MADVISE

Section: Linux Programmer's Manual (2)
Updated: 2020-04-11
Page Index

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 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/admin-guide/mm/ksm.rst 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/admin-guide/mm/transhuge.rst for more details.
: Most common kernels configurations provide MADV_HUGEPAGE-style behavior by default, and thus MADV_HUGEPAGE is normally not necessary. It is mostly intended for embedded systems, where MADV_HUGEPAGE-style behavior may not be enabled by default in the kernel. On such systems, this flag can be used in order to selectively enable THP. Whenever MADV_HUGEPAGE is used, it should always be in regions of memory with an access pattern that the developer knows in advance won't risk to increase the memory footprint of the application when transparent hugepages are enabled.
: 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 backed by transparent hugepages.
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)). In Linux before version 4.12, when freeing pages on a swapless system, the pages in the given range are freed 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.

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_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.

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 madvise() ignores them and applies the call to the rest (but returns ENOMEM from the system call, as it should).

COLOPHON

This page is part of release 5.10 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/.