linux-kernel - Re: [PATCH] writeback: prevent bandwidth calculation overflow

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Message-ID: <20101118163439.GA21318@localhost>
Date:	Fri, 19 Nov 2010 00:34:39 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, "Li, Shaohua" <shaohua.li@...el.com>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] writeback: prevent bandwidth calculation overflow

On Fri, Nov 19, 2010 at 12:29:00AM +0800, Peter Zijlstra wrote:
> On Fri, 2010-11-19 at 00:06 +0800, Wu Fengguang wrote:
> > On Fri, Nov 19, 2010 at 12:02:01AM +0800, Peter Zijlstra wrote:
> > > On Thu, 2010-11-18 at 23:44 +0800, Wu Fengguang wrote:
> > > > +               pause = HZ * pages_dirtied / (bw + 1);
> > > 
> > > Shouldn't that be using something like div64_u64 ?
> > 
> > OK, but a dumb question: gcc cannot handle this implicitly?
> 
> it could, but we chose not to implement the symbol it emits for these
> things so as to cause pain.. that was still assuming the world of 32bit
> computing was relevant and 64bit divides were expensive ;-)

Good to know that, thanks!  So let's avoid it totally :)
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