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From: Dave Chinner <david@fromorbit.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] vmscan: fix initial shrinker size handling
Date: Tue, 23 Aug 2011 10:00:54 +1000	[thread overview]
Message-ID: <20110823000054.GW3162@dastard> (raw)
In-Reply-To: <20110822163821.e746ab25.akpm@linux-foundation.org>

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@fromorbit.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@fromorbit.com

WARNING: multiple messages have this Message-ID (diff)
From: Dave Chinner <david@fromorbit.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] vmscan: fix initial shrinker size handling
Date: Tue, 23 Aug 2011 10:00:54 +1000	[thread overview]
Message-ID: <20110823000054.GW3162@dastard> (raw)
In-Reply-To: <20110822163821.e746ab25.akpm@linux-foundation.org>

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@fromorbit.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@fromorbit.com

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  reply	other threads:[~2011-08-23  0:01 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-08-22 11:17 [PATCH 1/2] vmscan: fix initial shrinker size handling Konstantin Khlebnikov
2011-08-22 11:17 ` Konstantin Khlebnikov
2011-08-22 11:17 ` [PATCH 2/2] vmscan: use atomic-long for shrinker batching Konstantin Khlebnikov
2011-08-22 11:17   ` Konstantin Khlebnikov
2011-08-22 21:39   ` Andrew Morton
2011-08-22 21:39     ` Andrew Morton
2011-08-22 23:26   ` Dave Chinner
2011-08-22 23:26     ` Dave Chinner
2011-08-23  6:56   ` Konstantin Khlebnikov
2011-08-23  6:56     ` Konstantin Khlebnikov
2011-08-22 21:30 ` [PATCH 1/2] vmscan: fix initial shrinker size handling Andrew Morton
2011-08-22 21:30   ` Andrew Morton
2011-08-22 23:28   ` Dave Chinner
2011-08-22 23:28     ` Dave Chinner
2011-08-23  6:47   ` Konstantin Khlebnikov
2011-08-23  6:47     ` Konstantin Khlebnikov
2011-08-22 23:22 ` Dave Chinner
2011-08-22 23:22   ` Dave Chinner
2011-08-22 23:38   ` Andrew Morton
2011-08-22 23:38     ` Andrew Morton
2011-08-23  0:00     ` Dave Chinner [this message]
2011-08-23  0:00       ` Dave Chinner
2011-09-13 18:38       ` Johannes Weiner
2011-09-13 18:38         ` Johannes Weiner
2011-10-05 21:52         ` Johannes Weiner
2011-10-05 21:52           ` Johannes Weiner
2011-10-05 21:58           ` Andrew Morton
2011-10-05 21:58             ` Andrew Morton

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