man2/madvise.2

   1 .\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
   6 .\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpages@gmail.com>
   7 .\"     Added notes on MADV_DONTNEED
   8 .\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and
   9 .\"     MADV_UNMERGEABLE
  10 .\" 2010-06-15, Andi Kleen, Add documentation of MADV_HWPOISON.
  11 .\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE.
  12 .\" 2011-09-18, Doug Goldstein <cardoe@cardoe.com>
  13 .\"     Document MADV_HUGEPAGE and MADV_NOHUGEPAGE
  14 .\"
  15 .TH madvise 2 (date) "Linux man-pages (unreleased)"
  16 .SH NAME
  17 madvise \- give advice about use of memory
  18 .SH LIBRARY
  19 Standard C library
  20 .RI ( libc ", " \-lc )
  21 .SH SYNOPSIS
  22 .nf
  23 .B #include <sys/mman.h>
  24 .PP
  25 .BI "int madvise(void " addr [. length "], size_t " length ", int " advice );
  26 .fi
  27 .PP
  28 .RS -4
  29 Feature Test Macro Requirements for glibc (see
  30 .BR feature_test_macros (7)):
  31 .RE
  32 .PP
  33 .BR madvise ():
  34 .nf
  35     Since glibc 2.19:
  36         _DEFAULT_SOURCE
  37     Up to and including glibc 2.19:
  38         _BSD_SOURCE
  39 .fi
  40 .SH DESCRIPTION
  41 The
  42 .BR madvise ()
  43 system call is used to give advice or directions to the kernel
  44 about the address range beginning at address
  45 .I addr
  46 and with size
  47 .IR length .
  48 .BR madvise ()
  49 only operates on whole pages, therefore
  50 .I addr
  51 must be page-aligned.
  52 The value of
  53 .I length
  54 is rounded up to a multiple of page size.
  55 In most cases,
  56 the goal of such advice is to improve system or application performance.
  57 .PP
  58 Initially, the system call supported a set of "conventional"
  59 .I advice
  60 values, which are also available on several other implementations.
  61 (Note, though, that
  62 .BR madvise ()
  63 is not specified in POSIX.)
  64 Subsequently, a number of Linux-specific
  65 .I advice
  66 values have been added.
  67 .\"
  68 .\" ======================================================================
  69 .\"
  70 .SS Conventional advice values
  71 The
  72 .I advice
  73 values listed below
  74 allow an application to tell the kernel how it expects to use
  75 some mapped or shared memory areas, so that the kernel can choose
  76 appropriate read-ahead and caching techniques.
  77 These
  78 .I advice
  79 values do not influence the semantics of the application
  80 (except in the case of
  81 .BR MADV_DONTNEED ),
  82 but may influence its performance.
  83 All of the
  84 .I advice
  85 values listed here have analogs in the POSIX-specified
  86 .BR posix_madvise (3)
  87 function, and the values have the same meanings, with the exception of
  88 .BR MADV_DONTNEED .
  89 .PP
  90 The advice is indicated in the
  91 .I advice
  92 argument, which is one of the following:
  93 .TP
  94 .B MADV_NORMAL
  95 No special treatment.
  96 This is the default.
  97 .TP
  98 .B MADV_RANDOM
  99 Expect page references in random order.
 100 (Hence, read ahead may be less useful than normally.)
 101 .TP
 102 .B MADV_SEQUENTIAL
 103 Expect page references in sequential order.
 104 (Hence, pages in the given range can be aggressively read ahead,
 105 and may be freed soon after they are accessed.)
 106 .TP
 107 .B MADV_WILLNEED
 108 Expect access in the near future.
 109 (Hence, it might be a good idea to read some pages ahead.)
 110 .TP
 111 .B MADV_DONTNEED
 112 Do not expect access in the near future.
 113 (For the time being, the application is finished with the given range,
 114 so the kernel can free resources associated with it.)
 115 .IP
 116 After a successful
 117 .B MADV_DONTNEED
 118 operation,
 119 the semantics of memory access in the specified region are changed:
 120 subsequent accesses of pages in the range will succeed, but will result
 121 in either repopulating the memory contents from the
 122 up-to-date contents of the underlying mapped file
 123 (for shared file mappings, shared anonymous mappings,
 124 and shmem-based techniques such as System V shared memory segments)
 125 or zero-fill-on-demand pages for anonymous private mappings.
