.\" Copyright (C) 2001 David Gómez .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Based on comments from mm/filemap.c. Last modified on 10-06-2001 .\" Modified, 25 Feb 2002, Michael Kerrisk, .\" Added notes on MADV_DONTNEED .\" 2010-06-19, mtk, Added documentation of MADV_MERGEABLE and .\" MADV_UNMERGEABLE .\" 2010-06-15, Andi Kleen, Add documentation of MADV_HWPOISON. .\" 2010-06-19, Andi Kleen, Add documentation of MADV_SOFT_OFFLINE. .\" 2011-09-18, Doug Goldstein .\" Document MADV_HUGEPAGE and MADV_NOHUGEPAGE .\" .TH madvise 2 2024-05-02 "Linux man-pages 6.8" .SH NAME madvise \- give advice about use of memory .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .P .BI "int madvise(void " addr [. length "], size_t " length ", int " advice ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR madvise (): .nf Since glibc 2.19: _DEFAULT_SOURCE Up to and including glibc 2.19: _BSD_SOURCE .fi .SH DESCRIPTION The .BR madvise () system call is used to give advice or directions to the kernel about the address range beginning at address .I addr and with size .IR length . .BR madvise () only operates on whole pages, therefore .I addr must be page-aligned. The value of .I length is rounded up to a multiple of page size. In most cases, the goal of such advice is to improve system or application performance. .P Initially, the system call supported a set of "conventional" .I advice values, which are also available on several other implementations. (Note, though, that .BR madvise () is not specified in POSIX.) Subsequently, a number of Linux-specific .I advice values have been added. .\" .\" ====================================================================== .\" .SS Conventional advice values The .I 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 .I advice values do not influence the semantics of the application (except in the case of .BR MADV_DONTNEED ), but may influence its performance. All of the .I advice values listed here have analogs in the POSIX-specified .BR posix_madvise (3) function, and the values have the same meanings, with the exception of .BR MADV_DONTNEED . .P The advice is indicated in the .I advice argument, which is one of the following: .TP .B MADV_NORMAL No special treatment. This is the default. .TP .B MADV_RANDOM Expect page references in random order. (Hence, read ahead may be less useful than normally.) .TP .B 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.) .TP .B MADV_WILLNEED Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.) .TP .B 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.) .IP After a successful .B 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. .IP Note that, when applied to shared mappings, .B 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. .IP .B MADV_DONTNEED cannot be applied to locked pages, or .B VM_PFNMAP pages. (Pages marked with the kernel-internal .B VM_PFNMAP .\" http://lwn.net/Articles/162860/ 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.) .IP Support for Huge TLB pages was added in Linux v5.18. Addresses within a mapping backed by Huge TLB pages must be aligned to the underlying Huge TLB page size, and the range length is rounded up to a multiple of the underlying Huge TLB page size. .\" .\" ====================================================================== .\" .SS Linux-specific advice values The following Linux-specific .I advice values have no counterparts in the POSIX-specified .BR posix_madvise (3), and may or may not have counterparts in the .BR madvise () interface available on other implementations. Note that some of these operations change the semantics of memory accesses. .TP .BR MADV_REMOVE " (since Linux 2.6.16)" .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349 Free up a given range of pages and its associated backing store. This is equivalent to punching a hole in the corresponding range of the backing store (see .BR fallocate (2)). Subsequent accesses in the specified address range will see data with a value of zero. .\" Databases want to use this feature to drop a section of their .\" bufferpool (shared memory segments) - without writing back to .\" disk/swap space. This feature is also useful for supporting .\" hot-plug memory on UML. .IP The specified address range must be mapped shared and writable. This flag cannot be applied to locked pages, or .B VM_PFNMAP pages. .IP In the initial implementation, only .BR tmpfs (5) supported .BR MADV_REMOVE ; but since Linux 3.5, .