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

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Message-Id: <20081211124330.ff9b7483.akpm@linux-foundation.org>
Date:	Thu, 11 Dec 2008 12:43:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	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, 11 Dec 2008 20:28:00 +0000
Al Viro <viro@...IV.linux.org.uk> wrote:

> 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?

No, not at all.  It's all too hard.  Which is why I'm looking for
simplification.

>  Do they actually cross the page boundaries?

Some flavours of slab have at times done an order-1 allocation for
objects which would fit into an order-0 page (etc) if it looks like
that will be beneficial from a packing POV.  I'm unsure whether that
still happens - I tried to get it stamped out for reliability reasons.

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