Ubuntu Manpage: madvise - give advice about use of memory

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).

COLOPHON

       This  page  is  part of release 5.05 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/.