netdev - Re: [bug, netconsole, SLUB] BUG skbuff_head

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Date:	Fri, 18 Jul 2008 02:16:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [bug, netconsole, SLUB] BUG skbuff_head_cache: Poison
	overwritten


* Vegard Nossum <vegard.nossum@...il.com> wrote:

> On Fri, Jul 18, 2008 at 1:52 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >  If only we had some kernel technology that could track and validate
> >  memory accesses, and point out the cases where we access uninitialized
> >  memory, just like Valgrind?
> >
> > ... something like kmemcheck? ;-)
> 
> Cool :)
> 
> > So i booted that box with tip/master and kmemcheck enabled. (plus a few
> > fixlets to make networking allocations be properly tracked by
> > kmemcheck.)
> >
> > It was a slow bootup and long wait, but it gave a few hits here:
> 
> Hm, if you think it was that slow, I am suspecting you were also using 
> SLUB debugging.

nope:

 # CONFIG_SLUB_DEBUG is not set
 CONFIG_SLUB=y

> This can actually be negative, since now SLUB will access the objects 
> (+redzone +padding) and possibly trick kmemcheck into thinking they 
> were initialized in the first place.
> 
> But what we are really looking for is "read from freed memory" 
> messages. So I would actually recommend this: Disable kmemcheck's 
> reporting of uninitialized memory, simply to make it easier to spot 
> the "freed" messages more easily.
> 
> Maybe something like this (warning: whitespace-munged):

ok, applied this too.

> If this only happens during boot, it would also be a good idea to 
> simply reboot the machine a lot...

yeah, i've got a script for that. Will try it overnight.

	Ingo
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