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

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Date:	Wed, 30 Apr 2014 14:32:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
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 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 :)
> 
> Masayoshi-san, does the bug still happen with this version, or does
> this fix the problem?

I assumed we wanted a reported-by in there.

> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
> 
> It is possible for "limit - setpoint + 1" to equal zero, after
> getting truncated to a 32 bit variable, and resulting in a divide
> by zero error.
> 
> Using the fully 64 bit divide functions avoids this problem.

This isn't the whole story, is it?  I added stuff:

: Using the fully 64 bit divide functions avoids this problem.  It also will
: cause pos_ratio_polynom() to return the correct value when (setpoint -
: limit) exceeds 2^32.


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