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

Rik van Riel
2014-04-30T14:29:12Z

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;
>>
>