man2/madvise.2 - pub/scm/docs/man-pages/man-pages - Git at Google

 .\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
 .\"
 .\" %%%LICENSE_START(VERBATIM)
 .\" Permission is granted to make and distribute verbatim copies of this
 .\" manual provided the copyright notice and this permission notice are
 .\" preserved on all copies.
 .\"
 .\" Permission is granted to copy and distribute modified versions of this
 .\" manual under the conditions for verbatim copying, provided that the
 .\" entire resulting derived work is distributed under the terms of a
 .\" permission notice identical to this one.
 .\"
 .\" Since the Linux kernel and libraries are constantly changing, this
 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
 .\" responsibility for errors or omissions, or for damages resulting from
 .\" the use of the information contained herein.  The author(s) may not
 .\" have taken the same level of care in the production of this manual,
 .\" which is licensed free of charge, as they might when working
 .\" professionally.
 .\"
 .\" Formatted or processed versions of this manual, if unaccompanied by
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" %%%LICENSE_END
 .\"
 .\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
 .\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpages@gmail.com>
 .\"	Added notes on MADV_DONTNEED
 .\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and
 .\"     MADV_UNMERGEABLE
 .\" 2010-06-15, Andi Kleen, Add documentation of MADV_HWPOISON.
 .\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE.
 .\" 2011-09-18, Doug Goldstein <cardoe@cardoe.com>
 .\"     Document MADV_HUGEPAGE and MADV_NOHUGEPAGE
 .\"
 .TH MADVISE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
 .SH NAME
 madvise \- give advice about use of memory
 .SH SYNOPSIS
 .nf
 .B #include <sys/mman.h>
 .PP
 .BI "int madvise(void *" addr ", size_t " length ", int " advice );
 .fi
 .PP
 .RS -4
 Feature Test Macro Requirements for glibc (see
 .BR feature_test_macros (7)):
 .RE
 .PP
 .BR madvise ():
 .nf
     Since glibc 2.19:
         _DEFAULT_SOURCE
     Up to and including glibc 2.19:
         _BSD_SOURCE
 .fi
 .SH DESCRIPTION
 The
 .BR madvise ()
 system call is used to give advice or directions to the kernel
 about the address range beginning at address
 .I addr
 and with size
 .I length
 bytes
 In most cases,
 the goal of such advice is to improve system or application performance.
 .PP
 Initially, the system call supported a set of "conventional"
 .I advice
 values, which are also available on several other implementations.
 (Note, though, that
 .BR madvise ()
 is not specified in POSIX.)
 Subsequently, a number of Linux-specific
 .IR advice
 values have been added.
 .\"
 .\" ======================================================================
 .\"
 .SS Conventional advice values
 The
 .I advice
 values listed below
 allow an application to tell the kernel how it expects to use
 some mapped or shared memory areas, so that the kernel can choose
 appropriate read-ahead and caching techniques.
 These
 .I advice
 values do not influence the semantics of the application
 (except in the case of
 .BR MADV_DONTNEED ),
 but may influence its performance.
 All of the
 .I advice
 values listed here have analogs in the POSIX-specified
 .BR posix_madvise (3)
 function, and the values have the same meanings, with the exception of
 .BR MADV_DONTNEED .
 .PP
 The advice is indicated in the
 .I advice
 argument, which is one of the following:
 .TP
 .B MADV_NORMAL
 No special treatment.
 This is the default.
 .TP
 .B MADV_RANDOM
 Expect page references in random order.
 (Hence, read ahead may be less useful than normally.)
 .TP
 .B MADV_SEQUENTIAL
 Expect page references in sequential order.
 (Hence, pages in the given range can be aggressively read ahead,
 and may be freed soon after they are accessed.)
 .TP
 .B MADV_WILLNEED
 Expect access in the near future.
 (Hence, it might be a good idea to read some pages ahead.)
 .TP
 .B MADV_DONTNEED
 Do not expect access in the near future.
 (For the time being, the application is finished with the given range,
 so the kernel can free resources associated with it.)
 .IP
 After a successful
 .B MADV_DONTNEED
 operation,
 the semantics of memory access in the specified region are changed:
 subsequent accesses of pages in the range will succeed, but will result
 in either repopulating the memory contents from the
 up-to-date contents of the underlying mapped file
 (for shared file mappings, shared anonymous mappings,
 and shmem-based techniques such as System V shared memory segments)
 or zero-fill-on-demand pages for anonymous private mappings.
 .IP
 Note that, when applied to shared mappings,
 .BR MADV_DONTNEED
 might not lead to immediate freeing of the pages in the range.
 The kernel is free to delay freeing the pages until an appropriate moment.
 The resident set size (RSS) of the calling process will be immediately
 reduced however.
 .IP
 .B MADV_DONTNEED
 cannot be applied to locked pages, Huge TLB pages, or
 .BR VM_PFNMAP
 pages.
 (Pages marked with the kernel-internal
 .B VM_PFNMAP
 .\" http://lwn.net/Articles/162860/
 flag are special memory areas that are not managed
 by the virtual memory subsystem.
