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():
Since glibc 2.19:
_DEFAULT_SOURCE
Up to and including glibc 2.19:
_BSD_SOURCE
DESCRIPTION
The madvise() system call is used to give advice or directions to the
kernel about the address range beginning at address addr and with size
length bytes In most cases, the goal of such advice is to improve sys-
tem or application performance.
Initially, the system call supported a set of "conventional" advice
values, which are also available on several other implementations.
(Note, though, that madvise() is not specified in POSIX.) Subse-
quently, a number of Linux-specific advice values have been added.
Conventional advice values
The 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
advice values do not influence the semantics of the application (except
in the case of MADV_DONTNEED), but may influence its performance. All
of the advice values listed here have analogs in the POSIX-specified
posix_madvise(3) function, and the values have the same meanings, with
the exception of MADV_DONTNEED.
The advice is indicated in the advice argument, which is one of the
following:
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.)
After a successful 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 con-
tents 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, 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.
MADV_DONTNEED cannot be applied to locked pages, Huge TLB pages,
or VM_PFNMAP pages. (Pages marked with the kernel-internal
VM_PFNMAP 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.)
Linux-specific advice values
The following Linux-specific advice values have no counterparts in the
POSIX-specified posix_madvise(3), and may or may not have counterparts
in the madvise() interface available on other implementations. Note
that some of these operations change the semantics of memory accesses.
MADV_REMOVE (since Linux 2.6.16)
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 fallocate(2)). Subsequent
accesses in the specified address range will see bytes contain-
ing zero.
The specified address range must be mapped shared and writable.
This flag cannot be applied to locked pages, Huge TLB pages, or
VM_PFNMAP pages.
In the initial implementation, only tmpfs(5) was supported
MADV_REMOVE; but since Linux 3.5, any filesystem which supports
the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also supports
MADV_REMOVE. Hugetlbfs fails with the error EINVAL and other
filesystems fail with the error EOPNOTSUPP.
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 if the parent
writes to it after a fork(2). (Such page relocations cause
problems for hardware that DMAs into the page.)
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 the pages in the range specified by addr and length and
handle subsequent references to those pages like a hardware mem-
ory 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 available only if the kernel was configured 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 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 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_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 configured with
CONFIG_MEMORY_FAILURE.
MADV_HUGEPAGE (since Linux 2.6.38)
Enable Transparent Huge Pages (THP) for pages in the range spec-
ified 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 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 Documenta-
tion/vm/transhuge.txt for more details.
The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are available
only if the kernel was configured with CONFIG_TRANSPAR-
ENT_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.
MADV_FREE (since Linux 4.5)
The application no longer requires the pages in the range speci-
fied by addr and 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 call-
er writes into the page. After a successful MADV_FREE opera-
tion, 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 subse-
quent page references.
The MADV_FREE operation can be applied only to private anonymous
pages (see mmap(2)). On a swapless system, freeing pages in a
given range happens instantly, regardless of memory pressure.
MADV_WIPEONFORK (since Linux 4.14)
Present the child process with zero-filled memory in this range
after a 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 pro-
cesses.
The MADV_WIPEONFORK operation can be applied only to private
anonymous pages (see mmap(2)).
Within the child created by fork(2), the MADV_WIPEONFORK setting
remains in place on the specified address range. This setting
is cleared during execve(2).
MADV_KEEPONFORK (since Linux 4.14)
Undo the effect of an earlier MADV_WIPEONFORK.
RETURN VALUE
On success, madvise() returns zero. On error, it returns -1 and errno
is set appropriately.
ERRORS
EACCES advice is MADV_REMOVE, but the specified address range is not a
shared writable mapping.
EAGAIN A kernel resource was temporarily unavailable.
EBADF The map exists, but the area maps something that isn't a file.
EINVAL addr is not page-aligned or length is negative.
EINVAL advice is not a valid.
EINVAL advice is MADV_DONTNEED or MADV_REMOVE and the specified address
range includes locked, Huge TLB pages, or VM_PFNMAP pages.
EINVAL advice is MADV_MERGEABLE or MADV_UNMERGEABLE, but the kernel was
not configured with CONFIG_KSM.
EINVAL advice is MADV_FREE or MADV_WIPEONFORK but the specified address
range includes file, Huge TLB, MAP_SHARED, or VM_PFNMAP ranges.
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.
EPERM advice is MADV_HWPOISON, but the caller does not have the
CAP_SYS_ADMIN capability.
VERSIONS
Since Linux 3.18, support for this system call is optional, depending
on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.
CONFORMING TO
madvise() is not specified by any standards. Versions of this system
call, implementing a wide variety of advice values, exist on many other
implementations. Other implementations typically implement at least
the flags listed above under Conventional advice flags, albeit with
some variation in semantics.
POSIX.1-2001 describes posix_madvise(3) with constants POSIX_MADV_NOR-
MAL, POSIX_MADV_RANDOM, POSIX_MADV_SEQUENTIAL, POSIX_MADV_WILLNEED, and
POSIX_MADV_DONTNEED, and so on, with behavior close to the similarly
named flags listed above.
NOTES
Linux notes
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),
prctl(2), posix_madvise(3), core(5)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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