#[repr(i32)]
pub enum Advice {
Show 16 variants
Normal,
Random,
Sequential,
WillNeed,
DontNeed,
Free,
Remove,
DontFork,
DoFork,
Mergeable,
Unmergeable,
HugePage,
NoHugePage,
DontDump,
DoDump,
HwPoison,
}Expand description
Values supported by Mmap::advise and MmapMut::advise functions. See madvise() map page.
Variants
Normal
MADV_NORMAL
No special treatment. This is the default.
Random
MADV_RANDOM
Expect page references in random order. (Hence, read ahead may be less useful than normally.)
Sequential
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.)
WillNeed
MADV_WILLNEED
Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.)
DontNeed
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.)
Free
MADV_FREE - Linux (since Linux 4.5) and Darwin
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.
Remove
MADV_REMOVE - Linux only (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.
DontFork
MADV_DONTFORK - Linux only (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.)
DoFork
MADV_DOFORK - Linux only (since Linux 2.6.16)
Undo the effect of MADV_DONTFORK, restoring the default behavior, whereby a mapping is inherited across fork(2).
Mergeable
MADV_MERGEABLE - Linux only (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.
Unmergeable
MADV_UNMERGEABLE - Linux only (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.
HugePage
MADV_HUGEPAGE - Linux only (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.
NoHugePage
MADV_NOHUGEPAGE - Linux only (since Linux 2.6.38)
Ensures that memory in the address range specified by addr and length will not be backed by transparent hugepages.
DontDump
MADV_DONTDUMP - Linux only (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)).
DoDump
MADV_DODUMP - Linux only (since Linux 3.4)
Undo the effect of an earlier MADV_DONTDUMP.
HwPoison
MADV_HWPOISON - Linux only (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.