linux-kernel - Re: slub: Lockout validation scans during freeing of object

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Message-ID: <20111122193231.GB1627@x4.trippels.de>
Date:	Tue, 22 Nov 2011 20:32:31 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Christian Kujau <lists@...dbynature.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Alex,Shi" <alex.shi@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: slub: Lockout validation scans during freeing of object

On 2011.11.22 at 13:20 -0600, Christoph Lameter wrote:
> On Tue, 22 Nov 2011, Markus Trippelsdorf wrote:
> 
> > On 2011.11.22 at 11:40 -0600, Christoph Lameter wrote:
> > > On Tue, 22 Nov 2011, Eric Dumazet wrote:
> > >
> > > > This seems better, but I still have some warnings :
> > >
> > > Trying to reproduce with a kernel configured to do preempt. This is
> > > actually quite interesting since its always off by 1.
> >
> > BTW there are some obvious overflows in the "slabinfo -l" output on my machine:
> 
> Could you get me the value of the "slabs" field for the slabs showing the
> wierd values. I.e. do
> 
> cat /sys/kernel/slab/signal_cache/slabs
> 
> > signal_cache               268     920   360.4K 18446744073709551614/7/24   17 2  31  68 A
> 

It's quite easy to explain. You're using unsigned ints in:
snprintf(dist_str, 40, "%lu/%lu/%d", s->slabs - s->cpu_slabs, s->partial, s->cpu_slabs);

and  (s->slabs - s->cpu_slabs) can get negative. For example:

task_struct                269    1504   557.0K 18446744073709551601/5/32   21 3  29  72

Here s-slabs is 17 and s->cpu_slabs is 32. 
That gives: 17-32=18446744073709551601.

-- 
Markus
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