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

[Maxim Patlasov]

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@xxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > ---
> > >   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;

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/