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SubjectRe: [PATCH 05/15] kmemleak: Add the slub memory allocation/freeing hooks
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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@arm.com>
> > Cc: Christoph Lameter <cl@linux-foundation.org>
> > Cc: Pekka Enberg <penberg@cs.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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