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Date: Fri, 19 Nov 2010 00:34:39 +0800
From: Wu Fengguang <fengguang.wu@intel.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rik van Riel <riel@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, "Li, Shaohua" <shaohua.li@intel.com>,
        Christoph Hellwig <hch@lst.de>, Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Chris Mason <chris.mason@oracle.com>,
        Mel Gorman <mel@csn.ul.ie>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        linux-mm <linux-mm@kvack.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] writeback: prevent bandwidth calculation overflow
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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 :)