linux-kernel - Re: [PATCH TRIVIAL] mm: Fix build warning in kmem_cache

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Date:	Wed, 25 Jul 2012 10:28:31 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Glauber Costa <glommer@...allels.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Shuah Khan <shuah.khan@...com>, js1304@...il.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	shuahkhan@...il.com
Subject: Re: [PATCH TRIVIAL] mm: Fix build warning in kmem_cache_create()

On Mon, 23 Jul 2012, Glauber Costa wrote:

> >> worth including unconditionally. Furthermore, the size related checks
> >> certainly make sense and I don't see any harm in having them as well.
> >
> > There is a WARN_ON() there and then it returns NULL!!! Crazy. Causes a
> > NULL pointer dereference later in the caller?
> >
>
> It obviously depends on the caller.

This is a violation of the calling convention to say the least. This means
if you have SLAB_PANIC set and accidentally set the name to NULL the
function will return despite the error and not panic!

> Although most of the calls to kmem_cache_create are made from static
> data, we can't assume that. Of course whoever is using static data
> should do those very same tests from the outside to be safe, but in case
> they do not, this seems to fall in the category of things that make
> debugging easier - even if we later on get to a NULL pointer dereference.
>
> Your mentioned bias towards minimum code size, however, is totally
> valid, IMHO. But I doubt those checks would introduce a huge footprint.
> I would imagine you being much more concerned about being able to wipe
> out entire subsystems like memcg, which will give you a lot more.

They are useless checks since any use of the name will also cause a NULL
pointer dereference. Same is true for interrupt checks. Checks like that
indicate a deterioration of the code base. People are afraid that
something goes wrong because they no longer understand the code so they
build a embroidery around it instead of relying on the already existing
checks at vital places. The embroidery can be useful for debugging thats
why I left it in for the CONFIG_DEBUG_VM but certainly should not be
included in production kernels.

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