linux-kernel - Re: [PATCH][v2] fork_init: fix division by zero

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Date:	Fri, 12 Dec 2008 11:48:29 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Yuri Tikhonov <yur@...raft.com>, wd@...x.de, dzu@...x.de,
	linux-kernel@...r.kernel.org, miltonm@....com,
	linuxppc-dev@...abs.org, viro@...iv.linux.org.uk,
	Geert.Uytterhoeven@...ycom.com, yanok@...raft.com
Subject: Re: [PATCH][v2] fork_init: fix division by zero

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.)

Paul.
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