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From: Rik van Riel <riel@redhat.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	sandeen@redhat.com, akpm@linux-foundation.org,
	jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com,
	fengguang.wu@intel.com, mpatlasov@parallels.com,
	Motohiro.Kosaki@us.fujitsu.com
Subject: Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom
Date: Wed, 30 Apr 2014 10:52:08 -0400	[thread overview]
Message-ID: <53610E18.8050809@redhat.com> (raw)
In-Reply-To: <20140430144903.GI4357@dhcp22.suse.cz>

On 04/30/2014 10:49 AM, Michal Hocko wrote:
> On Wed 30-04-14 10:31:29, Rik van Riel wrote:
>> On 04/30/2014 09:48 AM, Michal Hocko wrote:
>>> On Wed 30-04-14 09:30:35, Rik van Riel wrote:
>>> [...]
>>>> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
>>>>
>>>> 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.
>>>>
>>>> Signed-off-by: Rik van Riel <riel@redhat.com>
>>>> ---
>>>>   mm/page-writeback.c | 13 +++++++++++--
>>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>>>> index ef41349..f98a297 100644
>>>> --- 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 long divisor;
>>>>   	long long pos_ratio;
>>>>   	long x;
>>>>
>>>> +	divisor = limit - setpoint;
>>>> +	if (!divisor)
>>>> +		divisor = 1;	/* Avoid div-by-zero */
>>>> +
>>>
>>> This is still prone to u64 -> s32 issue, isn't it?
>>> What was the original problem anyway? Was it really setpoint > limit or
>>> rather the overflow?
>>
>> Thinking about it some more, is it possible that
>> limit and/or setpoint are larger than 32 bits, but
>> the difference between them is not?
>>
>> In that case, truncating both to 32 bits before
>> doing the subtraction would be troublesome, and
>> it would be better to do a cast in the comparison:
>>
>> if (!(s32)divisor)
>> 	divisor = 1;
>
> How is that any different than defining divisor as 32b directly?

For unsigned, it probably doesn't make a difference.

For signed int vs unsigned long, I wonder if there is a
corner case where casting the second to the first before
doing the "limit - setpoint" calculation can lead to a
different outcome...

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  reply	other threads:[~2014-04-30 15:31 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-04-29 19:19 [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom Rik van Riel
2014-04-29 19:43 ` Motohiro Kosaki
2014-04-29 22:39 ` Andrew Morton
2014-04-29 22:48   ` Rik van Riel
2014-04-29 22:53     ` Andrew Morton
2014-04-30  8:04 ` Maxim Patlasov
2014-04-30  8:12   ` Michal Hocko
2014-04-30  8:34     ` Maxim Patlasov
2014-04-30 10:01 ` Masayoshi Mizuma
2014-04-30 13:30   ` [PATCH v2] " Rik van Riel
2014-04-30 13:48     ` Michal Hocko
2014-04-30 14:26       ` Rik van Riel
2014-04-30 14:31       ` Rik van Riel
2014-04-30 14:49         ` Michal Hocko
2014-04-30 14:52           ` Rik van Riel [this message]
2014-04-30 14:41       ` [PATCH v3] " Rik van Riel
2014-04-30 19:00         ` Andrew Morton
2014-04-30 19:30           ` Rik van Riel
2014-04-30 19:35             ` Andrew Morton
2014-04-30 20:02               ` [PATCH v4] " Rik van Riel
2014-04-30 20:13                 ` Andrew Morton
2014-04-30 20:32                   ` Rik van Riel
2014-04-30 20:42                   ` [PATCH v5] " Rik van Riel
2014-04-30 21:00                     ` Andrew Morton
2014-04-30 21:21                       ` Rik van Riel
2014-04-30 21:32                     ` Andrew Morton
2014-05-02  9:16                     ` Michal Hocko
2014-05-08 10:17                     ` Masayoshi Mizuma

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