 126 .IP
 127 Note that, when applied to shared mappings,
 128 .B MADV_DONTNEED
 129 might not lead to immediate freeing of the pages in the range.
 130 The kernel is free to delay freeing the pages until an appropriate moment.
 131 The resident set size (RSS) of the calling process will be immediately
 132 reduced however.
 133 .IP
 134 .B MADV_DONTNEED
 135 cannot be applied to locked pages, or
 136 .B VM_PFNMAP
 137 pages.
 138 (Pages marked with the kernel-internal
 139 .B VM_PFNMAP
 140 .\" http://lwn.net/Articles/162860/
 141 flag are special memory areas that are not managed
 142 by the virtual memory subsystem.
 143 Such pages are typically created by device drivers that
 144 map the pages into user space.)
 145 .IP
 146 Support for Huge TLB pages was added in Linux v5.18.
 147 Addresses within a mapping backed by Huge TLB pages must be aligned
 148 to the underlying Huge TLB page size,
 149 and the range length is rounded up
 150 to a multiple of the underlying Huge TLB page size.
 151 .\"
 152 .\" ======================================================================
 153 .\"
 154 .SS Linux-specific advice values
 155 The following Linux-specific
 156 .I advice
 157 values have no counterparts in the POSIX-specified
 158 .BR posix_madvise (3),
 159 and may or may not have counterparts in the
 160 .BR madvise ()
 161 interface available on other implementations.
 162 Note that some of these operations change the semantics of memory accesses.
 163 .TP
 164 .BR MADV_REMOVE " (since Linux 2.6.16)"
 165 .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
 166 Free up a given range of pages
 167 and its associated backing store.
 168 This is equivalent to punching a hole in the corresponding
 169 range of the backing store (see
 170 .BR fallocate (2)).
 171 Subsequent accesses in the specified address range will see
 172 data with a value of zero.
 173 .\" Databases want to use this feature to drop a section of their
 174 .\" bufferpool (shared memory segments) - without writing back to
 175 .\" disk/swap space.  This feature is also useful for supporting
 176 .\" hot-plug memory on UML.
 177 .IP
 178 The specified address range must be mapped shared and writable.
 179 This flag cannot be applied to locked pages, or
 180 .B VM_PFNMAP
 181 pages.
 182 .IP
 183 In the initial implementation, only
 184 .BR tmpfs (5)
 185 supported
 186 .BR MADV_REMOVE ;
 187 but since Linux 3.5,
 188 .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
 189 any filesystem which supports the
 190 .BR fallocate (2)
 191 .B FALLOC_FL_PUNCH_HOLE
 192 mode also supports
 193 .BR MADV_REMOVE .
 194 Filesystems which do not support
 195 .B MADV_REMOVE
 196 fail with the error
 197 .BR EOPNOTSUPP .
 198 .IP
 199 Support for the Huge TLB filesystem was added in Linux v4.3.
 200 .TP
 201 .BR MADV_DONTFORK " (since Linux 2.6.16)"
 202 .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
 203 .\" See http://lwn.net/Articles/171941/
 204 Do not make the pages in this range available to the child after a
 205 .BR fork (2).
 206 This is useful to prevent copy-on-write semantics from changing
 207 the physical location of a page if the parent writes to it after a
 208 .BR fork (2).
 209 (Such page relocations cause problems for hardware that
 210 DMAs into the page.)
 211 .\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK
 212 .\" Currently, copy-on-write may change the physical address of
 213 .\" a page even if the user requested that the page is pinned in
 214 .\" memory (either by mlock or by get_user_pages).  This happens
 215 .\" if the process forks meanwhile, and the parent writes to that
 216 .\" page.  As a result, the page is orphaned: in case of
 217 .\" get_user_pages, the application will never see any data hardware
 218 .\" DMA's into this page after the COW.  In case of mlock'd memory,
 219 .\" the parent is not getting the realtime/security benefits of mlock.