 Such pages are typically created by device drivers that
 map the pages into user space.)
 .\"
 .\" ======================================================================
 .\"
 .SS Linux-specific advice values
 The following Linux-specific
 .I advice
 values have no counterparts in the POSIX-specified
 .BR posix_madvise (3),
 and may or may not have counterparts in the
 .BR madvise ()
 interface available on other implementations.
 Note that some of these operations change the semantics of memory accesses.
 .TP
 .BR MADV_REMOVE " (since Linux 2.6.16)"
 .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
 Free up a given range of pages
 and its associated backing store.
 This is equivalent to punching a hole in the corresponding byte
 range of the backing store (see
 .BR fallocate (2)).
 Subsequent accesses in the specified address range will see
 bytes containing zero.
 .\" Databases want to use this feature to drop a section of their
 .\" bufferpool (shared memory segments) - without writing back to
 .\" disk/swap space.  This feature is also useful for supporting
 .\" hot-plug memory on UML.
 .IP
 The specified address range must be mapped shared and writable.
 This flag cannot be applied to locked pages, Huge TLB pages, or
 .BR VM_PFNMAP
 pages.
 .IP
 In the initial implementation, only
 .BR tmpfs (5)
 was supported
 .BR MADV_REMOVE ;
 but since Linux 3.5,
 .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
 any filesystem which supports the
 .BR fallocate (2)
 .BR FALLOC_FL_PUNCH_HOLE
 mode also supports
 .BR MADV_REMOVE .
 Hugetlbfs fails with the error
 .BR EINVAL
 and other filesystems fail with the error
 .BR EOPNOTSUPP .
 .TP
 .BR MADV_DONTFORK " (since Linux 2.6.16)"
 .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
 .\" See http://lwn.net/Articles/171941/
 Do not make the pages in this range available to the child after a
 .BR fork (2).
 This is useful to prevent copy-on-write semantics from changing
 the physical location of a page if the parent writes to it after a
 .BR fork (2).
 (Such page relocations cause problems for hardware that
 DMAs into the page.)
 .\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK
 .\" Currently, copy-on-write may change the physical address of
 .\" a page even if the user requested that the page is pinned in
 .\" memory (either by mlock or by get_user_pages).  This happens
 .\" if the process forks meanwhile, and the parent writes to that
 .\" page.  As a result, the page is orphaned: in case of
 .\" get_user_pages, the application will never see any data hardware
 .\" DMA's into this page after the COW.  In case of mlock'd memory,
 .\" the parent is not getting the realtime/security benefits of mlock.
 .\"
 .\" In particular, this affects the Infiniband modules which do DMA from
 .\" and into user pages all the time.
 .\"
 .\" This patch adds madvise options to control whether memory range is
 .\" inherited across fork. Useful e.g. for when hardware is doing DMA
 .\" from/into these pages.  Could also be useful to an application
 .\" wanting to speed up its forks by cutting large areas out of
 .\" consideration.
 .\"
 .\" SEE ALSO: http://lwn.net/Articles/171941/
 .\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006
 .TP
 .BR MADV_DOFORK " (since Linux 2.6.16)"
 Undo the effect of
 .BR MADV_DONTFORK ,
 restoring the default behavior, whereby a mapping is inherited across
 .BR fork (2).
 .TP
 .BR MADV_HWPOISON " (since Linux 2.6.32)"
 .\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573
 Poison the pages in the range specified by
 .I addr
 and
 .IR length
 and handle subsequent references to those pages
 like a hardware memory corruption.
 This operation is available only for privileged
 .RB ( CAP_SYS_ADMIN )
 processes.
 This operation may result in the calling process receiving a
 .B SIGBUS
 and the page being unmapped.
 .IP
 This feature is intended for testing of memory error-handling code;
 it is available only if the kernel was configured with
 .BR CONFIG_MEMORY_FAILURE .
 .TP
 .BR MADV_MERGEABLE " (since Linux 2.6.32)"
 .\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5
 Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
 .I addr
 and
 .IR length .
 The kernel regularly scans those areas of user memory that have
 been marked as mergeable,
 looking for pages with identical content.
 These are replaced by a single write-protected page (which is automatically
 copied if a process later wants to update the content of the page).
 KSM merges only private anonymous pages (see
 .BR mmap (2)).
 .IP
 The KSM feature is intended for applications that generate many
 instances of the same data (e.g., virtualization systems such as KVM).
 It can consume a lot of processing power; use with care.
 See the Linux kernel source file
 .I Documentation/admin\-guide/mm/ksm.rst
 for more details.
 .IP
 The
 .BR MADV_MERGEABLE
 and
 .BR MADV_UNMERGEABLE
 operations are available only if the kernel was configured with
 .BR CONFIG_KSM .
 .TP
 .BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
 Undo the effect of an earlier
 .BR MADV_MERGEABLE
 operation on the specified address range;
 KSM unmerges whatever pages it had merged in the address range specified by
 .IR addr
 and
 .IR length .
