NAME
madvise - give advice about use of memory SYNOPSIS
#include
int madvise(void *addr, sizet length, int advice); Feature Test Macro Requirements for glibc (see featuretestmacros(7)): madvise(): BSDSOURCE 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 MADVDONTNEED), but may influence its performance. The kernel is free to ignore the advice. The advice is indicated in the advice argument which can be MADVNORMAL No special treatment. This is the default. MADVRANDOM Expect page references in random order. (Hence, read ahead may be less useful than normally.) MADVSEQUENTIAL 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.) MADVWILLNEED Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.) MADVDONTNEED 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. MADVREMOVE (Since Linux 2.6.16) Free up a given range of pages and its associated backing store. Originally, only shmfs/tmpfs supported this; but since Linux 3.5, any filesystem which supports the fallocate(2) mode FALLOCFLPUNCHHOLE also supports the madvise(2) advice MADVREMOVE. Other filesystems return with the EOPNOTSUPP error. MADVDONTFORK (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).) MADVDOFORK (Since Linux 2.6.16) Undo the effect of MADVDONTFORK, restoring the default behavior, whereby a mapping is inherited across fork(2). MADVHWPOISON (Since Linux 2.6.32) Poison a page and handle it like a hardware memory corruption. This operation is available only for privileged (CAPSYSADMIN) 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 CONFIGMEMORYFAILURE. MADVSOFTOFFLINE (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 MADVSOFTOFFLINE 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 CONFIGMEMORYFAILURE. MADVMERGEABLE (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 MADVMERGEABLE and MADVUNMERGEABLE operations are available only if the kernel was configured with CONFIGKSM. MADVUNMERGEABLE (since Linux 2.6.32) Undo the effect of an earlier MADVMERGEABLE operation on the specified address range; KSM unmerges whatever pages it had merged in the address range specified by addr and length. MADVHUGEPAGE (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 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 posixmemalign(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 Documentation/vm/transhuge.txt for more details. The MADVHUGEPAGE and MADVNOHUGEPAGE operations are available only if the kernel was configured with CONFIGTRANSPARENTHUGEPAGE. MADVNOHUGEPAGE (since Linux 2.6.38) Ensures that memory in the address range specified by addr and length will not be collapsed into huge pages. MADVDONTDUMP (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 MADVDONTDUMP takes precedence over the bit mask that is set via the /proc/PID/coredumpfilter file (see core(5)). MADVDODUMP (since Linux 3.4) Undo the effect of an earlier MADVDONTDUMP. MADVWIPEONFORK (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 MADVWIPEONFORK operation can be applied only to private anonymous pages (see mmap(2)). Within the child created by fork(2), the MADVWIPEONFORK setting remains in place on the specified address range. This setting is cleared during execve(2). MADVKEEPONFORK (since Linux 4.14) Undo the effect of an earlier MADVWIPEONFORK. 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 MADVDONTNEED). * MADVMERGEABLE or MADVUNMERGEABLE was specified in advice, but the kernel was not configured with CONFIGKSM. EINVAL advice is MADVFREE or MADVWIPEONFORK but the specified address range includes file, Huge TLB, MAPSHARED, or VMPFNMAP ranges. EIO (for MADVWILLNEED) Paging in this area would exceed the process's maximum resident set size. ENOMEM (for MADVWILLNEED) 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 posixmadvise(3) with constants POSIXMADVNORMAL, etc., with a behavior close to that described here. There is a similar posixfadvise(2) for file access. MADVREMOVE, MADVDONTFORK, MADVDOFORK, MADVHWPOISON, MADVMERGEABLE,
and MADVUNMERGEABLE 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 madvise() 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), fallocate(2) 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)