Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

[Rik van Riel <riel@redhat.com>]

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?

Good point. I guess we need these divisors to be "int" and
"unsigned int" respectively, since those are the arguments
given to div_s64 and div_u64.

Let me send a v3 right now.

>>   	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>> -		    limit - setpoint + 1);
>> +		    divisor);
>>   	pos_ratio = x;
>>   	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
>>   	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
>> @@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
>>   	x_intercept = bdi_setpoint + span;
>>
>>   	if (bdi_dirty < x_intercept - span / 4) {
>> +		unsigned long divisor = x_intercept - bdi_setpoint;
>
> Same here.
>
>> +		if (!divisor)
>> +			divisor = 1;	/* Avoid div-by-zero */
>> +
>>   		pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
>> -				    x_intercept - bdi_setpoint + 1);
>> +				    divisor);
>>   	} else
>>   		pos_ratio /= 4;
>>
>

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