linux-kernel - Re: [PATCH 1/2] vmscan: fix initial shrinker size handling

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Message-ID: <20110823000054.GW3162@dastard>
Date:	Tue, 23 Aug 2011 10:00:54 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Konstantin Khlebnikov <khlebnikov@...nvz.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] vmscan: fix initial shrinker size handling

On Mon, Aug 22, 2011 at 04:38:21PM -0700, Andrew Morton wrote:
> On Tue, 23 Aug 2011 09:22:57 +1000
> Dave Chinner <david@...morbit.com> wrote:
> 
> > On Mon, Aug 22, 2011 at 02:17:21PM +0300, Konstantin Khlebnikov wrote:
> > > Shrinker function can returns -1, it means it cannot do anything without a risk of deadlock.
> > > For example prune_super() do this if it cannot grab superblock refrence, even if nr_to_scan=0.
> > > Currenly we interpret this like ULONG_MAX size shrinker, evaluate total_scan according this,
> > > and next time this shrinker can get really big pressure. Let's skip such shrinkers instead.
> > > 
> > > Also make total_scan signed, otherwise check (total_scan < 0) below never works.
> > 
> > I've got a patch set I am going to post out today that makes this
> > irrelevant.
> 
> Well, how serious is the bug?  If it's a non-issue then we can leave
> the fix until 3.1.  If it's a non-non-issue then we'd need a minimal
> patch to fix up 3.1 and 3.0.x.

I'm pretty sure it's a non-issue. I'm pretty sure all of the
shrinkers return a count >= 0 rather than -1 when passed nr_to_scan
== 0 (i.e.  they skip the GFP_NOFS checking), so getting a max_pass
of -1 isn't going to happen very often....

And with total_scan being unsigned, the negative check is followed
by a "if (total_scan > max_pass * 2)" check, which will catch
numbers that would have gone negative anyway because max_pass won't
be negative....

So, grotty code but I don't think there is even a problem that can
be tripped right now.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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