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17 any filesystem which supports the .BR fallocate (2) .B FALLOC_FL_PUNCH_HOLE mode also supports .BR MADV_REMOVE . Filesystems which do not support .B MADV_REMOVE fail with the error .BR EOPNOTSUPP . .IP Support for the Huge TLB filesystem was added in Linux v4.3. .TP .BR MADV_DONTFORK " (since Linux 2.6.16)" .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4 .\" See http://lwn.net/Articles/171941/ Do not make the pages in this range available to the child after a .BR 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 .BR fork (2). (Such page relocations cause problems for hardware that DMAs into the page.) .\" [PATCH] madvise MADV_DONTFORK/MADV_DOFORK .\" Currently, copy-on-write may change the physical address of .\" a page even if the user requested that the page is pinned in .\" memory (either by mlock or by get_user_pages). This happens .\" if the process forks meanwhile, and the parent writes to that .\" page. As a result, the page is orphaned: in case of .\" get_user_pages, the application will never see any data hardware .\" DMA's into this page after the COW. In case of mlock'd memory, .\" the parent is not getting the realtime/security benefits of mlock. .\" .\" In particular, this affects the Infiniband modules which do DMA from .\" and into user pages all the time. .\" .\" This patch adds madvise options to control whether memory range is .\" inherited across fork. Useful e.g. for when hardware is doing DMA .\" from/into these pages. Could also be useful to an application .\" wanting to speed up its forks by cutting large areas out of .\" consideration. .\" .\" SEE ALSO: http://lwn.net/Articles/171941/ .\" "Tweaks to madvise() and posix_fadvise()", 14 Feb 2006 .TP .BR MADV_DOFORK " (since Linux 2.6.16)" Undo the effect of .BR MADV_DONTFORK , restoring the default behavior, whereby a mapping is inherited across .BR fork (2). .TP .BR MADV_HWPOISON " (since Linux 2.6.32)" .\" commit 9893e49d64a4874ea67849ee2cfbf3f3d6817573 Poison the pages in the range specified by .I addr and .I length and handle subsequent references to those pages like a hardware memory corruption. This operation is available only for privileged .RB ( CAP_SYS_ADMIN ) processes. This operation may result in the calling process receiving a .B SIGBUS and the page being unmapped. .IP This feature is intended for testing of memory error-handling code; it is available only if the kernel was configured with .BR CONFIG_MEMORY_FAILURE . .TP .BR MADV_MERGEABLE " (since Linux 2.6.32)" .\" commit f8af4da3b4c14e7267c4ffb952079af3912c51c5 Enable Kernel Samepage Merging (KSM) for the pages in the range specified by .I addr and .IR 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 .BR mmap (2)). .IP 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 .I Documentation/admin\-guide/mm/ksm.rst for more details. .IP The .B MADV_MERGEABLE and .B MADV_UNMERGEABLE operations are available only if the kernel was configured with .BR CONFIG_KSM . .TP .BR MADV_UNMERGEABLE " (since Linux 2.6.32)" Undo the effect of an earlier .B MADV_MERGEABLE operation on the specified address range; KSM unmerges whatever pages it had merged in the address range specified by .I addr and .IR length . .TP .BR MADV_SOFT_OFFLINE " (since Linux 2.6.33)" .\" commit afcf938ee0aac4ef95b1a23bac704c6fbeb26de6 Soft offline the pages in the range specified by .I addr and .IR 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 .B MADV_SOFT_OFFLINE operation is invisible to (i.e., does not change the semantics of) the calling process. .IP This feature is intended for testing of memory error-handling code; it is available only if the kernel was configured with .BR CONFIG_MEMORY_FAILURE . .TP .BR MADV_HUGEPAGE " (since Linux 2.6.38)" .\" commit 0af4e98b6b095c74588af04872f83d333c958c32 .\" http://lwn.net/Articles/358904/ .\" https://lwn.net/Articles/423584/ Enable Transparent Huge Pages (THP) for pages in the range specified by .I addr and .IR length . 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 .BR posix_memalign (2)). .IP 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 .I Documentation/admin\-guide/mm/transhuge.rst for more details. .IP Most common kernels configurations provide .BR MADV_HUGEPAGE -style behavior by default, and thus .B MADV_HUGEPAGE is normally not necessary. It is mostly intended for embedded systems, where .BR 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 .B 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. .IP .