linux-kernel - Transparent Hugepage Nit

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Message-ID: <20130115205041.GA12113@rockgeek.org>
Date:	Tue, 15 Jan 2013 13:50:41 -0700
From:	John McCorquodale <mcq@...kgeek.org>
To:	linux-kernel@...r.kernel.org
Subject: Transparent Hugepage Nit

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.

Cheers,

-mcq
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