 .TP
 .BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)"
 .\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6
 Soft offline the pages in the range specified by
 .I addr
 and
 .IR length .
 The memory of each page in the specified range is preserved
 (i.e., when next accessed, the same content will be visible,
 but in a new physical page frame),
 and the original page is offlined
 (i.e., no longer used, and taken out of normal memory management).
 The effect of the
 .B MADV_SOFT_OFFLINE
 operation is invisible to (i.e., does not change the semantics of)
 the calling process.
 .IP
 This feature is intended for testing of memory error-handling code;
 it is available only if the kernel was configured with
 .BR CONFIG_MEMORY_FAILURE .
 .TP
 .BR MADV_HUGEPAGE " (since Linux 2.6.38)"
 .\" commit 0af4e98b6b095c74588af04872f83d333c958c32
 .\" http://lwn.net/Articles/358904/
 .\" https://lwn.net/Articles/423584/
 Enable Transparent Huge Pages (THP) for pages in the range specified by
 .I addr
 and
 .IR length .
 Currently, Transparent Huge Pages work only with private anonymous pages (see
 .BR mmap (2)).
 The kernel will regularly scan the areas marked as huge page candidates
 to replace them with huge pages.
 The kernel will also allocate huge pages directly when the region is
 naturally aligned to the huge page size (see
 .BR posix_memalign (2)).
 .IP
 This feature is primarily aimed at applications that use large mappings of
 data and access large regions of that memory at a time (e.g., virtualization
 systems such as QEMU).
 It can very easily waste memory (e.g., a 2\ MB mapping that only ever accesses
 1 byte will result in 2\ MB of wired memory instead of one 4\ KB page).
 See the Linux kernel source file
 .I Documentation/admin\-guide/mm/transhuge.rst
 for more details.
 .IP
 Most common kernels configurations provide
 .BR MADV_HUGEPAGE -style
 behavior by default, and thus
 .BR MADV_HUGEPAGE
 is normally not necessary.
 It is mostly intended for embedded systems, where
 .BR MADV_HUGEPAGE -style
 behavior may not be enabled by default in the kernel.
 On such systems,
 this flag can be used in order to selectively enable THP.
 Whenever
 .BR MADV_HUGEPAGE
 is used, it should always be in regions of memory with
 an access pattern that the developer knows in advance won't risk
 to increase the memory footprint of the application when transparent
 hugepages are enabled.
 .IP
 The
 .BR MADV_HUGEPAGE
 and
 .BR MADV_NOHUGEPAGE
 operations are available only if the kernel was configured with
 .BR CONFIG_TRANSPARENT_HUGEPAGE .
 .TP
 .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
 Ensures that memory in the address range specified by
 .IR addr
 and
 .IR length
 will not be backed by transparent hugepages.
 .TP
 .BR MADV_DONTDUMP " (since Linux 3.4)"
 .\" commit 909af768e88867016f427264ae39d27a57b6a8ed
 .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
 Exclude from a core dump those pages in the range specified by
 .I addr
 and
 .IR length .
 This is useful in applications that have large areas of memory
 that are known not to be useful in a core dump.
 The effect of
 .BR MADV_DONTDUMP
 takes precedence over the bit mask that is set via the
 .I /proc/[pid]/coredump_filter
 file (see
 .BR core (5)).
 .TP
 .BR MADV_DODUMP " (since Linux 3.4)"
 Undo the effect of an earlier
 .BR MADV_DONTDUMP .
 .TP
 .BR MADV_FREE " (since Linux 4.5)"
 The application no longer requires the pages in the range specified by
 .IR addr
 and
 .IR len .
 The kernel can thus free these pages,
 but the freeing could be delayed until memory pressure occurs.
 For each of the pages that has been marked to be freed
 but has not yet been freed,
 the free operation will be canceled if the caller writes into the page.
 After a successful
 .B MADV_FREE
 operation, any stale data (i.e., dirty, unwritten pages) will be lost
 when the kernel frees the pages.
 However, subsequent writes to pages in the range will succeed
 and then kernel cannot free those dirtied pages,
 so that the caller can always see just written data.
 If there is no subsequent write,
 the kernel can free the pages at any time.
 Once pages in the range have been freed, the caller will
 see zero-fill-on-demand pages upon subsequent page references.
 .IP
 The
 .B MADV_FREE
 operation
 can be applied only to private anonymous pages (see
 .BR mmap (2)).
 In Linux before version 4.12,
 .\" commit 93e06c7a645343d222c9a838834a51042eebbbf7
 when freeing pages on a swapless system,
 the pages in the given range are freed instantly,
 regardless of memory pressure.
 .TP
 .BR MADV_WIPEONFORK " (since Linux 4.14)"
 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 Present the child process with zero-filled memory in this range after a
 .BR fork (2).
 This is useful in forking servers in order to ensure
 that sensitive per-process data
 (for example, PRNG seeds, cryptographic secrets, and so on)
 is not handed to child processes.
 .IP
 The
 .B MADV_WIPEONFORK
 operation can be applied only to private anonymous pages (see
 .BR mmap (2)).
