madvise(2) — Linux manual page

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madvise(2)                 System Calls Manual                madvise(2)

NAME         top

       madvise - give advice about use of memory

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/mman.h>

       int madvise(void addr[.length], 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         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.  madvise() only operates on whole pages,
       therefore addr must be page-aligned.  The value of 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.

       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, 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.)

              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.

   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 range of the backing store (see
              fallocate(2)).  Subsequent accesses in the specified
              address range will see data with a value of zero.

              The specified address range must be mapped shared and
              writable.  This flag cannot be applied to locked pages, or
              VM_PFNMAP pages.

              In the initial implementation, only tmpfs(5) supported
              MADV_REMOVE; but since Linux 3.5, any filesystem which
              supports the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also
              supports MADV_REMOVE.  Filesystems which do not support
              MADV_REMOVE fail with the error EOPNOTSUPP.

              Support for the Huge TLB filesystem was added in Linux
              v4.3.

       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.  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.

              Since Linux 5.4, automatic scan of eligible areas and
              replacement by huge pages works with private anonymous
              pages (see 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 —including tmpfs (see tmpfs(2))— 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.

              The VMA must not be marked VM_NOHUGEPAGE, VM_HUGETLB,
              VM_IO, VM_DONTEXPAND, VM_MIXEDMAP, or 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 PR_SET_THP_DISABLE set (see
              prctl(2)).

              The MADV_HUGEPAGE, MADV_NOHUGEPAGE, and MADV_COLLAPSE
              operations are available only if the kernel was configured
              with CONFIG_TRANSPARENT_HUGEPAGE and file/shmem memory is
              only supported if the kernel was configured with
              CONFIG_READ_ONLY_THP_FOR_FS.

       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_COLLAPSE (since Linux 6.1)
              Perform a best-effort synchronous collapse of the native
              pages mapped by the memory range into Transparent Huge
              Pages (THPs).  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.

              MADV_COLLAPSE supports private anonymous pages (see
              mmap(2)), shmem pages, and file-backed pages.  See
              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.  MADV_COLLAPSE will automatically clamp
              the provided range to be hugepage-aligned.

              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.

              MADV_COLLAPSE is independent of any sysfs (see sysfs(5))
              setting under /sys/kernel/mm/transparent_hugepage, both in
              terms of determining THP eligibility, and allocation
              semantics.  See Linux kernel source file
              Documentation/admin-guide/mm/transhuge.rst for more
              information.  MADV_COLLAPSE also ignores 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 VM_NOHUGEPAGE is still
              respected).

              When the system has multiple NUMA nodes, the hugepage will
              be allocated from the node providing the most native
              pages.

              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–failed code will be set in errno.

       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)).  Before Linux 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.

       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.

              In contrast to MAP_POPULATE, 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.

              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 SIGBUS signal is not generated;
              instead, an error is returned.

              If MADV_POPULATE_READ succeeds, all page tables have been
              populated (prefaulted) readable once.  If
              MADV_POPULATE_READ fails, some page tables might have been
              populated.

              MADV_POPULATE_READ cannot be applied to mappings without
              read permissions and special mappings, for example,
              mappings marked with kernel-internal flags such as
              VM_PFNMAP or VM_IO, or secret memory regions created using
              memfd_secret(2).

              Note that with MADV_POPULATE_READ, the process can be
              killed at any moment when the system runs out of memory.

       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.

              In contrast to 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).

              Depending on the underlying mapping, preallocate memory or
              read the underlying file; files with holes will
              preallocate blocks.  If populating fails, a SIGBUS signal
              is not generated; instead, an error is returned.

              If MADV_POPULATE_WRITE succeeds, all page tables have been
              populated (prefaulted) writable once.  If
              MADV_POPULATE_WRITE fails, some page tables might have
              been populated.

              MADV_POPULATE_WRITE cannot be applied to mappings without
              write permissions and special mappings, for example,
              mappings marked with kernel-internal flags such as
              VM_PFNMAP or VM_IO, or secret memory regions created using
              memfd_secret(2).

              Note that with MADV_POPULATE_WRITE, the process can be
              killed at any moment when the system runs out of memory.

RETURN VALUE         top

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

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.

       EBUSY  (for MADV_COLLAPSE) Could not charge hugepage to cgroup:
              cgroup limit exceeded.

       EFAULT advice is MADV_POPULATE_READ or MADV_POPULATE_WRITE, and
              populating (prefaulting) page tables failed because a
              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 MADV_HWPOISON
              flag described elsewhere in this page).

       EINVAL addr is not page-aligned or length is negative.

       EINVAL advice is not a valid.

       EINVAL advice is MADV_COLD or MADV_PAGEOUT and the specified
              address range includes locked, Huge TLB pages, or
              VM_PFNMAP pages.

       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.

       EINVAL advice is MADV_POPULATE_READ or 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 VM_IO or
              VM_PFNMAP, or secret memory regions created using
              memfd_secret(2).

       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 (for MADV_COLLAPSE) Not enough memory: could not allocate
              hugepage.

       ENOMEM Addresses in the specified range are not currently mapped,
              or are outside the address space of the process.

       ENOMEM advice is MADV_POPULATE_READ or MADV_POPULATE_WRITE, and
              populating (prefaulting) page tables failed because there
              was not enough memory.

       EPERM  advice is MADV_HWPOISON, but the caller does not have the
              CAP_SYS_ADMIN capability.

       EHWPOISON
              advice is MADV_POPULATE_READ or MADV_POPULATE_WRITE, and
              populating (prefaulting) page tables failed because a HW
              poisoned page (HW poisoned pages can, for example, be
              created using the MADV_HWPOISON flag described elsewhere
              in this page) was encountered.

VERSIONS         top

       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.

   Linux
       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).

       madvise(0, 0, advice) will return zero iff advice is supported by
       the kernel and can be relied on to probe for support.

STANDARDS         top

       None.

HISTORY         top

       First appeared in 4.4BSD.

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

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                     madvise(2)

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