linux-kernel - Re: Transparent Hugepage Nit

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Message-ID: <m2zk0ayw65.fsf@firstfloor.org>
Date:	Tue, 15 Jan 2013 13:26:58 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	John McCorquodale <mcq@...kgeek.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Transparent Hugepage Nit

John McCorquodale <mcq@...kgeek.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.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only
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