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

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Message-ID: <20081211202800.GQ28946@ZenIV.linux.org.uk>
Date:	Thu, 11 Dec 2008 20:28:00 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Yuri Tikhonov <yur@...raft.com>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Geert.Uytterhoeven@...ycom.com,
	dhowells@...hat.com, miltonm@....com, wd@...x.de, dzu@...x.de,
	yanok@...raft.com
Subject: Re: [PATCH][v2] fork_init: fix division by zero

On Thu, Dec 11, 2008 at 12:16:35PM -0800, Andrew Morton wrote:
> > +#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
> 
> 	max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
> 
> so how about avoiding the nasty ifdefs and doing

Are you sure?  Do they actually cross the page boundaries?
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