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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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MADVISE(2) Linux Programmer's Manual MADVISE(2)
madvise - give advice about use of memory
#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
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 system 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.)
Subsequently, 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
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, 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 containing 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
behavior, 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
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 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 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 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
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
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
CONFIG_MEMORY_FAILURE.
MADV_HUGEPAGE (since Linux 2.6.38)
Enable 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 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 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
Documentation/vm/transhuge.txt for more details.
The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are available
only if the kernel was configured with
CONFIG_TRANSPARENT_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
specified 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 caller writes into the page. After a
successful 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.
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 processes.
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.
On success, madvise() returns zero. On error, it returns -1 and
errno is set appropriately.
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.
Since Linux 3.18, support for this system call is optional, depending
on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.
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_NORMAL, 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.
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
madvise() ignores them and applies the call to the rest (but returns
ENOMEM from the system call, as it should).
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2),
prctl(2), posix_madvise(3), core(5)
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/.
Linux 2017-09-15 MADVISE(2)
Pages that refer to this page: fork(2), getrlimit(2), ioctl_userfaultfd(2), mincore(2), open(2), prctl(2), readahead(2), syscalls(2), userfaultfd(2), posix_madvise(3), core(5), capabilities(7)
Copyright and license for this manual page
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