linux-kernel - Re: [PATCH] slab: deal with NULL pointers passed to kmem_cache

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Date:	Wed, 21 Mar 2007 15:11:09 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Jarek Poplawski <jarkao2@...pl>,
	Eric Dumazet <dada1@...mosbay.com>,
	Andrew Morton <akpm@...ux-foundation.org>, mpm@...enic.com,
	Christoph Lameter <clameter@....com>,
	"ast\\@domdv\\.de" <ast@...dv.de>,
	"linux-kernel\\@vger\\.kernel\\.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slab: deal with NULL pointers passed to kmem_cache_free

On Wednesday, 21 March 2007 14:36, Pekka J Enberg wrote:
> On Wed, 21 Mar 2007, Jarek Poplawski wrote:
> > With  __kmem_cache_free you would set #1 I hope, but if
> > nobody would use this - debugging time wouldn't change.
> 
> I think you got it backwards. I suggested making the _current_ 
> kmem_cache_free() deal with NULL (so everyone will get it) and add a new 
> optimized __kmem_cache_free() for those call-sites that really need it.
> 
> On Wed, 21 Mar 2007, Jarek Poplawski wrote:
> > This could be acceptable, if there were no problems
> > with fixing the errors. But there are problems - bugs
> > like this aren't fixed on time - maybe because people
> > waste too much time per bug?
> 
> You're barking up the wrong tree here, Jarek. I strongly feel that we 
> should be more defensive in the slab for the exact reasons you outlined. 
> There's bunch of bug reports people seem to dismiss as slab errors where 
> in fact it's caused by a buggy caller.
> 
> That said, Eric and Andrew make a good point about kmem_cache_free() being 
> in super-hot paths which clearly must be addressed. The only reason 
> holding me back is the fact that I don't know what those super-hot 
> call-sites are (with the exception of network skb allocation) so I am 
> really in no position to make that patch.

IMHO one way to find them is to actually slow down kmem_cache_free() and see
where the performance is hurt.

Greetings,
Rafael
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