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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 slighly more memory in this mode. This is the
+ default.
+1 - Always overcommit. Appropriate for some scientific
+ applications.
+2 - Don't overcommit. The total address space commit
+ for the system is not permitted to exceed swap + a
+ configurable percentage (default is 50) of physical RAM.
+ Depending on the percentage 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.
+The overcommit policy is set via the sysctl `vm.overcommit_memory'.
+The overcommit percentage is set via `vm.overcommit_ratio'.
+The current overcommit limit and amount committed are viewable in
+/proc/meminfo as CommitLimit and Committed_AS respectively.
+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
+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)