linux-kernel - Re: [PATCH 14/20] mm: Remove i_mmap

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Date:	Thu, 21 Apr 2011 15:32:35 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH 14/20] mm: Remove i_mmap_lock lockbreak

On Tue, 2011-04-19 at 13:07 -0700, Andrew Morton wrote:
> On Fri, 01 Apr 2011 14:13:12 +0200
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > Hugh says:
> >  "The only significant loser, I think, would be page reclaim (when
> >   concurrent with truncation): could spin for a long time waiting for
> >   the i_mmap_mutex it expects would soon be dropped? "
> > 
> > Counter points:
> >  - cpu contention makes the spin stop (need_resched())
> >  - zap pages should be freeing pages at a higher rate than reclaim
> >    ever can
> > 
> > I think the simplification of the truncate code is definately worth it.
> 
> Well, we don't need to guess.  These things are testable!

I suppose you're right, but I'm having a bit of a hard time coming up
with a sensible (reproducible) test case for the page reclaim part of
this problem set.

I'll try running 3 cyclic file scanners sized such that 2 exceed the
memory footprint of the machine and truncate the 3rd's file after
warming up.

That is, unless someone has a saner idea..

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