linux-kernel - Re: [PATCH v5] mm,writeback: fix divide by zero in pos_ratio

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Date:	Wed, 30 Apr 2014 17:21:27 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Michal Hocko <mhocko@...e.cz>,
	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v5] mm,writeback: fix divide by zero in pos_ratio_polynom

On 04/30/2014 05:00 PM, Andrew Morton wrote:
> On Wed, 30 Apr 2014 16:42:55 -0400 Rik van Riel <riel@...hat.com> wrote:
>
>> On Wed, 30 Apr 2014 13:13:53 -0700
>> Andrew Morton <akpm@...ux-foundation.org> wrote:
>>
>>> This was a consequence of 64->32 truncation and it can't happen any
>>> more, can it?
>>
>> Andrew, this is cleaner indeed :)
>
> I'm starting to get worried about 32-bit wraparound in the patch
> version number ;)
>
>> Masayoshi-san, does the bug still happen with this version, or does
>> this fix the problem?
>>
>
> We could put something like
>
> 	if (WARN_ON_ONCE(setpoint == limit))
> 		setpoint--;
>
> in there if we're not sure.  But it's better to be sure!

The more I look at the code, the more I am convinced that
Michal is right, and we cannot actually hit the case that
"limit - setpoint + 1 == 0".

Setpoint always seems to be some in-between point.

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