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

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Message-Id: <20140429155353.8fe070101d3b4faa0c825d99@linux-foundation.org>
Date:	Tue, 29 Apr 2014 15:53:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, mhocko@...e.cz,
	fengguang.wu@...el.com, mpatlasov@...allels.com
Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom

On Tue, 29 Apr 2014 18:48:11 -0400 Rik van Riel <riel@...hat.com> wrote:

> On 04/29/2014 06:39 PM, Andrew Morton wrote:
> > On Tue, 29 Apr 2014 15:19:10 -0400 Rik van Riel <riel@...hat.com> wrote:
> > 
> >> It is possible for "limit - setpoint + 1" to equal zero, leading to a
> >> divide by zero error. Blindly adding 1 to "limit - setpoint" is not
> >> working, so we need to actually test the divisor before calling div64.
> >>
> >> ...
> >>
> >> --- a/mm/page-writeback.c
> >> +++ b/mm/page-writeback.c
> >> @@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
> >>  					  unsigned long dirty,
> >>  					  unsigned long limit)
> >>  {
> >> +	unsigned int divisor;
> > 
> > I'm thinking this would be better as a ulong so I don't have to worry
> > my pretty head over truncation things?
> 
> I looked at div_*64, and the second argument is a 32 bit
> variable. I guess a long would be ok, since if we are
> dividing by more than 4 billion we don't really care :)
> 
> static inline s64 div_s64(s64 dividend, s32 divisor)

ah, good point.  Switching to ulong is perhaps a bit misleading then.

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