linux-kernel - Re: Memory overcommit

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Message-ID: <Pine.LNX.4.64.0910301558050.3974@sister.anvils>
Date:	Fri, 30 Oct 2009 16:24:26 +0000 (GMT)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	Vedran Furač <vedran.furac@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	minchan.kim@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Memory overcommit

On Fri, 30 Oct 2009, Andrea Arcangeli wrote:
> 
> It is a guess in the sense to guarantee no ENOMEM it has to take into
> account the worst possible case, that is all shared lib MAP_PRIVATE
> mappings are cowed, which is very far from reality.

A MAP_PRIVATE area is only counted into Committed_AS when it is or
has in the past been PROT_WRITE.  I think it's up to the ELF header
of the shared library whether a section is PROT_WRITE or not; but it
looks like many are not, so Committed_AS should be (a little) nearer
reality than you fear.

Though we do account for Committed_AS, even while allowing overcommit,
we do not at present account for Committed_AS per mm.  Seeing David
and KAMEZAWA-san debating over total_vm versus rss versus anon_rss,
I wonder whether such a "commit" count might be a better measure for
OOM choices (but shmem is as usual awkward: though accounted just once
in Committed_AS, it would probably have to be accounted to every mm
that maps it).  Just an idea to throw into the mix.

Hugh
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