Re: Q, slab, kmemleak_erase() and redzone?

[Pekka Enberg]

Catalin Marinas kirjoitti:
> On Tue, 2009-12-01 at 11:49 +0000, Pekka Enberg wrote:
> > hooanon05@xxxxxxxxxxx kirjoitti:
> > > Pekka Enberg:
> > > > We are setting an element in the per CPU array to NULL so the the
> > > > kmemleak code in ____cache_alloc() is safe. Red-zoning is done at the
> > > > _object_ which is not touched by kmemleak. Looking at the oops, it
> > > > does seem likely that you have a bug in your module (or in some other
> > > > part of the kernel).
> > > Thanks for reply.
> > > In ____cache_alloc(), the variable 'ac' is assigned before
> > > cache_alloc_refill() call, and it is used for the parameter of
> > > kmemleak_erase(). The value may be changed by cache_alloc_refill(),
> > > isn't it?
> > No. The pointer returned by cpu_cache_get() is not changed by
> > cache_alloc_refill(). The contents of the array might change, yes. That
> > said, we should check if objp is NULL before calling kmemleak_erase().
>
> Possibly but I don't understand why that's needed. The kmemleak_erase()
> call just sets the ac->entry[ac->avail] to NULL. If ac->avail is 0, it
> doesn't cause any harm.

No, you are absolutely correct. Can you please send an updated patch to 
Catalin that adds a comment on top of the cpu_cache_get() call that 
explains why we need it there?

			Pekka
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