 .IP
 Within the child created by
 .BR fork (2),
 the
 .B MADV_WIPEONFORK
 setting remains in place on the specified address range.
 This setting is cleared during
 .BR execve (2).
 .TP
 .BR MADV_KEEPONFORK " (since Linux 4.14)"
 .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
 Undo the effect of an earlier
 .BR MADV_WIPEONFORK .
 .TP
 .BR MADV_COLD " (since Linux 5.4)"
 .\" commit 9c276cc65a58faf98be8e56962745ec99ab87636
 Deactivate a given range of pages.
 This will make the pages a more probable
 reclaim target should there be a memory pressure.
 This is a nondestructive operation.
 The advice might be ignored for some pages in the range when it is not
 applicable.
 .TP
 .BR MADV_PAGEOUT " (since Linux 5.4)"
 .\" commit 1a4e58cce84ee88129d5d49c064bd2852b481357
 Reclaim a given range of pages.
 This is done to free up memory occupied by these pages.
 If a page is anonymous, it will be swapped out.
 If a page is file-backed and dirty, it will be written back to the backing
 storage.
 The advice might be ignored for some pages in the range when it is not
 applicable.
 .SH RETURN VALUE
 On success,
 .BR madvise ()
 returns zero.
 On error, it returns \-1 and
 .I errno
 is set to indicate the error.
 .SH ERRORS
 .TP
 .B EACCES
 .I advice
 is
 .BR MADV_REMOVE ,
 but the specified address range is not a shared writable mapping.
 .TP
 .B EAGAIN
 A kernel resource was temporarily unavailable.
 .TP
 .B EBADF
 The map exists, but the area maps something that isn't a file.
 .TP
 .B EINVAL
 .I addr
 is not page-aligned or
 .I length
 is negative.
 .\" .I length
 .\" is zero,
 .TP
 .B EINVAL
 .I advice
 is not a valid.
 .TP
 .B EINVAL
 .I advice
 is
 .B MADV_DONTNEED
 or
 .BR MADV_REMOVE
 and the specified address range includes locked, Huge TLB pages, or
 .B VM_PFNMAP
 pages.
 .TP
 .B EINVAL
 .I advice
 is
 .BR MADV_MERGEABLE
 or
 .BR MADV_UNMERGEABLE ,
 but the kernel was not configured with
 .BR CONFIG_KSM .
 .TP
 .B EINVAL
 .I advice
 is
 .BR MADV_FREE
 or
 .BR MADV_WIPEONFORK
 but the specified address range includes file, Huge TLB,
 .BR MAP_SHARED ,
 or
 .BR VM_PFNMAP
 ranges.
 .TP
 .B EIO
 (for
 .BR MADV_WILLNEED )
 Paging in this area would exceed the process's
 maximum resident set size.
 .TP
 .B ENOMEM
 (for
 .BR MADV_WILLNEED )
 Not enough memory: paging in failed.
 .TP
 .B ENOMEM
 Addresses in the specified range are not currently
 mapped, or are outside the address space of the process.
 .TP
 .B EPERM
 .I advice
 is
 .BR MADV_HWPOISON ,
 but the caller does not have the
 .B CAP_SYS_ADMIN
 capability.
 .SH VERSIONS
 Since Linux 3.18,
 .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
 support for this system call is optional,
 depending on the setting of the
 .B CONFIG_ADVISE_SYSCALLS
 configuration option.
 .SH CONFORMING TO
 .BR madvise ()
 is not specified by any standards.
 Versions of this system call, implementing a wide variety of
 .I advice
 values, exist on many other implementations.
 Other implementations typically implement at least the flags listed
 above under
 .IR "Conventional advice flags" ,
 albeit with some variation in semantics.
 .PP
 POSIX.1-2001 describes
 .BR posix_madvise (3)
 with constants
 .BR POSIX_MADV_NORMAL ,
 .BR POSIX_MADV_RANDOM ,
 .BR POSIX_MADV_SEQUENTIAL ,
 .BR POSIX_MADV_WILLNEED ,
 and
 .BR POSIX_MADV_DONTNEED ,
 and so on, with behavior close to the similarly named flags listed above.
 .SH NOTES
 .SS Linux notes
 The Linux implementation requires that the address
 .I addr
 be page-aligned, and allows
 .I length
 to be zero.
 If there are some parts of the specified address range
 that are not mapped, the Linux version of
 .BR madvise ()
 ignores them and applies the call to the rest (but returns
 .B ENOMEM
 from the system call, as it should).
 .\" .SH HISTORY
 .\" The
 .\" .BR madvise ()
 .\" function first appeared in 4.4BSD.
 .SH SEE ALSO
 .BR getrlimit (2),
 .BR mincore (2),
 .BR mmap (2),
 .BR mprotect (2),
 .BR msync (2),
 .BR munmap (2),
 .BR prctl (2),
 .BR process_madvise (2),
 .BR posix_madvise (3),
 .BR core (5)
	.\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
	.\"
	.\" %%%LICENSE_START(VERBATIM)
	.\" Permission is granted to make and distribute verbatim copies of this
	.\" manual provided the copyright notice and this permission notice are
	.\" preserved on all copies.
