madvise(2)



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 sys-
       tem 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, 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 ac-
              cesses 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 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/vm/ksm.txt 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.  Currently, Transparent Huge Pages
              work only with private anonymous pages (see mmap(2)).  The  ker-
              nel 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  Documenta-
              tion/vm/transhuge.txt for more details.

              The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations  are  available
              only   if   the  kernel  was  configured  with  CONFIG_TRANSPAR-
              ENT_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 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)).  On a swapless system, freeing pages  in  a
              given range happens 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  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.

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_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.

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  mad-
       vise()  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 4.16 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                             2017-09-15                        MADVISE(2)

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