linux-kernel - Re: [PATCH] Fix array overflow in CFQ

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Message-ID: <20101019150516.GE857@redhat.com>
Date:	Tue, 19 Oct 2010 11:05:16 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Andi Kleen <andi@...stfloor.org>,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix array overflow in CFQ

On Tue, Oct 19, 2010 at 03:23:22PM +0200, Andi Kleen wrote:
> 
> >         slice = group_slice * count /
> >                 max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
> >                       cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd,
> >cfqg));
> >
> >So for IDLE class, we return immediately from the function and never
> >execute cfqg->busy_queues_avg[IDLE].
> 
> Hmm that's true. But why do you put this into a global variable
> anyways, can't it
> just be a local?

We keep track of average number of queues per group per prio class. So it
can't be local as it historical data.

> >Now to remove the gcc warning we can increase the size of busy_queues_avg[]
> >array but third field should always remain unused.
> >
> It's better to increase the field still I think.

Agreed.

Jens, do you want me to regenerate your patch so that we increase the
size of ->busy_queues_avg[CFQ_PRIO_NR] but not ->service_trees[][].

Thanks
Vivek
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