	.\"
	.\" Permission is granted to copy and distribute modified versions of this
	.\" manual under the conditions for verbatim copying, provided that the
	.\" entire resulting derived work is distributed under the terms of a
	.\" permission notice identical to this one.
	.\"
	.\" Since the Linux kernel and libraries are constantly changing, this
	.\" manual page may be incorrect or out-of-date. The author(s) assume no
	.\" responsibility for errors or omissions, or for damages resulting from
	.\" the use of the information contained herein. The author(s) may not
	.\" have taken the same level of care in the production of this manual,
	.\" which is licensed free of charge, as they might when working
	.\" professionally.
	.\"
	.\" Formatted or processed versions of this manual, if unaccompanied by
	.\" the source, must acknowledge the copyright and authors of this work.
	.\" %%%LICENSE_END
	.\"
	.\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
	.\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpages@gmail.com>
	.\" Added notes on MADV_DONTNEED
	.\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and
	.\" MADV_UNMERGEABLE
	.\" 2010-06-15, Andi Kleen, Add documentation of MADV_HWPOISON.
	.\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE.
	.\" 2011-09-18, Doug Goldstein <cardoe@cardoe.com>
	.\" Document MADV_HUGEPAGE and MADV_NOHUGEPAGE
	.\"
	.TH MADVISE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
	.SH NAME
	madvise \- give advice about use of memory
	.SH SYNOPSIS
	.nf
	.B #include <sys/mman.h>
	.PP
	.BI "int madvise(void *" addr ", size_t " length ", int " advice );
	.fi
	.PP
	.RS -4
	Feature Test Macro Requirements for glibc (see
	.BR feature_test_macros (7)):
	.RE
	.PP
	.BR madvise ():
	.nf
	Since glibc 2.19:
	_DEFAULT_SOURCE
	Up to and including glibc 2.19:
	_BSD_SOURCE
	.fi
	.SH DESCRIPTION
	The
	.BR madvise ()
	system call is used to give advice or directions to the kernel
	about the address range beginning at address
	.I addr
	and with size
	.I length
	bytes
	In most cases,
	the goal of such advice is to improve system or application performance.
	.PP
	Initially, the system call supported a set of "conventional"
	.I advice
	values, which are also available on several other implementations.
	(Note, though, that
	.BR madvise ()
	is not specified in POSIX.)
	Subsequently, a number of Linux-specific
	.IR advice
	values have been added.
	.\"
	.\" ======================================================================
	.\"
	.SS Conventional advice values
	The
	.I advice
	values listed below
	allow an application to tell the kernel how it expects to use
	some mapped or shared memory areas, so that the kernel can choose
	appropriate read-ahead and caching techniques.
	These
	.I advice
	values do not influence the semantics of the application
	(except in the case of
	.BR MADV_DONTNEED ),
	but may influence its performance.
	All of the
	.I advice
	values listed here have analogs in the POSIX-specified
	.BR posix_madvise (3)
	function, and the values have the same meanings, with the exception of
	.BR MADV_DONTNEED .
	.PP
	The advice is indicated in the
	.I advice
	argument, which is one of the following:
	.TP
	.B MADV_NORMAL
	No special treatment.
	This is the default.
	.TP
	.B MADV_RANDOM
	Expect page references in random order.
	(Hence, read ahead may be less useful than normally.)
	.TP
	.B MADV_SEQUENTIAL
	Expect page references in sequential order.
	(Hence, pages in the given range can be aggressively read ahead,
	and may be freed soon after they are accessed.)
	.TP
	.B MADV_WILLNEED
	Expect access in the near future.
	(Hence, it might be a good idea to read some pages ahead.)
	.TP
	.B MADV_DONTNEED
	Do not expect access in the near future.
	(For the time being, the application is finished with the given range,
	so the kernel can free resources associated with it.)
	.IP
	After a successful
	.B MADV_DONTNEED
	operation,
	the semantics of memory access in the specified region are changed:
	subsequent accesses of pages in the range will succeed, but will result
	in either repopulating the memory contents from the
	up-to-date contents of the underlying mapped file
	(for shared file mappings, shared anonymous mappings,
	and shmem-based techniques such as System V shared memory segments)
	or zero-fill-on-demand pages for anonymous private mappings.
	.IP
	Note that, when applied to shared mappings,
	.BR MADV_DONTNEED
	might not lead to immediate freeing of the pages in the range.
	The kernel is free to delay freeing the pages until an appropriate moment.
	The resident set size (RSS) of the calling process will be immediately
	reduced however.
	.IP
	.B MADV_DONTNEED
	cannot be applied to locked pages, Huge TLB pages, or
	.BR VM_PFNMAP
	pages.
	(Pages marked with the kernel-internal
	.B VM_PFNMAP
	.\" http://lwn.net/Articles/162860/
	flag are special memory areas that are not managed
	by the virtual memory subsystem.
	Such pages are typically created by device drivers that
	map the pages into user space.)
