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madvise(2)                    System Calls Manual                   madvise(2)

NAME
       madvise - give advice about use of memory

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #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
       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.)   Subse-
       quently, 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 con-
              tents  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_RE-
              MOVE;  but  since  Linux  3.5, any filesystem which supports the
              fallocate(2) FALLOC_FL_PUNCH_HOLE mode  also  supports  MADV_RE-
              MOVE.   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  behav-
              ior, 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 mem-
              ory 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  sin-
              gle  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/ad-
              min-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 pre-
              served (i.e., when next accessed, the same content will be visi-
              ble, 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  call-
              ing 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 spec-
              ified 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 ac-
              cesses 1 byte will result in 2 MB of wired memory instead of one
              4 KB  page).  See the Linux kernel source file Documentation/ad-
              min-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 nec-
              essary.  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 replace-
              ment 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  map-
              ping 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  CON-
              FIG_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  gen-
              eral  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  de-
              termining  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  map-
              pings   of   the   memory.   In  the  event  multiple  hugepage-
              aligned/sized  areas  fail  to  collapse,  only   the   most-re-
              cently-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 speci-
              fied 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 oper-
              ation, 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 subse-
              quent 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 in-
              stantly, 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 pro-
              cesses.

              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 ig-
              nored 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; how-
              ever, avoid the actual memory access that would have  been  per-
              formed 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  popu-
              lated  (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; how-
              ever, avoid the actual memory access that would have  been  per-
              formed 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 popu-
              lated (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
       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.

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

       EFAULT advice  is  MADV_POPULATE_READ or MADV_POPULATE_WRITE, and popu-
              lating (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 per-
              missions or special mappings, for example, mappings marked  with
              kernel-internal  flags such a VM_IO or VM_PFNMAP, or secret mem-
              ory 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 popu-
              lating (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  popu-
              lating  (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 encoun-
              tered.

VERSIONS
       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_NOR-
       MAL, 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  mad-
       vise()  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
       None.

HISTORY
       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
       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)

Linux man-pages 6.04              2023-04-03                        madvise(2)

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