linux-kernel - Re: [BUG] mm/page-writeback.c: divide by zero in pos_ratio

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Date:	Tue, 11 Nov 2014 22:16:15 +0100
From:	Jan Kara <jack@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jan Kara <jack@...e.cz>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG] mm/page-writeback.c: divide by zero in pos_ratio_polynom
 not fixed

On Tue 11-11-14 16:04:48, Steven Rostedt wrote:
> On Tue, 11 Nov 2014 21:15:39 +0100
> Jan Kara <jack@...e.cz> wrote:
> 
> 
> >   So I was looking into this but I have to say I don't understand where is
> > the problem. The registers clearly show that x_intercept - bdi_setpoint + 1
> > == 0 (in 32-bit arithmetics). Given:
> >    x_intercept = bdi_setpoint + span
> > 
> > We have that span + 1 == 0 and that means that:
> > ((thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16) == -1 (again in
> > 32-bit arithmetics). But I don't see how that can realistically happen...
> > 
> > Is this reproducible at all?
> > 
> 
> Unfortunately not. It only happened once, and I haven't been able to
> reproduce it again.
  BTW, how much memory does the machine have and what is
/proc/sys/vm/dirty_ratio and /proc/sys/vm/dirty_background_ratio (or
corresponding dirty_bytes, dirty_background_bytes if you are using them)?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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