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

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Date:	Fri, 12 Dec 2008 13:45:43 +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

On Thu, 2008-12-11 at 23:30 +0200, Pekka Enberg wrote:
> Catalin Marinas wrote:
> > This patch adds the callbacks to memleak_(alloc|free) functions from the
> > slub allocator.
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> > Cc: Christoph Lameter <cl@...ux-foundation.org>
> > Cc: Pekka Enberg <penberg@...helsinki.fi>
> 
> Hmm, I'm not sure I understand why struct kmem_cache_cpu ->freelist is 
> never scanned. 

Did you get any false positives? Or were you expecting false negatives
because of freelist scanning which never occurred?

> For SMP, I suppose kmemleak doesn't scan the per-CPU 
> areas?

It should scan the per-CPU areas in the memleak_scan() function:

#ifdef CONFIG_SMP
	/* per-cpu sections scanning */
	for_each_possible_cpu(i)
		scan_block(__per_cpu_start + per_cpu_offset(i),
			   __per_cpu_end + per_cpu_offset(i), NULL);
#endif

>  But for UP, struct kmem_cache is allocated with kmalloc() and 
> that contains struct kmem_cache_cpu as well.

They should be scanned as well.

> And I suppose we never scan struct pages either. Otherwise ->freelist 
> there would be a problem as well.

It was scanning the mem_map arrays in the past but removed this part and
haven't seen any problems (on ARM).

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.

-- 
Catalin

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