Common/MemArena: Set MAP_NORESERVE in LazyMemoryRegion on Linux. #12177
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Oh that's fun, FreeBSD apparently doesn't implement |
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Googling around suggests FreeBSD also by-default just lets all allocations go through, so we can just not set the flag there and be fine. |
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Source/Core/Common/MemArenaUnix.cpp
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| @@ -117,14 +117,21 @@ LazyMemoryRegion::~LazyMemoryRegion() | |||
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| #if defined __FreeBSD__ || defined __OpenBSD__ || defined __NetBSD__ | |||
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Maybe we should also check if MAP_NORESERVE is defined? Just in case it gets implemented in the future.
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This came up during the discussion that led to the #12173 fixes.
In short: When
/proc/sys/vm/overcommit_memoryis 0 (which appears to be the default in the Linux kernel), Linux refusesmmaprequests of private anonymous memory that it knows cannot actually be fulfilled by the current amount of free RAM and swap. This causes the fast block lookup optimization to fail on low-memory (<= 4 GB) devices. This behavior makes sense (more or less) if you assume that all the allocated memory will actually be written to, but with our LazyMemoryRegion here, we assume that only a tiny amount will actually be touched before the memory will be reset or free'd. So the suggestion came up to just setMAP_NORESERVE-- which supresses the check and allows the allocation anyway.I think this is reasonable and basically matches what Windows and Android do, so might as well. (For reference, on Android, the default of
/proc/sys/vm/overcommit_memoryis 1, so it never checks for anything and just allows the request to go through. We could setMAP_NORESERVEthere too but it wouldn't really change anything unless someone messed with that setting.)