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

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Message-ID: <1290097740.2109.1527.camel@laptop>
Date:	Thu, 18 Nov 2010 17:29:00 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
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, 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 ;-)
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