madvise

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



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.

       MADV_COLD (since Linux 5.4)
              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.

       MADV_PAGEOUT (since Linux 5.4)
              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.

RETURN VALUE
       On success, madvise() returns zero.  On error, it returns -1 and errno is
       set to indicate the error.

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).

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

COLOPHON
       This page is part of release 5.12 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                              2021-03-22                         MADVISE(2)