 220 .\"
 221 .\" In particular, this affects the Infiniband modules which do DMA from
 222 .\" and into user pages all the time.
 223 .\"
 224 .\" This patch adds madvise options to control whether memory range is
 225 .\" inherited across fork. Useful e.g. for when hardware is doing DMA
 226 .\" from/into these pages.  Could also be useful to an application
 227 .\" wanting to speed up its forks by cutting large areas out of
 228 .\" consideration.
 229 .\"
 230 .\" SEE ALSO: http://lwn.net/Articles/171941/
 231 .\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006
 232 .TP
 233 .BR MADV_DOFORK " (since Linux 2.6.16)"
 234 Undo the effect of
 235 .BR MADV_DONTFORK ,
 236 restoring the default behavior, whereby a mapping is inherited across
 237 .BR fork (2).
 238 .TP
 239 .BR MADV_HWPOISON " (since Linux 2.6.32)"
 240 .\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573
 241 Poison the pages in the range specified by
 242 .I addr
 243 and
 244 .I length
 245 and handle subsequent references to those pages
 246 like a hardware memory corruption.
 247 This operation is available only for privileged
 248 .RB ( CAP_SYS_ADMIN )
 249 processes.
 250 This operation may result in the calling process receiving a
 251 .B SIGBUS
 252 and the page being unmapped.
 253 .IP
 254 This feature is intended for testing of memory error-handling code;
 255 it is available only if the kernel was configured with
 256 .BR CONFIG_MEMORY_FAILURE .
 257 .TP
 258 .BR MADV_MERGEABLE " (since Linux 2.6.32)"
 259 .\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5
 260 Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
 261 .I addr
 262 and
 263 .IR length .
 264 The kernel regularly scans those areas of user memory that have
 265 been marked as mergeable,
 266 looking for pages with identical content.
 267 These are replaced by a single write-protected page (which is automatically
 268 copied if a process later wants to update the content of the page).
 269 KSM merges only private anonymous pages (see
 270 .BR mmap (2)).
 271 .IP
 272 The KSM feature is intended for applications that generate many
 273 instances of the same data (e.g., virtualization systems such as KVM).
 274 It can consume a lot of processing power; use with care.
 275 See the Linux kernel source file
 276 .I Documentation/admin\-guide/mm/ksm.rst
 277 for more details.
 278 .IP
 279 The
 280 .B MADV_MERGEABLE
 281 and
 282 .B MADV_UNMERGEABLE
 283 operations are available only if the kernel was configured with
 284 .BR CONFIG_KSM .
 285 .TP
 286 .BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
 287 Undo the effect of an earlier
 288 .B MADV_MERGEABLE
 289 operation on the specified address range;
 290 KSM unmerges whatever pages it had merged in the address range specified by
 291 .I addr
 292 and
 293 .IR length .
 294 .TP
 295 .BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)"
 296 .\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6
 297 Soft offline the pages in the range specified by
 298 .I addr
 299 and
 300 .IR length .
 301 The memory of each page in the specified range is preserved
 302 (i.e., when next accessed, the same content will be visible,
 303 but in a new physical page frame),
 304 and the original page is offlined
 305 (i.e., no longer used, and taken out of normal memory management).
 306 The effect of the
 307 .B MADV_SOFT_OFFLINE
 308 operation is invisible to (i.e., does not change the semantics of)
 309 the calling process.
 310 .IP
 311 This feature is intended for testing of memory error-handling code;
 312 it is available only if the kernel was configured with
 313 .BR CONFIG_MEMORY_FAILURE .
 314 .TP
 315 .BR MADV_HUGEPAGE " (since Linux 2.6.38)"
 316 .\" commit 0af4e98b6b095c74588af04872f83d333c958c32
 317 .\" http://lwn.net/Articles/358904/
 318 .\" https://lwn.net/Articles/423584/
 319 Enable Transparent Huge Pages (THP) for pages in the range specified by
 320 .I addr
 321 and
 322 .IR length .
 323 The kernel will regularly scan the areas marked as huge page candidates
 324 to replace them with huge pages.
 325 The kernel will also allocate huge pages directly when the region is
 326 naturally aligned to the huge page size (see
 327 .BR posix_memalign (2)).
