Date: Wed, 30 Apr 2014 14:32:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Rik van Riel <riel@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>, Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com,
        jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com,
        fengguang.wu@intel.com, mpatlasov@parallels.com,
        Motohiro.Kosaki@us.fujitsu.com
Subject: Re: [PATCH v5] mm,writeback: fix divide by zero in
 pos_ratio_polynom
Message-Id: <20140430143259.022534b79b01c5234df79dc7@linux-foundation.org>
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On Wed, 30 Apr 2014 16:42:55 -0400 Rik van Riel <riel@redhat.com> wrote:

> On Wed, 30 Apr 2014 13:13:53 -0700
> Andrew Morton <akpm@linux-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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