	.\"
	.\" ======================================================================
	.\"
	.SS Linux-specific advice values
	The following Linux-specific
	.I advice
	values have no counterparts in the POSIX-specified
	.BR posix_madvise (3),
	and may or may not have counterparts in the
	.BR madvise ()
	interface available on other implementations.
	Note that some of these operations change the semantics of memory accesses.
	.TP
	.BR MADV_REMOVE " (since Linux 2.6.16)"
	.\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
	Free up a given range of pages
	and its associated backing store.
	This is equivalent to punching a hole in the corresponding byte
	range of the backing store (see
	.BR fallocate (2)).
	Subsequent accesses in the specified address range will see
	bytes containing zero.
	.\" Databases want to use this feature to drop a section of their
	.\" bufferpool (shared memory segments) - without writing back to
	.\" disk/swap space. This feature is also useful for supporting
	.\" hot-plug memory on UML.
	.IP
	The specified address range must be mapped shared and writable.
	This flag cannot be applied to locked pages, Huge TLB pages, or
	.BR VM_PFNMAP
	pages.
	.IP
	In the initial implementation, only
	.BR tmpfs (5)
	was supported
	.BR MADV_REMOVE ;
	but since Linux 3.5,
	.\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
	any filesystem which supports the
	.BR fallocate (2)
	.BR FALLOC_FL_PUNCH_HOLE
	mode also supports
	.BR MADV_REMOVE .
	Hugetlbfs fails with the error
	.BR EINVAL
	and other filesystems fail with the error
	.BR EOPNOTSUPP .
	.TP
	.BR MADV_DONTFORK " (since Linux 2.6.16)"
	.\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
	.\" See http://lwn.net/Articles/171941/
	Do not make the pages in this range available to the child after a
	.BR fork (2).
	This is useful to prevent copy-on-write semantics from changing
	the physical location of a page if the parent writes to it after a
	.BR fork (2).
	(Such page relocations cause problems for hardware that
	DMAs into the page.)
	.\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK
	.\" Currently, copy-on-write may change the physical address of
	.\" a page even if the user requested that the page is pinned in
	.\" memory (either by mlock or by get_user_pages). This happens
	.\" if the process forks meanwhile, and the parent writes to that
	.\" page. As a result, the page is orphaned: in case of
	.\" get_user_pages, the application will never see any data hardware
	.\" DMA's into this page after the COW. In case of mlock'd memory,
	.\" the parent is not getting the realtime/security benefits of mlock.
	.\"
	.\" In particular, this affects the Infiniband modules which do DMA from
	.\" and into user pages all the time.
	.\"
	.\" This patch adds madvise options to control whether memory range is
	.\" inherited across fork. Useful e.g. for when hardware is doing DMA
	.\" from/into these pages. Could also be useful to an application
	.\" wanting to speed up its forks by cutting large areas out of
	.\" consideration.
	.\"
	.\" SEE ALSO: http://lwn.net/Articles/171941/
	.\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006
	.TP
	.BR MADV_DOFORK " (since Linux 2.6.16)"
	Undo the effect of
	.BR MADV_DONTFORK ,
	restoring the default behavior, whereby a mapping is inherited across
	.BR fork (2).
	.TP
	.BR MADV_HWPOISON " (since Linux 2.6.32)"
	.\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573
	Poison the pages in the range specified by
	.I addr
	and
	.IR length
	and handle subsequent references to those pages
	like a hardware memory corruption.
	This operation is available only for privileged
	.RB ( CAP_SYS_ADMIN )
	processes.
	This operation may result in the calling process receiving a
	.B SIGBUS
	and the page being unmapped.
	.IP
	This feature is intended for testing of memory error-handling code;
	it is available only if the kernel was configured with
	.BR CONFIG_MEMORY_FAILURE .
	.TP
	.BR MADV_MERGEABLE " (since Linux 2.6.32)"
	.\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5
	Enable Kernel Samepage Merging (KSM) for the pages in the range specified by
	.I addr
	and
	.IR length .
	The kernel regularly scans those areas of user memory that have
	been marked as mergeable,
	looking for pages with identical content.
	These are replaced by a single write-protected page (which is automatically
	copied if a process later wants to update the content of the page).
	KSM merges only private anonymous pages (see
	.BR mmap (2)).
	.IP
	The KSM feature is intended for applications that generate many
	instances of the same data (e.g., virtualization systems such as KVM).
	It can consume a lot of processing power; use with care.
	See the Linux kernel source file
	.I Documentation/admin\-guide/mm/ksm.rst
	for more details.
	.IP
	The
	.BR MADV_MERGEABLE
	and
	.BR MADV_UNMERGEABLE
	operations are available only if the kernel was configured with
	.BR CONFIG_KSM .
	.TP
	.BR MADV_UNMERGEABLE " (since Linux 2.6.32)"
	Undo the effect of an earlier
	.BR MADV_MERGEABLE
	operation on the specified address range;
	KSM unmerges whatever pages it had merged in the address range specified by
	.IR addr
	and
	.IR length .