 328 .IP
 329 This feature is primarily aimed at applications that use large mappings of
 330 data and access large regions of that memory at a time (e.g., virtualization
 331 systems such as QEMU).
 332 It can very easily waste memory (e.g., a 2\ MB mapping that only ever accesses
 333 1 byte will result in 2\ MB of wired memory instead of one 4\ KB page).
 334 See the Linux kernel source file
 335 .I Documentation/admin\-guide/mm/transhuge.rst
 336 for more details.
 337 .IP
 338 Most common kernels configurations provide
 339 .BR MADV_HUGEPAGE -style
 340 behavior by default, and thus
 341 .B MADV_HUGEPAGE
 342 is normally not necessary.
 343 It is mostly intended for embedded systems, where
 344 .BR MADV_HUGEPAGE -style
 345 behavior may not be enabled by default in the kernel.
 346 On such systems,
 347 this flag can be used in order to selectively enable THP.
 348 Whenever
 349 .B MADV_HUGEPAGE
 350 is used, it should always be in regions of memory with
 351 an access pattern that the developer knows in advance won't risk
 352 to increase the memory footprint of the application when transparent
 353 hugepages are enabled.
 354 .IP
 355 .\" commit 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0
 356 Since Linux 5.4,
 357 automatic scan of eligible areas and replacement by huge pages works with
 358 private anonymous pages (see
 359 .BR mmap (2)),
 360 shmem pages,
 361 and file-backed pages.
 362 For all memory types,
 363 memory may only be replaced by huge pages on hugepage-aligned boundaries.
 364 For file-mapped memory
 365 \[em]including tmpfs (see
 366 .BR tmpfs (2))\[em]
 367 the mapping must also be naturally hugepage-aligned within the file.
 368 Additionally,
 369 for file-backed,
 370 non-tmpfs memory,
 371 the file must not be open for write and the mapping must be executable.
 372 .IP
 373 The VMA must not be marked
 374 .BR VM_NOHUGEPAGE ,
 375 .BR VM_HUGETLB ,
 376 .BR VM_IO ,
 377 .BR VM_DONTEXPAND ,
 378 .BR VM_MIXEDMAP ,
 379 or
 380 .BR VM_PFNMAP ,
 381 nor can it be stack memory or backed by a DAX-enabled device
 382 (unless the DAX device is hot-plugged as System RAM).
 383 The process must also not have
 384 .B PR_SET_THP_DISABLE
 385 set (see
 386 .BR prctl (2)).
 387 .IP
 388 The
 389 .BR MADV_HUGEPAGE ,
 390 .BR MADV_NOHUGEPAGE ,
 391 and
 392 .B MADV_COLLAPSE
 393 operations are available only if the kernel was configured with
 394 .B CONFIG_TRANSPARENT_HUGEPAGE
 395 and file/shmem memory is only supported if the kernel was configured with
 396 .BR CONFIG_READ_ONLY_THP_FOR_FS .
 397 .TP
 398 .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
 399 Ensures that memory in the address range specified by
 400 .I addr
 401 and
 402 .I length
 403 will not be backed by transparent hugepages.
 404 .TP
 405 .BR MADV_COLLAPSE " (since Linux 6.1)"
 406 .\" commit 7d8faaf155454f8798ec56404faca29a82689c77
 407 .\" commit 34488399fa08faaf664743fa54b271eb6f9e1321
 408 Perform a best-effort synchronous collapse of
 409 the native pages mapped by the memory range
 410 into Transparent Huge Pages (THPs).
 411 .B MADV_COLLAPSE
 412 operates on the current state of memory of the calling process and
 413 makes no persistent changes or guarantees on how pages will be mapped,
 414 constructed,
 415 or faulted in the future.
 416 .IP
 417 .B MADV_COLLAPSE
 418 supports private anonymous pages (see
 419 .BR mmap (2)),
 420 shmem pages,
 421 and file-backed pages.
 422 See
 423 .B MADV_HUGEPAGE
 424 for general information on memory requirements for THP.
 425 If the range provided spans multiple VMAs,
 426 the semantics of the collapse over each VMA is independent from the others.
