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authorLinus Torvalds <torvalds@linux-foundation.org>2018-06-04 12:34:27 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-06-04 12:34:27 -0700
commiteeee3149aaa022145b2659e3b0601dc705d69402 (patch)
tree1b537cccc059cab62d7d2e22b7583b192d9e8c15 /Documentation/vm/overcommit-accounting
parentc5e7a7ea22d5677f7c70028908372cff6948ecdc (diff)
parenta49d9c0ae46e149a22aefa8251d07dddd5611851 (diff)
Merge tag 'docs-4.18' of git://git.lwn.net/linux
Pull documentation updates from Jonathan Corbet: "There's been a fair amount of work in the docs tree this time around, including: - Extensive RST conversions and organizational work in the memory-management docs thanks to Mike Rapoport. - An update of Documentation/features from Andrea Parri and a script to keep it updated. - Various LICENSES updates from Thomas, along with a script to check SPDX tags. - Work to fix dangling references to documentation files; this involved a fair number of one-liner comment changes outside of Documentation/ ... and the usual list of documentation improvements, typo fixes, etc" * tag 'docs-4.18' of git://git.lwn.net/linux: (103 commits) Documentation: document hung_task_panic kernel parameter docs/admin-guide/mm: add high level concepts overview docs/vm: move ksm and transhuge from "user" to "internals" section. docs: Use the kerneldoc comments for memalloc_no*() doc: document scope NOFS, NOIO APIs docs: update kernel versions and dates in tables docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge docs/vm: transhuge: minor updates docs/vm: transhuge: change sections order Documentation: arm: clean up Marvell Berlin family info Documentation: gpio: driver: Fix a typo and some odd grammar docs: ranoops.rst: fix location of ramoops.txt scripts/documentation-file-ref-check: rewrite it in perl with auto-fix mode docs: uio-howto.rst: use a code block to solve a warning mm, THP, doc: Add document for thp_swpout/thp_swpout_fallback w1: w1_io.c: fix a kernel-doc warning Documentation/process/posting: wrap text at 80 cols docs: admin-guide: add cgroup-v2 documentation Revert "Documentation/features/vm: Remove arch support status file for 'pte_special'" Documentation: refcount-vs-atomic: Update reference to LKMM doc. ...
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-The Linux kernel supports the following overcommit handling modes
-
-0 - Heuristic overcommit handling. Obvious overcommits of
- address space are refused. Used for a typical system. It
- ensures a seriously wild allocation fails while allowing
- overcommit to reduce swap usage. root is allowed to
- allocate slightly more memory in this mode. This is the
- default.
-
-1 - Always overcommit. Appropriate for some scientific
- applications. Classic example is code using sparse arrays
- and just relying on the virtual memory consisting almost
- entirely of zero pages.
-
-2 - Don't overcommit. The total address space commit
- for the system is not permitted to exceed swap + a
- configurable amount (default is 50%) of physical RAM.
- Depending on the amount you use, in most situations
- this means a process will not be killed while accessing
- pages but will receive errors on memory allocation as
- appropriate.
-
- Useful for applications that want to guarantee their
- memory allocations will be available in the future
- without having to initialize every page.
-
-The overcommit policy is set via the sysctl `vm.overcommit_memory'.
-
-The overcommit amount can be set via `vm.overcommit_ratio' (percentage)
-or `vm.overcommit_kbytes' (absolute value).
-
-The current overcommit limit and amount committed are viewable in
-/proc/meminfo as CommitLimit and Committed_AS respectively.
-
-Gotchas
--------
-
-The C language stack growth does an implicit mremap. If you want absolute
-guarantees and run close to the edge you MUST mmap your stack for the
-largest size you think you will need. For typical stack usage this does
-not matter much but it's a corner case if you really really care
-
-In mode 2 the MAP_NORESERVE flag is ignored.
-
-
-How It Works
-------------
-
-The overcommit is based on the following rules
-
-For a file backed map
- SHARED or READ-only - 0 cost (the file is the map not swap)
- PRIVATE WRITABLE - size of mapping per instance
-
-For an anonymous or /dev/zero map
- SHARED - size of mapping
- PRIVATE READ-only - 0 cost (but of little use)
- PRIVATE WRITABLE - size of mapping per instance
-
-Additional accounting
- Pages made writable copies by mmap
- shmfs memory drawn from the same pool
-
-Status
-------
-
-o We account mmap memory mappings
-o We account mprotect changes in commit
-o We account mremap changes in size
-o We account brk
-o We account munmap
-o We report the commit status in /proc
-o Account and check on fork
-o Review stack handling/building on exec
-o SHMfs accounting
-o Implement actual limit enforcement
-
-To Do
------
-o Account ptrace pages (this is hard)