Messages in this thread | | | Date | Wed, 30 Apr 2014 12:34:45 +0400 | From | Maxim Patlasov <> | Subject | Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On 04/30/2014 12:12 PM, Michal Hocko wrote: > On Wed 30-04-14 12:04:04, Maxim Patlasov wrote: >> Hi Rik! >> >> On 04/29/2014 11:19 PM, Rik van Riel 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. >> The patch looks correct, but I'm afraid it can hide an actual bug in a >> caller of pos_ratio_polynom(). The latter is not intended for setpoint > >> limit. All callers take pains to ensure that setpoint <= limit. Look, for >> example, at global_dirty_limits(): > The bug might trigger even if setpoint < limit because the result is > trucated to s32 and I guess this is what is going on here? > Is (limit - setpoint + 1) > 4G possible?
Yes, you are right. Probably the problem came from s32 overflow.
> >>> if (background >= dirty) >>> background = dirty / 2; >> If you ever encountered "limit - setpoint + 1" equal zero, it may be worthy >> to investigate how you came to setpoint greater than limit. >> >> Thanks, >> Maxim >> >>> Signed-off-by: Rik van Riel <riel@redhat.com> >>> Cc: stable@vger.kernel.org >>> --- >>> mm/page-writeback.c | 7 ++++++- >>> 1 file changed, 6 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c >>> index ef41349..2682516 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 int divisor; >>> long long pos_ratio; >>> long x; >>> + divisor = limit - setpoint; >>> + if (!divisor) >>> + divisor = 1; >>> + >>> 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;
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