MADVISE(2) Linux Programmer's Manual MADVISE(2)
NAME
madvise - give advice about use of memory
SYNOPSIS
#include <sys/mman.h>
int madvise(void *addr, size_t length, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
madvise(): _BSD_SOURCE
DESCRIPTION
The madvise() system call advises the kernel about how to handle paging
input/output in the address range beginning at address addr and with
size 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 MADV_DONTNEED), but may influence its performance. The kernel is
free to ignore the advice.
The advice is indicated in the advice argument which can be
MADV_NORMAL
No special treatment. This is the default.
MADV_RANDOM
Expect page references in random order. (Hence, read ahead may
be less useful than normally.)
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.)
MADV_WILLNEED
Expect access in the near future. (Hence, it might be a good
idea to read some pages ahead.)
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 mmap(2)) or zero-fill-on-demand pages for mappings without
an underlying file.
MADV_REMOVE (Since Linux 2.6.16)
Free up a given range of pages and its associated backing store.
Currently, only shmfs/tmpfs supports this; other file systems
return with the error ENOSYS.
MADV_DONTFORK (Since Linux 2.6.16)
Do not make the pages in this range available to the child after
a 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 fork(2). (Such page relocations cause
problems for hardware that DMAs into the page(s).)
MADV_DOFORK (Since Linux 2.6.16)
Undo the effect of MADV_DONTFORK, restoring the default behav-
ior, whereby a mapping is inherited across fork(2).
MADV_HWPOISON (Since Linux 2.6.32)
Poison a page and handle it like a hardware memory corruption.
This operation is available only for privileged (CAP_SYS_ADMIN)
processes. This operation may result in the calling process
receiving a SIGBUS and the page being unmapped. This feature is
intended for testing of memory error-handling code; it is avail-
able only if the kernel was configured with CONFIG_MEMORY_FAIL-
URE.
MADV_SOFT_OFFLINE (Since Linux 2.6.33)
Soft offline the pages in the range specified by addr and
length. The memory of each page in the specified range is pre-
served (i.e., when next accessed, the same content will be visi-
ble, 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 MADV_SOFT_OFFLINE operation is
invisible to (i.e., does not change the semantics of) the call-
ing process. This feature is intended for testing of memory
error-handling code; it is available only if the kernel was con-
figured with CONFIG_MEMORY_FAILURE.
MADV_MERGEABLE (since Linux 2.6.32)
Enable Kernel Samepage Merging (KSM) for the pages in the range
specified by addr and 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 sin-
gle 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 mmap(2)). The KSM fea-
ture 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 Documentation/vm/ksm.txt for more
details. The MADV_MERGEABLE and MADV_UNMERGEABLE operations are
available only if the kernel was configured with CONFIG_KSM.
MADV_UNMERGEABLE (since Linux 2.6.32)
Undo the effect of an earlier MADV_MERGEABLE operation on the
specified address range; KSM unmerges whatever pages it had
merged in the address range specified by addr and length.
MADV_HUGEPAGE (since Linux 2.6.38)
Enables Transparent Huge Pages (THP) for pages in the range
specified by addr and length. Currently, Transparent Huge Pages
work only with private anonymous pages (see mmap(2)). The ker-
nel 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 posix_memalign(2)). This feature is primar-
ily aimed at applications that use large mappings of data and
access large regions of that memory at a time (e.g., virtualiza-
tion 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 Documentation/vm/transhuge.txt for more
details. The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are
available only if the kernel was configured with CONFIG_TRANS-
PARENT_HUGEPAGE.
MADV_NOHUGEPAGE (since Linux 2.6.38)
Ensures that memory in the address range specified by addr and
length will not be collapsed into huge pages.
MADV_DONTDUMP (since Linux 3.4)
Exclude from a core dump those pages in the range specified by
addr and 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 MADV_DONTDUMP takes precedence over the bit mask
that is set via the /proc/PID/coredump_filter file (see
core(5)).
MADV_DODUMP (since Linux 3.4)
Undo the effect of an earlier MADV_DONTDUMP.
RETURN VALUE
On success madvise() returns zero. On error, it returns -1 and errno
is set appropriately.
ERRORS
EAGAIN A kernel resource was temporarily unavailable.
EBADF The map exists, but the area maps something that isn't a file.
EINVAL This error can occur for the following reasons:
* The value len is negative.
* addr is not page-aligned.
* advice is not a valid value
* The application is attempting to release locked or shared
pages (with MADV_DONTNEED).
* MADV_MERGEABLE or MADV_UNMERGEABLE was specified in advice,
but the kernel was not configured with CONFIG_KSM.
EIO (for MADV_WILLNEED) Paging in this area would exceed the
process's maximum resident set size.
ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.
ENOMEM Addresses in the specified range are not currently mapped, or
are outside the address space of the process.
CONFORMING TO
POSIX.1b. POSIX.1-2001 describes posix_madvise(3) with constants
POSIX_MADV_NORMAL, etc., with a behavior close to that described here.
There is a similar posix_fadvise(2) for file access.
MADV_REMOVE, MADV_DONTFORK, MADV_DOFORK, MADV_HWPOISON, MADV_MERGEABLE,
and MADV_UNMERGEABLE are Linux-specific.
NOTES
Linux notes
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.
The Linux implementation requires that the address addr be page-
aligned, and allows length to be zero. If there are some parts of the
specified address range that are not mapped, the Linux version of mad-
vise() ignores them and applies the call to the rest (but returns
ENOMEM from the system call, as it should).
SEE ALSO
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2),
core(5)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-04-28 MADVISE(2)
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