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Message-ID: <18753.46301.944770.779390@cargo.ozlabs.ibm.com>
Date: Fri, 12 Dec 2008 11:48:29 +1100
From: Paul Mackerras <paulus@samba.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Yuri Tikhonov <yur@emcraft.com>, wd@denx.de, dzu@denx.de,
       linux-kernel@vger.kernel.org, miltonm@bga.com, linuxppc-dev@ozlabs.org,
       viro@zeniv.linux.org.uk, Geert.Uytterhoeven@sonycom.com,
       yanok@emcraft.com
Subject: Re: [PATCH][v2] fork_init: fix division by zero
In-Reply-To: <20081211121635.ff58193f.akpm@linux-foundation.org>
References: <200812101950.51958.yur@emcraft.com>
	<20081211121635.ff58193f.akpm@linux-foundation.org>
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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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