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.

Linux¶

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.