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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.

              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
              collapsed into huge pages.

       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.

RETURN VALUE

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

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),
       posix_madvise(3), core(5)

COLOPHON

       This  page  is  part of release 5.02 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/.