	.TP
	.BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)"
	.\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6
	Soft offline the pages in the range specified by
	.I addr
	and
	.IR length .
	The memory of each page in the specified range is preserved
	(i.e., when next accessed, the same content will be visible,
	but in a new physical page frame),
	and the original page is offlined
	(i.e., no longer used, and taken out of normal memory management).
	The effect of the
	.B MADV_SOFT_OFFLINE
	operation is invisible to (i.e., does not change the semantics of)
	the calling process.
	.IP
	This feature is intended for testing of memory error-handling code;
	it is available only if the kernel was configured with
	.BR CONFIG_MEMORY_FAILURE .
	.TP
	.BR MADV_HUGEPAGE " (since Linux 2.6.38)"
	.\" commit 0af4e98b6b095c74588af04872f83d333c958c32
	.\" http://lwn.net/Articles/358904/
	.\" https://lwn.net/Articles/423584/
	Enable Transparent Huge Pages (THP) for pages in the range specified by
	.I addr
	and
	.IR length .
	Currently, Transparent Huge Pages work only with private anonymous pages (see
	.BR mmap (2)).
	The kernel will regularly scan the areas marked as huge page candidates
	to replace them with huge pages.
	The kernel will also allocate huge pages directly when the region is
	naturally aligned to the huge page size (see
	.BR posix_memalign (2)).
	.IP
	This feature is primarily aimed at applications that use large mappings of
	data and access large regions of that memory at a time (e.g., virtualization
	systems such as QEMU).
	It can very easily waste memory (e.g., a 2\ MB mapping that only ever accesses
	1 byte will result in 2\ MB of wired memory instead of one 4\ KB page).
	See the Linux kernel source file
	.I Documentation/admin\-guide/mm/transhuge.rst
	for more details.
	.IP
	Most common kernels configurations provide
	.BR MADV_HUGEPAGE -style
	behavior by default, and thus
	.BR MADV_HUGEPAGE
	is normally not necessary.
	It is mostly intended for embedded systems, where
	.BR MADV_HUGEPAGE -style
	behavior may not be enabled by default in the kernel.
	On such systems,
	this flag can be used in order to selectively enable THP.
	Whenever
	.BR MADV_HUGEPAGE
	is used, it should always be in regions of memory with
	an access pattern that the developer knows in advance won't risk
	to increase the memory footprint of the application when transparent
	hugepages are enabled.
	.IP
	The
	.BR MADV_HUGEPAGE
	and
	.BR MADV_NOHUGEPAGE
	operations are available only if the kernel was configured with
	.BR CONFIG_TRANSPARENT_HUGEPAGE .
	.TP
	.BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
	Ensures that memory in the address range specified by
	.IR addr
	and
	.IR length
	will not be backed by transparent hugepages.
	.TP
	.BR MADV_DONTDUMP " (since Linux 3.4)"
	.\" commit 909af768e88867016f427264ae39d27a57b6a8ed
	.\" commit accb61fe7bb0f5c2a4102239e4981650f9048519
	Exclude from a core dump those pages in the range specified by
	.I addr
	and
	.IR length .
	This is useful in applications that have large areas of memory
	that are known not to be useful in a core dump.
	The effect of
	.BR MADV_DONTDUMP
	takes precedence over the bit mask that is set via the
	.I /proc/[pid]/coredump_filter
	file (see
	.BR core (5)).
	.TP
	.BR MADV_DODUMP " (since Linux 3.4)"
	Undo the effect of an earlier
	.BR MADV_DONTDUMP .
	.TP
	.BR MADV_FREE " (since Linux 4.5)"
	The application no longer requires the pages in the range specified by
	.IR addr
	and
	.IR len .
	The kernel can thus free these pages,
	but the freeing could be delayed until memory pressure occurs.
	For each of the pages that has been marked to be freed
	but has not yet been freed,
	the free operation will be canceled if the caller writes into the page.
	After a successful
	.B MADV_FREE
	operation, any stale data (i.e., dirty, unwritten pages) will be lost
	when the kernel frees the pages.
	However, subsequent writes to pages in the range will succeed
	and then kernel cannot free those dirtied pages,
	so that the caller can always see just written data.
	If there is no subsequent write,
	the kernel can free the pages at any time.
	Once pages in the range have been freed, the caller will
	see zero-fill-on-demand pages upon subsequent page references.
	.IP
	The
	.B MADV_FREE
	operation
	can be applied only to private anonymous pages (see
	.BR mmap (2)).
	In Linux before version 4.12,
	.\" commit 93e06c7a645343d222c9a838834a51042eebbbf7
	when freeing pages on a swapless system,
	the pages in the given range are freed instantly,
	regardless of memory pressure.
	.TP
	.BR MADV_WIPEONFORK " (since Linux 4.14)"
	.\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
	Present the child process with zero-filled memory in this range after a
	.BR fork (2).
	This is useful in forking servers in order to ensure
	that sensitive per-process data
	(for example, PRNG seeds, cryptographic secrets, and so on)
	is not handed to child processes.
