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.
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.
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 backed by transparent hugepages.
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.
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.