 427 If collapse of a given huge page-aligned/sized region fails,
 428 the operation may continue to attempt collapsing
 429 the remainder of the specified memory.
 430 .B MADV_COLLAPSE
 431 will automatically clamp the provided range to be hugepage-aligned.
 432 .IP
 433 All non-resident pages covered by the range
 434 will first be swapped/faulted-in,
 435 before being copied onto a freshly allocated hugepage.
 436 If the native pages compose the same PTE-mapped hugepage,
 437 and are suitably aligned,
 438 allocation of a new hugepage may be elided and
 439 collapse may happen in-place.
 440 Unmapped pages will have their data directly initialized to 0
 441 in the new hugepage.
 442 However,
 443 for every eligible hugepage-aligned/sized region to be collapsed,
 444 at least one page must currently be backed by physical memory.
 445 .IP
 446 .B MADV_COLLAPSE
 447 is independent of any sysfs
 448 (see
 449 .BR sysfs (5))
 450 setting under
 451 .IR /sys/kernel/mm/transparent_hugepage ,
 452 both in terms of determining THP eligibility,
 453 and allocation semantics.
 454 See Linux kernel source file
 455 .I Documentation/admin\-guide/mm/transhuge.rst
 456 for more information.
 457 .B MADV_COLLAPSE
 458 also ignores
 459 .B huge=
 460 tmpfs mount when operating on tmpfs files.
 461 Allocation for the new hugepage may enter direct reclaim and/or compaction,
 462 regardless of VMA flags
 463 (though
 464 .B VM_NOHUGEPAGE
 465 is still respected).
 466 .IP
 467 When the system has multiple NUMA nodes,
 468 the hugepage will be allocated from
 469 the node providing the most native pages.
 470 .IP
 471 If all hugepage-sized/aligned regions covered by the provided range were
 472 either successfully collapsed,
 473 or were already PMD-mapped THPs,
 474 this operation will be deemed successful.
 475 Note that this doesn't guarantee anything about
 476 other possible mappings of the memory.
 477 In the event multiple hugepage-aligned/sized areas fail to collapse,
 478 only the most-recently\[en]failed code will be set in
 479 .IR errno .
 480 .TP
 481 .BR MADV_DONTDUMP " (since Linux 3.4)"
 482 .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
 483 .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
 484 Exclude from a core dump those pages in the range specified by
 485 .I addr
 486 and
 487 .IR length .
 488 This is useful in applications that have large areas of memory
 489 that are known not to be useful in a core dump.
 490 The effect of
 491 .B MADV_DONTDUMP
 492 takes precedence over the bit mask that is set via the
 493 .IR /proc/ pid /coredump_filter
 494 file (see
 495 .BR core (5)).
 496 .TP
 497 .BR MADV_DODUMP " (since Linux 3.4)"
 498 Undo the effect of an earlier
 499 .BR MADV_DONTDUMP .
 500 .TP
 501 .BR MADV_FREE " (since Linux 4.5)"
 502 The application no longer requires the pages in the range specified by
 503 .I addr
 504 and
 505 .IR len .
 506 The kernel can thus free these pages,
 507 but the freeing could be delayed until memory pressure occurs.
 508 For each of the pages that has been marked to be freed
 509 but has not yet been freed,
 510 the free operation will be canceled if the caller writes into the page.
 511 After a successful
 512 .B MADV_FREE
 513 operation, any stale data (i.e., dirty, unwritten pages) will be lost
 514 when the kernel frees the pages.
 515 However, subsequent writes to pages in the range will succeed
 516 and then kernel cannot free those dirtied pages,
 517 so that the caller can always see just written data.
 518 If there is no subsequent write,
 519 the kernel can free the pages at any time.
 520 Once pages in the range have been freed, the caller will
 521 see zero-fill-on-demand pages upon subsequent page references.
 522 .IP
 523 The
 524 .B MADV_FREE
 525 operation
 526 can be applied only to private anonymous pages (see
 527 .BR mmap (2)).
 528 Before Linux 4.12,
 529 .\" commit 93e06c7a645343d222c9a838834a51042eebbbf7
 530 when freeing pages on a swapless system,
 531 the pages in the given range are freed instantly,
 532 regardless of memory pressure.
