madvise.2: Improve MADV_DONTNEED description

[thirdparty/man-pages.git] / man2 / madvise.2

.\" 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.\" manual page may be incorrect or out-of-date.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 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
madvise \- give advice about use of memory
.SH SYNOPSIS
.B #include <sys/mman.h>
.sp
.BI "int madvise(void *" addr ", size_t " length ", int " advice );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR madvise ():
_BSD_SOURCE
.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.
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 .
.LP
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.)

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.

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.

.B MADV_DONTNEED
cannot be applied to locked pages or Huge TLB pages.
.\"
.\" ======================================================================
.\"
.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.

The specified address range must be mapped shared and writable.
This flag cannot be applied to locked pages or Huge TLB pages.

In the initial implementation, only shmfs/tmpfs 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 .
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 a page and handle it 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.

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_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.

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)).

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/vm/ksm.txt
for more details.

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_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)).

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 2MB mapping that only ever accesses
1 byte will result in 2MB of wired memory instead of one 4KB page).
See the Linux kernel source file
.I Documentation/vm/transhuge.txt
for more details.

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 collapsed into huge pages.
.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 .
.SH RETURN VALUE
On success,
.BR madvise ()
returns zero.
On error, it returns \-1 and
.I errno
is set appropriately.
.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 or Huge TLB 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 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.

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.
(POSIX.1-2008 adds a further flag,
.BR POSIX_MADV_NOREUSE ,
that has no analog in
.BR madvise (2).)
.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 posix_fadvise (2),
.BR prctl (2),
.BR core (5)