\" commit 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0 Since Linux 5.4, automatic scan of eligible areas and replacement by huge pages works with private anonymous pages (see .BR mmap (2)), shmem pages, and file-backed pages. For all memory types, memory may only be replaced by huge pages on hugepage-aligned boundaries. For file-mapped memory \[em]including tmpfs (see .BR tmpfs (2))\[em] the mapping must also be naturally hugepage-aligned within the file. Additionally, for file-backed, non-tmpfs memory, the file must not be open for write and the mapping must be executable. .IP The VMA must not be marked .BR VM_NOHUGEPAGE , .BR VM_HUGETLB , .BR VM_IO , .BR VM_DONTEXPAND , .BR VM_MIXEDMAP , or .BR VM_PFNMAP , nor can it be stack memory or backed by a DAX-enabled device (unless the DAX device is hot-plugged as System RAM). The process must also not have .B PR_SET_THP_DISABLE set (see .BR prctl (2)). .IP The .BR MADV_HUGEPAGE , .BR MADV_NOHUGEPAGE , and .B MADV_COLLAPSE operations are available only if the kernel was configured with .B CONFIG_TRANSPARENT_HUGEPAGE and file/shmem memory is only supported if the kernel was configured with .BR CONFIG_READ_ONLY_THP_FOR_FS . .TP .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)" Ensures that memory in the address range specified by .I addr and .I length will not be backed by transparent hugepages. .TP .BR MADV_COLLAPSE " (since Linux 6.1)" .\" commit 7d8faaf155454f8798ec56404faca29a82689c77 .\" commit 34488399fa08faaf664743fa54b271eb6f9e1321 Perform a best-effort synchronous collapse of the native pages mapped by the memory range into Transparent Huge Pages (THPs). .B MADV_COLLAPSE operates on the current state of memory of the calling process and makes no persistent changes or guarantees on how pages will be mapped, constructed, or faulted in the future. .IP .B MADV_COLLAPSE supports private anonymous pages (see .BR mmap (2)), shmem pages, and file-backed pages. See .B MADV_HUGEPAGE for general information on memory requirements for THP. If the range provided spans multiple VMAs, the semantics of the collapse over each VMA is independent from the others. If collapse of a given huge page-aligned/sized region fails, the operation may continue to attempt collapsing the remainder of the specified memory. .B MADV_COLLAPSE will automatically clamp the provided range to be hugepage-aligned. .IP All non-resident pages covered by the range will first be swapped/faulted-in, before being copied onto a freshly allocated hugepage. If the native pages compose the same PTE-mapped hugepage, and are suitably aligned, allocation of a new hugepage may be elided and collapse may happen in-place. Unmapped pages will have their data directly initialized to 0 in the new hugepage. However, for every eligible hugepage-aligned/sized region to be collapsed, at least one page must currently be backed by physical memory. .IP .B MADV_COLLAPSE is independent of any sysfs (see .BR sysfs (5)) setting under .IR /sys/kernel/mm/transparent_hugepage , both in terms of determining THP eligibility, and allocation semantics. See Linux kernel source file .I Documentation/admin\-guide/mm/transhuge.rst for more information. .B MADV_COLLAPSE also ignores .B huge= tmpfs mount when operating on tmpfs files. Allocation for the new hugepage may enter direct reclaim and/or compaction, regardless of VMA flags (though .B VM_NOHUGEPAGE is still respected). .IP When the system has multiple NUMA nodes, the hugepage will be allocated from the node providing the most native pages. .IP If all hugepage-sized/aligned regions covered by the provided range were either successfully collapsed, or were already PMD-mapped THPs, this operation will be deemed successful. Note that this doesn't guarantee anything about other possible mappings of the memory. In the event multiple hugepage-aligned/sized areas fail to collapse, only the most-recently\[en]failed code will be set in .IR errno . .TP .BR MADV_DONTDUMP " (since Linux 3.4)" .\" commit 909af768e88867016f427264ae39d27a57b6a8ed .\" commit accb61fe7bb0f5c2a4102239e4981650f9048519 Exclude from a core dump those pages in the range specified by .I addr and .IR 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 .B MADV_DONTDUMP takes precedence over the bit mask that is set via the .IR /proc/ pid /coredump_filter file (see .BR core (5)). .TP .BR MADV_DODUMP " (since Linux 3.4)" Undo the effect of an earlier .BR MADV_DONTDUMP . .TP .BR MADV_FREE " (since Linux 4.5)" The application no longer requires the pages in the range specified by .I addr and .IR 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 .B 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. .IP The .B MADV_FREE operation can be applied only to private anonymous pages (see .BR mmap (2)). Before Linux 4.12, .