Messages in this thread | | | Date | Wed, 16 Jan 2013 09:50:20 -0600 | From | Rob Landley <> | Subject | Re: Transparent Hugepage Nit |
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On 01/15/2013 03:26:58 PM, Andi Kleen wrote: > John McCorquodale <mcq@rockgeek.org> writes: > > > Suppose a hugepage-aligned mmap(MAP_ANONYMOUS) mapping has been > madvise()d > > HUGEPAGE. If a subeqeuent call to mremap() grows the mapping and > has to > > move the mapping, the hugepage-alignment is not preserved in the > choice of > > new address (in 3.7.2). > > > > I can workaround this by doing a 1-hugepage-oversized remap to find > a new > > aligned address and then size it back down MREMAP_FIXED, but that's > probably > > a lot of frags to 4k pages and back that aren't necessary. > > > > Should it not be the case that mremap(MAYMOVE) on something advised > hugepage > > ALWAYS chooses a hugepage-aligned address? This would be handy > when doing the > > initial allocation too: mmap, madvise, mremap (to the same size) to > get > > alignment. > > The hole searching currently doesn't know anything about transparent > huge pages. There were some discussions on fixing it. But it's > essentially a trade off between memory fragmentation and huge page > optimization: aggressively aligning to 2MB can lose address space > in holes. > > Usually if the program uses large enough mappings and enough memory > it shouldn't be a problem.
Possibly we should just document that doing mremap() on a hugepage loses the hugepageness, and consider it pilot error to do that?
Rob
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