	.IP
	The
	.B MADV_WIPEONFORK
	operation can be applied only to private anonymous pages (see
	.BR mmap (2)).
	.IP
	Within the child created by
	.BR fork (2),
	the
	.B MADV_WIPEONFORK
	setting remains in place on the specified address range.
	This setting is cleared during
	.BR execve (2).
	.TP
	.BR MADV_KEEPONFORK " (since Linux 4.14)"
	.\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19
	Undo the effect of an earlier
	.BR MADV_WIPEONFORK .
	.TP
	.BR MADV_COLD " (since Linux 5.4)"
	.\" commit 9c276cc65a58faf98be8e56962745ec99ab87636
	Deactivate a given range of pages.
	This will make the pages a more probable
	reclaim target should there be a memory pressure.
	This is a nondestructive operation.
	The advice might be ignored for some pages in the range when it is not
	applicable.
	.TP
	.BR MADV_PAGEOUT " (since Linux 5.4)"
	.\" commit 1a4e58cce84ee88129d5d49c064bd2852b481357
	Reclaim a given range of pages.
	This is done to free up memory occupied by these pages.
	If a page is anonymous, it will be swapped out.
	If a page is file-backed and dirty, it will be written back to the backing
	storage.
	The advice might be ignored for some pages in the range when it is not
	applicable.
	.SH RETURN VALUE
	On success,
	.BR madvise ()
	returns zero.
	On error, it returns \-1 and
	.I errno
	is set to indicate the error.
	.SH ERRORS
	.TP
	.B EACCES
	.I advice
	is
	.BR MADV_REMOVE ,
	but the specified address range is not a shared writable mapping.
	.TP
	.B EAGAIN
	A kernel resource was temporarily unavailable.
	.TP
	.B EBADF
	The map exists, but the area maps something that isn't a file.
	.TP
	.B EINVAL
	.I addr
	is not page-aligned or
	.I length
	is negative.
	.\" .I length
	.\" is zero,
	.TP
	.B EINVAL
	.I advice
	is not a valid.
	.TP
	.B EINVAL
	.I advice
	is
	.B MADV_DONTNEED
	or
	.BR MADV_REMOVE
	and the specified address range includes locked, Huge TLB pages, or
	.B VM_PFNMAP
	pages.
	.TP
	.B EINVAL
	.I advice
	is
	.BR MADV_MERGEABLE
	or
	.BR MADV_UNMERGEABLE ,
	but the kernel was not configured with
	.BR CONFIG_KSM .
	.TP
	.B EINVAL
	.I advice
	is
	.BR MADV_FREE
	or
	.BR MADV_WIPEONFORK
	but the specified address range includes file, Huge TLB,
	.BR MAP_SHARED ,
	or
	.BR VM_PFNMAP
	ranges.
	.TP
	.B EIO
	(for
	.BR MADV_WILLNEED )
	Paging in this area would exceed the process's
	maximum resident set size.
	.TP
	.B ENOMEM
	(for
	.BR MADV_WILLNEED )
	Not enough memory: paging in failed.
	.TP
	.B ENOMEM
	Addresses in the specified range are not currently
	mapped, or are outside the address space of the process.
	.TP
	.B EPERM
	.I advice
	is
	.BR MADV_HWPOISON ,
	but the caller does not have the
	.B CAP_SYS_ADMIN
	capability.
	.SH VERSIONS
	Since Linux 3.18,
	.\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
	support for this system call is optional,
	depending on the setting of the
	.B CONFIG_ADVISE_SYSCALLS
	configuration option.
	.SH CONFORMING TO
	.BR madvise ()
	is not specified by any standards.
	Versions of this system call, implementing a wide variety of
	.I advice
	values, exist on many other implementations.
	Other implementations typically implement at least the flags listed
	above under
	.IR "Conventional advice flags" ,
	albeit with some variation in semantics.
	.PP
	POSIX.1-2001 describes
	.BR posix_madvise (3)
	with constants
	.BR POSIX_MADV_NORMAL ,
	.BR POSIX_MADV_RANDOM ,
	.BR POSIX_MADV_SEQUENTIAL ,
	.BR POSIX_MADV_WILLNEED ,
	and
	.BR POSIX_MADV_DONTNEED ,
	and so on, with behavior close to the similarly named flags listed above.
	.SH NOTES
	.SS Linux notes
	The Linux implementation requires that the address
	.I addr
	be page-aligned, and allows
	.I length
	to be zero.
	If there are some parts of the specified address range
	that are not mapped, the Linux version of
	.BR madvise ()
	ignores them and applies the call to the rest (but returns
	.B ENOMEM
	from the system call, as it should).
	.\" .SH HISTORY
	.\" The
	.\" .BR madvise ()
	.\" function first appeared in 4.4BSD.
	.SH SEE ALSO
	.BR getrlimit (2),
	.BR mincore (2),
	.BR mmap (2),
	.BR mprotect (2),
	.BR msync (2),
	.BR munmap (2),
	.BR prctl (2),
	.BR process_madvise (2),
	.BR posix_madvise (3),
	.BR core (5)