 533 .TP
 534 .BR MADV_WIPEONFORK " (since Linux 4.14)"
 535 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 536 Present the child process with zero-filled memory in this range after a
 537 .BR fork (2).
 538 This is useful in forking servers in order to ensure
 539 that sensitive per-process data
 540 (for example, PRNG seeds, cryptographic secrets, and so on)
 541 is not handed to child processes.
 542 .IP
 543 The
 544 .B MADV_WIPEONFORK
 545 operation can be applied only to private anonymous pages (see
 546 .BR mmap (2)).
 547 .IP
 548 Within the child created by
 549 .BR fork (2),
 550 the
 551 .B MADV_WIPEONFORK
 552 setting remains in place on the specified address range.
 553 This setting is cleared during
 554 .BR execve (2).
 555 .TP
 556 .BR MADV_KEEPONFORK " (since Linux 4.14)"
 557 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 558 Undo the effect of an earlier
 559 .BR MADV_WIPEONFORK .
 560 .TP
 561 .BR MADV_COLD " (since Linux 5.4)"
 562 .\" commit 9c276cc65a58faf98be8e56962745ec99ab87636
 563 Deactivate a given range of pages.
 564 This will make the pages a more probable
 565 reclaim target should there be a memory pressure.
 566 This is a nondestructive operation.
 567 The advice might be ignored for some pages in the range when it is not
 568 applicable.
 569 .TP
 570 .BR MADV_PAGEOUT " (since Linux 5.4)"
 571 .\" commit 1a4e58cce84ee88129d5d49c064bd2852b481357
 572 Reclaim a given range of pages.
 573 This is done to free up memory occupied by these pages.
 574 If a page is anonymous, it will be swapped out.
 575 If a page is file-backed and dirty, it will be written back to the backing
 576 storage.
 577 The advice might be ignored for some pages in the range when it is not
 578 applicable.
 579 .TP
 580 .BR MADV_POPULATE_READ " (since Linux 5.14)"
 581 "Populate (prefault) page tables readable,
 582 faulting in all pages in the range just as if manually reading from each page;
 583 however,
 584 avoid the actual memory access that would have been performed after handling
 585 the fault.
 586 .IP
 587 In contrast to
 588 .BR MAP_POPULATE ,
 589 .B MADV_POPULATE_READ
 590 does not hide errors,
 591 can be applied to (parts of) existing mappings and will always populate
 592 (prefault) page tables readable.
 593 One example use case is prefaulting a file mapping,
 594 reading all file content from disk;
 595 however,
 596 pages won't be dirtied and consequently won't have to be written back to disk
 597 when evicting the pages from memory.
 598 .IP
 599 Depending on the underlying mapping,
 600 map the shared zeropage,
 601 preallocate memory or read the underlying file;
 602 files with holes might or might not preallocate blocks.
 603 If populating fails,
 604 a
 605 .B SIGBUS
 606 signal is not generated; instead, an error is returned.
 607 .IP
 608 If
 609 .B MADV_POPULATE_READ
 610 succeeds,
 611 all page tables have been populated (prefaulted) readable once.
 612 If
 613 .B MADV_POPULATE_READ
 614 fails,
 615 some page tables might have been populated.
 616 .IP
 617 .B MADV_POPULATE_READ
 618 cannot be applied to mappings without read permissions
 619 and special mappings,
 620 for example,
 621 mappings marked with kernel-internal flags such as
 622 .B VM_PFNMAP
 623 or
 624 .BR VM_IO ,
 625 or secret memory regions created using
 626 .BR memfd_secret(2) .
 627 .IP
 628 Note that with
 629 .BR MADV_POPULATE_READ ,
 630 the process can be killed at any moment when the system runs out of memory.
 631 .TP
 632 .BR MADV_POPULATE_WRITE " (since Linux 5.14)"
 633 Populate (prefault) page tables writable,
 634 faulting in all pages in the range just as if manually writing to each
 635 each page;
 636 however,
 637 avoid the actual memory access that would have been performed after handling
 638 the fault.
