Messages in this thread Patch in this message | | | Date | Wed, 30 Apr 2014 16:42:55 -0400 | From | Rik van Riel <> | Subject | [PATCH v5] mm,writeback: fix divide by zero in pos_ratio_polynom |
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On Wed, 30 Apr 2014 13:13:53 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> This was a consequence of 64->32 truncation and it can't happen any > more, can it?
Andrew, this is cleaner indeed :)
Masayoshi-san, does the bug still happen with this version, or does this fix the problem?
---8<---
Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
It is possible for "limit - setpoint + 1" to equal zero, after getting truncated to a 32 bit variable, and resulting in a divide by zero error.
Using the fully 64 bit divide functions avoids this problem.
Also uninline pos_ratio_polynom, at Andrew's request.
Signed-off-by: Rik van Riel <riel@redhat.com> --- mm/page-writeback.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index ef41349..a4317da 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -593,14 +593,14 @@ 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) { long long pos_ratio; long x; - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, + x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, limit - setpoint + 1); pos_ratio = x; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; @@ -842,7 +842,7 @@ 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), + pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), x_intercept - bdi_setpoint + 1); } else pos_ratio /= 4;
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