\" commit 93e06c7a645343d222c9a838834a51042eebbbf7 when freeing pages on a swapless system, the pages in the given range are freed instantly, regardless of memory pressure. .TP .BR MADV_WIPEONFORK " (since Linux 4.14)" .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19 Present the child process with zero-filled memory in this range after a .BR 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. .IP The .B MADV_WIPEONFORK operation can be applied only to private anonymous pages (see .BR mmap (2)). .IP Within the child created by .BR fork (2), the .B MADV_WIPEONFORK setting remains in place on the specified address range. This setting is cleared during .BR execve (2). .TP .BR MADV_KEEPONFORK " (since Linux 4.14)" .\" commit d2cd9ede6e193dd7d88b6d27399e96229a551b19 Undo the effect of an earlier .BR MADV_WIPEONFORK . .TP .BR MADV_COLD " (since Linux 5.4)" .\" commit 9c276cc65a58faf98be8e56962745ec99ab87636 Deactivate a given range of pages. This will make the pages a more probable reclaim target should there be a memory pressure. This is a nondestructive operation. The advice might be ignored for some pages in the range when it is not applicable. .TP .BR MADV_PAGEOUT " (since Linux 5.4)" .\" commit 1a4e58cce84ee88129d5d49c064bd2852b481357 Reclaim a given range of pages. This is done to free up memory occupied by these pages. If a page is anonymous, it will be swapped out. If a page is file-backed and dirty, it will be written back to the backing storage. The advice might be ignored for some pages in the range when it is not applicable. .TP .BR MADV_POPULATE_READ " (since Linux 5.14)" "Populate (prefault) page tables readable, faulting in all pages in the range just as if manually reading from each page; however, avoid the actual memory access that would have been performed after handling the fault. .IP In contrast to .BR MAP_POPULATE , .B MADV_POPULATE_READ does not hide errors, can be applied to (parts of) existing mappings and will always populate (prefault) page tables readable. One example use case is prefaulting a file mapping, reading all file content from disk; however, pages won't be dirtied and consequently won't have to be written back to disk when evicting the pages from memory. .IP Depending on the underlying mapping, map the shared zeropage, preallocate memory or read the underlying file; files with holes might or might not preallocate blocks. If populating fails, a .B SIGBUS signal is not generated; instead, an error is returned. .IP If .B MADV_POPULATE_READ succeeds, all page tables have been populated (prefaulted) readable once. If .B MADV_POPULATE_READ fails, some page tables might have been populated. .IP .B MADV_POPULATE_READ cannot be applied to mappings without read permissions and special mappings, for example, mappings marked with kernel-internal flags such as .B VM_PFNMAP or .BR VM_IO , or secret memory regions created using .BR memfd_secret(2) . .IP Note that with .BR MADV_POPULATE_READ , the process can be killed at any moment when the system runs out of memory. .TP .BR MADV_POPULATE_WRITE " (since Linux 5.14)" Populate (prefault) page tables writable, faulting in all pages in the range just as if manually writing to each each page; however, avoid the actual memory access that would have been performed after handling the fault. .IP In contrast to .BR MAP_POPULATE , MADV_POPULATE_WRITE does not hide errors, can be applied to (parts of) existing mappings and will always populate (prefault) page tables writable. One example use case is preallocating memory, breaking any CoW (Copy on Write). .IP Depending on the underlying mapping, preallocate memory or read the underlying file; files with holes will preallocate blocks. If populating fails, a .B SIGBUS signal is not generated; instead, an error is returned. .IP If .B MADV_POPULATE_WRITE succeeds, all page tables have been populated (prefaulted) writable once. If .B MADV_POPULATE_WRITE fails, some page tables might have been populated. .IP .B MADV_POPULATE_WRITE cannot be applied to mappings without write permissions and special mappings, for example, mappings marked with kernel-internal flags such as .B VM_PFNMAP or .BR VM_IO , or