
 316 the calling process.
 317 .IP
 318 This feature is intended for testing of memory error-handling code;
 319 it is available only if the kernel was configured with
 320 .BR CONFIG_MEMORY_FAILURE .
 321 .TP
 322 .BR MADV_HUGEPAGE " (since Linux 2.6.38)"
 323 .\" commit 0af4e98b6b095c74588af04872f83d333c958c32
 324 .\" http://lwn.net/Articles/358904/
 325 .\" https://lwn.net/Articles/423584/
 326 Enable Transparent Huge Pages (THP) for pages in the range specified by
 327 .I addr
 328 and
 329 .IR length .
 330 Currently, Transparent Huge Pages work only with private anonymous pages (see
 331 .BR mmap (2)).
 332 The kernel will regularly scan the areas marked as huge page candidates
 333 to replace them with huge pages.
 334 The kernel will also allocate huge pages directly when the region is
 335 naturally aligned to the huge page size (see
 336 .BR posix_memalign (2)).
 337 .IP
 338 This feature is primarily aimed at applications that use large mappings of
 339 data and access large regions of that memory at a time (e.g., virtualization
 340 systems such as QEMU).
 341 It can very easily waste memory (e.g., a 2\ MB mapping that only ever accesses
 342 1 byte will result in 2\ MB of wired memory instead of one 4\ KB page).
 343 See the Linux kernel source file
 344 .I Documentation/admin-guide/mm/transhuge.rst
 345 for more details.
 346 .IP
 347 Most common kernels configurations provide
 348 .BR MADV_HUGEPAGE -style
 349 behavior by default, and thus
 350 .BR MADV_HUGEPAGE
 351 is normally not necessary.
 352 It is mostly intended for embedded systems, where
 353 .BR MADV_HUGEPAGE -style
 354 behavior may not be enabled by default in the kernel.
 355 On such systems,
 356 this flag can be used in order to selectively enable THP.
 357 Whenever
 358 .BR MADV_HUGEPAGE
 359 is used, it should always be in regions of memory with
 360 an access pattern that the developer knows in advance won't risk
 361 to increase the memory footprint of the application when transparent
 362 hugepages are enabled.
 363 .IP
 364 The
 365 .BR MADV_HUGEPAGE
 366 and
 367 .BR MADV_NOHUGEPAGE
 368 operations are available only if the kernel was configured with
 369 .BR CONFIG_TRANSPARENT_HUGEPAGE .
 370 .TP
 371 .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
 372 Ensures that memory in the address range specified by
 373 .IR addr
 374 and
 375 .IR length
 376 will not be backed by transparent hugepages.
 377 .TP
 378 .BR MADV_DONTDUMP " (since Linux 3.4)"
 379 .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
 380 .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
 381 Exclude from a core dump those pages in the range specified by
 382 .I addr
 383 and
 384 .IR length .
 385 This is useful in applications that have large areas of memory
 386 that are known not to be useful in a core dump.
 387 The effect of
 388 .BR MADV_DONTDUMP
 389 takes precedence over the bit mask that is set via the
 390 .I /proc/[pid]/coredump_filter
 391 file (see
 392 .BR core (5)).
 393 .TP
 394 .BR MADV_DODUMP " (since Linux 3.4)"
 395 Undo the effect of an earlier
 396 .BR MADV_DONTDUMP .
 397 .TP
 398 .BR MADV_FREE " (since Linux 4.5)"
 399 The application no longer requires the pages in the range specified by
 400 .IR addr
 401 and
 402 .IR len .
 403 The kernel can thus free these pages,
 404 but the freeing could be delayed until memory pressure occurs.
 405 For each of the pages that has been marked to be freed
 406 but has not yet been freed,
 407 the free operation will be canceled if the caller writes into the page.
 408 After a successful
 409 .B MADV_FREE
 410 operation, any stale data (i.e., dirty, unwritten pages) will be lost
 411 when the kernel frees the pages.
 412 However, subsequent writes to pages in the range will succeed
 413 and then kernel cannot free those dirtied pages,
 414 so that the caller can always see just written data.
 415 If there is no subsequent write,
 416 the kernel can free the pages at any time.
 417 Once pages in the range have been freed, the caller will
 418 see zero-fill-on-demand pages upon subsequent page references.
 419 .IP
 420 The
 421 .B MADV_FREE
 422 operation
 423 can be applied only to private anonymous pages (see
 424 .BR mmap (2)).
 425 In Linux before version 4.12,
 426 .\" commit 93e06c7a645343d222c9a838834a51042eebbbf7
 427 when freeing pages on a swapless system,
 428 the pages in the given range are freed instantly,
 429 regardless of memory pressure.
 430 .TP
 431 .BR MADV_WIPEONFORK " (since Linux 4.14)"
 432 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 433 Present the child process with zero-filled memory in this range after a
 434 .BR fork (2).
 435 This is useful in forking servers in order to ensure
 436 that sensitive per-process data
 437 (for example, PRNG seeds, cryptographic secrets, and so on)
 438 is not handed to child processes.
