Messages in this thread Patch in this message | | | Date | Wed, 30 Apr 2014 16:02:18 -0400 | From | Rik van Riel <> | Subject | [PATCH v4] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Wed, 30 Apr 2014 12:35:26 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> > The easy way would be by calling div64_s64 and div64_u64, > > which are 64 bit all the way through. > > > > Any objections? > > Sounds good to me. > > > The inlined bits seem to be stubs calling the _rem variants > > of the functions, and discarding the remainder. > > I was referring to pos_ratio_polynom(). The compiler will probably be > uninlining it anyway, but still...
I believe this should do the trick.
---8<---
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 | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index ef41349..37f56bb 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -593,15 +593,20 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty) * (5) the closer to setpoint, the smaller |df/dx| (and the reverse) * => fast response on large errors; small oscillation near setpoint */ -static inline long long pos_ratio_polynom(unsigned long setpoint, +static long long pos_ratio_polynom(unsigned long setpoint, unsigned long dirty, unsigned long limit) { + unsigned long divisor; long long pos_ratio; long x; - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, - limit - setpoint + 1); + divisor = limit - setpoint; + if (!divisor) + divisor = 1; /* Avoid div-by-zero */ + + x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, + 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) { - pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), - x_intercept - bdi_setpoint + 1); + unsigned long divisor = x_intercept - bdi_setpoint; + if (!divisor) + divisor = 1; /* Avoid div-by-zero */ + + pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), + divisor); } else pos_ratio /= 4;
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