madvise(2) — Linux manual page


MADVISE(2)                Linux Programmer's Manual               MADVISE(2)

NAME         top

       madvise - give advice about use of memory

SYNOPSIS         top

       #include <sys/mman.h>

       int madvise(void *addr, size_t length, int advice);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           Since glibc 2.19:
           Up to and including glibc 2.19:

DESCRIPTION         top

       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

   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

              No special treatment.  This is the default.

              Expect page references in random order.  (Hence, read ahead
              may be less useful than normally.)

              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.)

              Expect access in the near future.  (Hence, it might be a good
              idea to read some pages ahead.)

              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

       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

       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

       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

       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         top

       On success, madvise() returns zero.  On error, it returns -1 and
       errno is set appropriately.

ERRORS         top

       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

       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         top

       Since Linux 3.18, support for this system call is optional, depending
       on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.

CONFORMING TO         top

       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_WILLNEED, and POSIX_MADV_DONTNEED, and so on, with
       behavior close to the similarly named flags listed above.

NOTES         top

   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).

SEE ALSO         top

       getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2),
       prctl(2), posix_madvise(3), core(5)

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-04-11                       MADVISE(2)

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