secret memory regions created using .BR memfd_secret(2) . .IP Note that with .BR MADV_POPULATE_WRITE , the process can be killed at any moment when the system runs out of memory. .SH RETURN VALUE On success, .BR madvise () returns zero. On error, it returns \-1 and .I errno is set to indicate the error. .SH ERRORS .TP .B EACCES .I advice is .BR MADV_REMOVE , but the specified address range is not a shared writable mapping. .TP .B EAGAIN A kernel resource was temporarily unavailable. .TP .B EBADF The map exists, but the area maps something that isn't a file. .TP .B EBUSY (for .BR MADV_COLLAPSE ) Could not charge hugepage to cgroup: cgroup limit exceeded. .TP .B EFAULT .I advice is .B MADV_POPULATE_READ or .BR MADV_POPULATE_WRITE , and populating (prefaulting) page tables failed because a .B SIGBUS would have been generated on actual memory access and the reason is not a HW poisoned page (HW poisoned pages can, for example, be created using the .B MADV_HWPOISON flag described elsewhere in this page). .TP .B EINVAL .I addr is not page-aligned or .I length is negative. .\" .I length .\" is zero, .TP .B EINVAL .I advice is not a valid. .TP .B EINVAL .I advice is .B MADV_COLD or .B MADV_PAGEOUT and the specified address range includes locked, Huge TLB pages, or .B VM_PFNMAP pages. .TP .B EINVAL .I advice is .B MADV_DONTNEED or .B MADV_REMOVE and the specified address range includes locked, Huge TLB pages, or .B VM_PFNMAP pages. .TP .B EINVAL .I advice is .B MADV_MERGEABLE or .BR MADV_UNMERGEABLE , but the kernel was not configured with .BR CONFIG_KSM . .TP .B EINVAL .I advice is .B MADV_FREE or .B MADV_WIPEONFORK but the specified address range includes file, Huge TLB, .BR MAP_SHARED , or .B VM_PFNMAP ranges. .TP .B EINVAL .I advice is .B MADV_POPULATE_READ or .BR MADV_POPULATE_WRITE , but the specified address range includes ranges with insufficient permissions or special mappings, for example, mappings marked with kernel-internal flags such a .B VM_IO or .BR VM_PFNMAP , or secret memory regions created using .BR memfd_secret(2) . .TP .B EIO (for .BR MADV_WILLNEED ) Paging in this area would exceed the process's maximum resident set size. .TP .B ENOMEM (for .BR MADV_WILLNEED ) Not enough memory: paging in failed. .TP .B ENOMEM (for .BR MADV_COLLAPSE ) Not enough memory: could not allocate hugepage. .TP .B ENOMEM Addresses in the specified range are not currently mapped, or are outside the address space of the process. .TP .B ENOMEM .I advice is .B MADV_POPULATE_READ or .BR MADV_POPULATE_WRITE , and populating (prefaulting) page tables failed because there was not enough memory. .TP .B EPERM .I advice is .BR MADV_HWPOISON , but the caller does not have the .B CAP_SYS_ADMIN capability. .TP .B EHWPOISON .I advice is .B MADV_POPULATE_READ or .BR MADV_POPULATE_WRITE , and populating (prefaulting) page tables failed because a HW poisoned page (HW poisoned pages can, for example, be created using the .B MADV_HWPOISON flag described elsewhere in this page) was encountered. .SH VERSIONS Versions of this system call, implementing a wide variety of .I advice values, exist on many other implementations. Other implementations typically implement at least the flags listed above under .IR "Conventional advice flags" , albeit with some variation in semantics. .P POSIX.1-2001 describes .BR posix_madvise (3) with constants .BR POSIX_MADV_NORMAL , .BR POSIX_MADV_RANDOM , .BR POSIX_MADV_SEQUENTIAL , .BR POSIX_MADV_WILLNEED , and .BR POSIX_MADV_DONTNEED , and so on, with behavior close to the similarly named flags listed above. .SS Linux The Linux implementation requires that the address .I addr be page-aligned, and allows .I length to be zero. If there are some parts of the specified address range that are not mapped, the Linux version of .BR madvise () ignores them and applies the call to the rest (but returns .B ENOMEM from the system call, as it should). .P .I madvise(0,\ 0,\ advice) will return zero iff .I advice is supported by the kernel and can be relied on to probe for support. .SH STANDARDS None. .SH HISTORY First appeared in 4.4BSD. .P Since Linux 3.18, .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb support for this system call is optional, depending on the setting of the .B CONFIG_ADVISE_SYSCALLS configuration option. .SH SEE ALSO .BR getrlimit (2), .BR memfd_secret (2), .BR mincore (2), .BR mmap (2), .BR mprotect (2), .BR msync (2), .BR munmap (2), .BR prctl (2), .BR process_madvise (2), .BR posix_madvise (3), .BR core (5)