Ubuntu Manpage: madvise - give advice about use of memory

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.) 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, 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_REMOVE; but since Linux
3.5, any filesystem which supports the fallocate(2) FALLOC_FL_PUNCH_HOLE mode also
supports MADV_REMOVE. 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 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. 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.

Since Linux 5.4, automatic scan of eligible areas and replacement 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 mapping 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 CONFIG_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 general 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 determining 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
mappings of the memory. In the event multiple hugepage-aligned/sized areas fail to
collapse, only the most-recently–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 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)). Before Linux 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.

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; however, avoid the actual memory access that
would have been performed 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 populated (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; however, avoid the actual memory access
that would have been performed 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 populated (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 populating (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 permissions or special mappings, for
example, mappings marked with kernel-internal flags such a VM_IO or VM_PFNMAP, or
secret memory 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 populating (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 populating (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
encountered.

VERSIONS

       Since  Linux  3.18,  support for this system call is optional, depending on the setting of
       the CONFIG_ADVISE_SYSCALLS configuration option.

STANDARDS

       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).

       madvise(0, 0, advice) will return zero iff advice is supported by the kernel  and  can  be
       relied on to probe for support.