 439 .IP
 440 The
 441 .B MADV_WIPEONFORK
 442 operation can be applied only to private anonymous pages (see
 443 .BR mmap (2)).
 444 .IP
 445 Within the child created by
 446 .BR fork (2),
 447 the
 448 .B MADV_WIPEONFORK
 449 setting remains in place on the specified address range.
 450 This setting is cleared during
 451 .BR execve (2).
 452 .TP
 453 .BR MADV_KEEPONFORK " (since Linux 4.14)"
 454 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 455 Undo the effect of an earlier
 456 .BR MADV_WIPEONFORK .
 457 .SH RETURN VALUE
 458 On success,
 459 .BR madvise ()
 460 returns zero.
 461 On error, it returns \-1 and
 462 .I errno
 463 is set appropriately.
 464 .SH ERRORS
 465 .TP
 466 .B EACCES
 467 .I advice
 468 is
 469 .BR MADV_REMOVE ,
 470 but the specified address range is not a shared writable mapping.
 471 .TP
 472 .B EAGAIN
 473 A kernel resource was temporarily unavailable.
 474 .TP
 475 .B EBADF
 476 The map exists, but the area maps something that isn't a file.
 477 .TP
 478 .B EINVAL
 479 .I addr
 480 is not page-aligned or
 481 .I length
 482 is negative.
 483 .\" .I length
 484 .\" is zero,
 485 .TP
 486 .B EINVAL
 487 .I advice
 488 is not a valid.
 489 .TP
 490 .B EINVAL
 491 .I advice
 492 is
 493 .B MADV_DONTNEED
 494 or
 495 .BR MADV_REMOVE
 496 and the specified address range includes locked, Huge TLB pages, or
 497 .B VM_PFNMAP
 498 pages.
 499 .TP
 500 .B EINVAL
 501 .I advice
 502 is
 503 .BR MADV_MERGEABLE
 504 or
 505 .BR MADV_UNMERGEABLE ,
 506 but the kernel was not configured with
 507 .BR CONFIG_KSM .
 508 .TP
 509 .B EINVAL
 510 .I advice
 511 is
 512 .BR MADV_FREE
 513 or
 514 .BR MADV_WIPEONFORK
 515 but the specified address range includes file, Huge TLB,
 516 .BR MAP_SHARED ,
 517 or
 518 .BR VM_PFNMAP
 519 ranges.
 520 .TP
 521 .B EIO
 522 (for
 523 .BR MADV_WILLNEED )
 524 Paging in this area would exceed the process's
 525 maximum resident set size.
 526 .TP
 527 .B ENOMEM
 528 (for
 529 .BR MADV_WILLNEED )
 530 Not enough memory: paging in failed.
 531 .TP
 532 .B ENOMEM
 533 Addresses in the specified range are not currently
 534 mapped, or are outside the address space of the process.
 535 .TP
 536 .B EPERM
 537 .I advice
 538 is
 539 .BR MADV_HWPOISON ,
 540 but the caller does not have the
 541 .B CAP_SYS_ADMIN
 542 capability.
 543 .SH VERSIONS
 544 Since Linux 3.18,
 545 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
 546 support for this system call is optional,
 547 depending on the setting of the
 548 .B CONFIG_ADVISE_SYSCALLS
 549 configuration option.
 550 .SH CONFORMING TO
 551 .BR madvise ()
 552 is not specified by any standards.
 553 Versions of this system call, implementing a wide variety of
 554 .I advice
 555 values, exist on many other implementations.
 556 Other implementations typically implement at least the flags listed
 557 above under
 558 .IR "Conventional advice flags" ,
 559 albeit with some variation in semantics.
 560 .PP
 561 POSIX.1-2001 describes
 562 .BR posix_madvise (3)
 563 with constants
 564 .BR POSIX_MADV_NORMAL ,
 565 .BR POSIX_MADV_RANDOM ,
 566 .BR POSIX_MADV_SEQUENTIAL ,
 567 .BR POSIX_MADV_WILLNEED ,
 568 and
 569 .BR POSIX_MADV_DONTNEED ,
 570 and so on, with behavior close to the similarly named flags listed above.
 571 .SH NOTES
 572 .SS Linux notes
 573 The Linux implementation requires that the address
 574 .I addr
 575 be page-aligned, and allows
 576 .I length
 577 to be zero.
 578 If there are some parts of the specified address range
 579 that are not mapped, the Linux version of
 580 .BR madvise ()
 581 ignores them and applies the call to the rest (but returns
 582 .B ENOMEM
 583 from the system call, as it should).
 584 .\" .SH HISTORY
 585 .\" The
 586 .\" .BR madvise ()
 587 .\" function first appeared in 4.4BSD.
 588 .SH SEE ALSO
 589 .BR getrlimit (2),
 590 .BR mincore (2),
 591 .BR mmap (2),
 592 .BR mprotect (2),
 593 .BR msync (2),
 594 .BR munmap (2),
 595 .BR prctl (2),
 596 .BR posix_madvise (3),
 597 .BR core (5)