 639 .IP
 640 In contrast to
 641 .BR MAP_POPULATE ,
 642 MADV_POPULATE_WRITE does not hide errors,
 643 can be applied to (parts of) existing mappings and will always populate
 644 (prefault) page tables writable.
 645 One example use case is preallocating memory,
 646 breaking any CoW (Copy on Write).
 647 .IP
 648 Depending on the underlying mapping,
 649 preallocate memory or read the underlying file;
 650 files with holes will preallocate blocks.
 651 If populating fails,
 652 a
 653 .B SIGBUS
 654 signal is not generated; instead, an error is returned.
 655 .IP
 656 If
 657 .B MADV_POPULATE_WRITE
 658 succeeds,
 659 all page tables have been populated (prefaulted) writable once.
 660 If
 661 .B MADV_POPULATE_WRITE
 662 fails,
 663 some page tables might have been populated.
 664 .IP
 665 .B MADV_POPULATE_WRITE
 666 cannot be applied to mappings without write permissions
 667 and special mappings,
 668 for example,
 669 mappings marked with kernel-internal flags such as
 670 .B VM_PFNMAP
 671 or
 672 .BR VM_IO ,
 673 or secret memory regions created using
 674 .BR memfd_secret(2) .
 675 .IP
 676 Note that with
 677 .BR MADV_POPULATE_WRITE ,
 678 the process can be killed at any moment when the system runs out of memory.
 679 .SH RETURN VALUE
 680 On success,
 681 .BR madvise ()
 682 returns zero.
 683 On error, it returns \-1 and
 684 .I errno
 685 is set to indicate the error.
 686 .SH ERRORS
 687 .TP
 688 .B EACCES
 689 .I advice
 690 is
 691 .BR MADV_REMOVE ,
 692 but the specified address range is not a shared writable mapping.
 693 .TP
 694 .B EAGAIN
 695 A kernel resource was temporarily unavailable.
 696 .TP
 697 .B EBADF
 698 The map exists, but the area maps something that isn't a file.
 699 .TP
 700 .B EBUSY
 701 (for
 702 .BR MADV_COLLAPSE )
 703 Could not charge hugepage to cgroup: cgroup limit exceeded.
 704 .TP
 705 .B EFAULT
 706 .I advice
 707 is
 708 .B MADV_POPULATE_READ
 709 or
 710 .BR MADV_POPULATE_WRITE ,
 711 and populating (prefaulting) page tables failed because a
 712 .B SIGBUS
 713 would have been generated on actual memory access and the reason is not a
 714 HW poisoned page
 715 (HW poisoned pages can,
 716 for example,
 717 be created using the
 718 .B MADV_HWPOISON
 719 flag described elsewhere in this page).
 720 .TP
 721 .B EINVAL
 722 .I addr
 723 is not page-aligned or
 724 .I length
 725 is negative.
 726 .\" .I length
 727 .\" is zero,
 728 .TP
 729 .B EINVAL
 730 .I advice
 731 is not a valid.
 732 .TP
 733 .B EINVAL
 734 .I advice
 735 is
 736 .B MADV_COLD
 737 or
 738 .B MADV_PAGEOUT
 739 and the specified address range includes locked, Huge TLB pages, or
 740 .B VM_PFNMAP
 741 pages.
 742 .TP
 743 .B EINVAL
 744 .I advice
 745 is
 746 .B MADV_DONTNEED
 747 or
 748 .B MADV_REMOVE
 749 and the specified address range includes locked, Huge TLB pages, or
 750 .B VM_PFNMAP
 751 pages.
 752 .TP
 753 .B EINVAL
 754 .I advice
 755 is
 756 .B MADV_MERGEABLE
 757 or
 758 .BR MADV_UNMERGEABLE ,
 759 but the kernel was not configured with
 760 .BR CONFIG_KSM .
 761 .TP
 762 .B EINVAL
 763 .I advice
 764 is
 765 .B MADV_FREE
 766 or
 767 .B MADV_WIPEONFORK
 768 but the specified address range includes file, Huge TLB,
 769 .BR MAP_SHARED ,
 770 or
 771 .B VM_PFNMAP
 772 ranges.
