madvise
Section: System Calls (2)
Updated: 2023-02-05
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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.
SEE ALSO
getrlimit(2),
memfd_secret(2),
mincore(2),
mmap(2),
mprotect(2),
msync(2),
munmap(2),
prctl(2),
process_madvise(2),
posix_madvise(3),
core(5)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- Conventional advice values
-
- Linux-specific advice values
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- STANDARDS
-
- NOTES
-
- Linux notes
-
- SEE ALSO
-
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Time: 01:12:54 GMT, April 25, 2024