man-pages/man2/madvise.2

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.\" Copyright (C) 2001 David Gómez <davidge@jazzfree.com>
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.\" 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 2012-04-28 "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 advises the kernel about how to handle paging input/output in
the address range beginning at address
.I addr
and with size
.I length
bytes.
It allows 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.
This call does not influence the semantics of the application
(except in the case of
.BR MADV_DONTNEED ),
but
may influence its performance.
The kernel is free to ignore the advice.
.LP
The advice is indicated in the
.I advice
argument which can be
.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.)
Subsequent accesses of pages in this range will succeed, but will result
either in reloading of the memory contents from the underlying mapped file
(see
.BR mmap (2))
or zero-fill-on-demand pages for mappings
without an underlying file.
.TP
.BR MADV_REMOVE " (Since Linux 2.6.16)"
Free up a given range of pages
and its associated backing store.
Currently,
.\" 2.6.18-rc5
only shmfs/tmpfs supports this; other file systems return with the
error
.BR ENOSYS .
.\" 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.
.TP
.BR MADV_DONTFORK " (Since Linux 2.6.16)"
.\" 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(s) if the parent writes to it after a
.BR fork (2).
(Such page relocations cause problems for hardware that
DMAs into the page(s).)
.\" [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)
Poison a page and handle it like a hardware memory corruption.
This operation is only available 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 only available if the kernel was configured with
.BR CONFIG_MEMORY_FAILURE .
.TP
.BR MADV_SOFT_OFFLINE " (Since Linux 2.6.33)
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 only available if the kernel was configured with
.BR CONFIG_MEMORY_FAILURE .
.TP
.BR MADV_MERGEABLE " (since Linux 2.6.32)"
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 only merges 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 only available 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)"
.\" http://lwn.net/Articles/358904/
.\" https://lwn.net/Articles/423584/
Enables Transparent Huge Pages (THP) for pages in the range specified by
.I addr
and
.IR length .
Currently, Transparent Huge Pages only work 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 only available 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)"
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 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
This error can occur for the following reasons:
.RS
.IP * 3
The value
.I len
is negative.
.\" .I len
.\" is zero,
.IP *
.I addr
is not page-aligned.
.IP *
.I advice
is not a valid value
.IP *
The application is attempting to release locked or shared pages (with
.BR MADV_DONTNEED ).
.IP *
.BR MADV_MERGEABLE
or
.BR MADV_UNMERGEABLE
was specified in
.IR advice ,
but the kernel was not configured with
.BR CONFIG_KSM .
.RE
.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.
.SH "CONFORMING TO"
POSIX.1b.
POSIX.1-2001 describes
.BR posix_madvise (3)
.\" FIXME . Write a posix_madvise(3) page.
with constants
.BR POSIX_MADV_NORMAL ,
etc.,
with a behavior close to that described here.
There is a similar
.BR posix_fadvise (2)
for file access.

.BR MADV_REMOVE ,
.BR MADV_DONTFORK ,
.BR MADV_DOFORK ,
.BR MADV_HWPOISON ,
.BR MADV_MERGEABLE ,
and
.BR MADV_UNMERGEABLE
are Linux-specific.
.SH NOTES
.SS Linux notes
.LP
The current Linux implementation (2.4.0) views this system call
more as a command than as advice and hence may return an error
when it cannot do what it usually would do in response to this
advice.
(See the ERRORS description above.)
This is nonstandard behavior.
.LP
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 core (5)