 773 .TP
 774 .B EINVAL
 775 .I advice
 776 is
 777 .B MADV_POPULATE_READ
 778 or
 779 .BR MADV_POPULATE_WRITE ,
 780 but the specified address range includes ranges with insufficient permissions
 781 or special mappings,
 782 for example,
 783 mappings marked with kernel-internal flags such a
 784 .B VM_IO
 785 or
 786 .BR VM_PFNMAP ,
 787 or secret memory regions created using
 788 .BR memfd_secret(2) .
 789 .TP
 790 .B EIO
 791 (for
 792 .BR MADV_WILLNEED )
 793 Paging in this area would exceed the process's
 794 maximum resident set size.
 795 .TP
 796 .B ENOMEM
 797 (for
 798 .BR MADV_WILLNEED )
 799 Not enough memory: paging in failed.
 800 .TP
 801 .B ENOMEM
 802 (for
 803 .BR MADV_COLLAPSE )
 804 Not enough memory: could not allocate hugepage.
 805 .TP
 806 .B ENOMEM
 807 Addresses in the specified range are not currently
 808 mapped, or are outside the address space of the process.
 809 .TP
 810 .B ENOMEM
 811 .I advice
 812 is
 813 .B MADV_POPULATE_READ
 814 or
 815 .BR MADV_POPULATE_WRITE ,
 816 and populating (prefaulting) page tables failed because there was not enough
 817 memory.
 818 .TP
 819 .B EPERM
 820 .I advice
 821 is
 822 .BR MADV_HWPOISON ,
 823 but the caller does not have the
 824 .B CAP_SYS_ADMIN
 825 capability.
 826 .TP
 827 .B EHWPOISON
 828 .I advice
 829 is
 830 .B MADV_POPULATE_READ
 831 or
 832 .BR MADV_POPULATE_WRITE ,
 833 and populating (prefaulting) page tables failed because a HW poisoned page
 834 (HW poisoned pages can,
 835 for example,
 836 be created using the
 837 .B MADV_HWPOISON
 838 flag described elsewhere in this page)
 839 was encountered.
 840 .SH VERSIONS
 841 Versions of this system call, implementing a wide variety of
 842 .I advice
 843 values, exist on many other implementations.
 844 Other implementations typically implement at least the flags listed
 845 above under
 846 .IR "Conventional advice flags" ,
 847 albeit with some variation in semantics.
 848 .PP
 849 POSIX.1-2001 describes
 850 .BR posix_madvise (3)
 851 with constants
 852 .BR POSIX_MADV_NORMAL ,
 853 .BR POSIX_MADV_RANDOM ,
 854 .BR POSIX_MADV_SEQUENTIAL ,
 855 .BR POSIX_MADV_WILLNEED ,
 856 and
 857 .BR POSIX_MADV_DONTNEED ,
 858 and so on, with behavior close to the similarly named flags listed above.
 859 .SS Linux
 860 The Linux implementation requires that the address
 861 .I addr
 862 be page-aligned, and allows
 863 .I length
 864 to be zero.
 865 If there are some parts of the specified address range
 866 that are not mapped, the Linux version of
 867 .BR madvise ()
 868 ignores them and applies the call to the rest (but returns
 869 .B ENOMEM
 870 from the system call, as it should).
 871 .PP
 872 .I madvise(0,\ 0,\ advice)
 873 will return zero iff
 874 .I advice
 875 is supported by the kernel and can be relied on to probe for support.
 876 .SH STANDARDS
 877 None.
 878 .SH HISTORY
 879 First appeared in 4.4BSD.
 880 .PP
 881 Since Linux 3.18,
 882 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
 883 support for this system call is optional,
 884 depending on the setting of the
 885 .B CONFIG_ADVISE_SYSCALLS
 886 configuration option.
 887 .SH SEE ALSO
 888 .BR getrlimit (2),
 889 .BR memfd_secret (2),
 890 .BR mincore (2),
 891 .BR mmap (2),
 892 .BR mprotect (2),
 893 .BR msync (2),
 894 .BR munmap (2),
 895 .BR prctl (2),
 896 .BR process_madvise (2),
 897 .BR posix_madvise (3),
 898 .BR core (5)