linux-kernel - Re: [RFC] kmemcheck: trap uses of uninitialized memory (v2)

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Message-ID: <Pine.LNX.4.64.0711291127470.26543@sbz-30.cs.Helsinki.FI>
Date:	Thu, 29 Nov 2007 11:39:37 +0200 (EET)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Vegard Nossum <vegard.nossum@...il.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] kmemcheck: trap uses of uninitialized memory (v2)

Hi,

On Nov 29, 2007 9:02 AM, Pekka Enberg <penberg@...helsinki.fi> wrote:
> > Is it really necessary to track every memory address? Tracking slab
> > objects would require far less memory. You might also want to make
> > kzalloc() and GFP_ZERO mark the memory area as initialized to avoid
> > some page faults.

On Thu, 29 Nov 2007, Vegard Nossum wrote:
> Yes, we are in fact only tracking the memory within SLUB allocations
> (minus what SLUB itself needs for bookkeeping -- like the caches).

Yeah but you didn't answer my question: why do we track every memory 
address instead of slab objects? What's the benefit? Like I already said, 
tracking slab objects would require much less memory which makes the 
thing more practical. It also reduces the number of false positives (the 
CONFIG_OPTIMIZE_FOR_SIZE problem). And we already have slab poisoning to 
cover the cases we would not catch with this scheme.

On Thu, 29 Nov 2007, Vegard Nossum wrote:
> As for the kzalloc() and GFP_ZERO, I believe these will write zeros to
> the data in question before the memory is returned to the caller. In
> that case, the area will be "automatically" set to initialized since
> these writes are also intercepted by kmemcheck.

Yes, and what I proposed is as a potential optimization. Debugging aids 
need to be fast enough to be practical.

			Pekka
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