linux-kernel - Re: [PATCH 05/15] kmemleak: Add the slub memory allocation/freeing hooks

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Message-Id: <1229614135.16418.33.camel@pc1117.cambridge.arm.com>
Date:	Thu, 18 Dec 2008 15:28:55 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 05/15] kmemleak: Add the slub memory allocation/freeing
	hooks

Hi Pekka,

On Thu, 2008-12-18 at 12:51 +0200, Pekka Enberg wrote:
> On Fri, 2008-12-12 at 13:45 +0000, Catalin Marinas wrote:
> > Pekka Enberg wrote:
> > > Hmm, I'm not sure I understand why struct kmem_cache_cpu ->freelist is 
> > > never scanned. 
> >
> > Why would the ->freelist be a problem? I don't fully understand the slub
> > allocator. Aren't objects added to the freelist only after they were
> > freed? In __slab_alloc there seems to be a line:
> > 
> > c->page->freelist = NULL;
> > 
> > so the freelist won't count as a reference anymore. After freeing an
> > object, kmemleak no longer cares about references to it.
> 
> I think we're talking about two different things here. Don't we then
> have false negatives because we reach ->freelist of struct
> kmem_cache_cpu which contains a pointer to an object that is free'd
> (take a look at slab_free() fast-path)?

Just to make sure I understand it correctly, the slab_free() fast path
stores the pointer to the freed object into c->freelist. However, this
object is no longer tracked by kmemleak because of the
kmemleak_free_recursive() call at the beginning of this function (false
negatives make sense only for allocated objects).

On the slab_alloc() fast path, the pointer to an allocated object is
obtained from the c->freelist pointer but this seems to be overridden by
the pointer to the next free object, object[c->offset], which isn't yet
tracked by kmemleak. So, during a memory scan, it shouldn't matter that
the kmem_cache_cpu structures are called as they don't contain any
pointer to an allocated (not free) object.

The new slabs are allocated with alloc_pages() and these are not tracked
by kmemleak.

Is my understanding correct? Thanks.

-- 
Catalin

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