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