Re: [PATCH][v2] fork_init: fix division by zero

From: Andrew Morton <akpm@linux-foundation.org>
To: Paul Mackerras <paulus@samba.org>
Cc: wd@denx.de, dzu@denx.de, linux-kernel@vger.kernel.org,
	miltonm@bga.com, linuxppc-dev@ozlabs.org,
	Geert.Uytterhoeven@sonycom.com, yanok@emcraft.com,
	viro@zeniv.linux.org.uk
Subject: Re: [PATCH][v2] fork_init: fix division by zero
Date: Thu, 11 Dec 2008 17:07:02 -0800	[thread overview]
Message-ID: <20081211170702.fc880a5c.akpm@linux-foundation.org> (raw)
In-Reply-To: <18753.46301.944770.779390@cargo.ozlabs.ibm.com>

On Fri, 12 Dec 2008 11:48:29 +1100 Paul Mackerras <paulus@samba.org> wrote:

> Andrew Morton writes:
> 
> > > +#if (8 * THREAD_SIZE) > PAGE_SIZE
> > >  	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> > > +#else
> > > +	max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
> > > +#endif
> > 
> > The expression you've chosen here can be quite inacccurate, because
> > ((PAGE_SIZE / (8 * THREAD_SIZE)) is a small number.  The way to
> > preserve accuracy is
> 
> The assumption is that THREAD_SIZE is a power of 2, as is PAGE_SIZE.
> 
> I think Yuri should be increasing THREAD_SIZE for the larger page
> sizes he's implementing, because we have on-stack arrays whose size
> depends on the page size.  I suspect that having THREAD_SIZE less than
> 1/8 of PAGE_SIZE risks stack overflows, and the better fix is for Yuri
> to make sure THREAD_SIZE is at least 1/8 of PAGE_SIZE.  (In fact, more
> may be needed - someone should work out what fraction is actually
> needed.)

OK, yes.

It's the MAX_BUF_PER_PAGE arrays which will hurt.  iirc they nest
three-deep on some codepaths.