linux-kernel - Re: [uml-devel] [PATCH 5/6] slab: Annotate slab

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Message-ID: <84144f020809030227jce7d3c2wab9e9c00b2fcdfe5@mail.gmail.com>
Date:	Wed, 3 Sep 2008 12:27:12 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Steve VanDeBogart" <vandebo-lkml@...dbox.net>
Cc:	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	"Ingo Molnar" <mingo@...e.hu>, dkegel@...gle.com,
	jiayingz@...gle.com, "John Reiser" <jreiser@...wagon.com>
Subject: Re: [uml-devel] [PATCH 5/6] slab: Annotate slab

Hi Steve,

On Wed, Sep 3, 2008 at 8:08 AM, Steve VanDeBogart
<vandebo-lkml@...dbox.net> wrote:
> Thank you for pointing out the location of the kmemcheck code.  I looked
> around briefly but didn't find the latest version.  It does seem that
> kmemcheck and Valgrind annotations want to live in the same places so it
> makes perfect sense to combine them where possible.  I'm not exactly sure
> what you find so ugly about the Valgrind annotations, please elaborate.

Upper case macros and littering #ifdefs in the code. See the kmemcheck
hooks how to do it in a clean way.

On Wed, Sep 3, 2008 at 8:08 AM, Steve VanDeBogart
<vandebo-lkml@...dbox.net> wrote:
> It is true that code above the allocator should not be touching free'd
> slab objects.  However, it is also true that objects from slabs that
> have a constructor should retain their per byte un/initialized state
> through allocation and free cycles (just the semantic of slabs with
> constructors AFAICT).

Sorry for being unclear, sure, object should be marked as initialized
when they're returned from kmem_cache_alloc(). However, what I
disagree with is *where* you're marking them as initialized. Surely
they're not semantically initialized when the slabs are allocated
(although technically they are).

On Wed, Sep 3, 2008 at 8:08 AM, Steve VanDeBogart
<vandebo-lkml@...dbox.net> wrote:
> Ideally, we'd tell Valgrind that the bytes of a free'd slab object are
> no longer accessible, but the initialized state should remain the same
> until the object is made accessible again by the next allocation of
> the object.  Unfortunately, the compression method for A & V bits in
> Valgrind doesn't allow a region to be inaccessible and retain validness
> bits.

I don't see why you should mark them initialized all the time. Just
mark them as uninitialized on kmem_cache_free() and again as
initialized when they're about to be returned from kmem_cache_alloc()
like we do in kmemcheck.

                              Pekka
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