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Subject[PATCH] mm/page-writeback.c: avoid potential division by zero
Date
The variables 'min', 'max' and 'bw' are unsigned long and
do_div truncates them to 32 bits, which means it can test
non-zero and be truncated to zero for division.
Fix this issue by using div64_ul() instead.

For the two variables 'numerator' and 'denominator',
though they are declared as long, they should actually be
unsigned long (according to the implementation of
the fprop_fraction_percpu() function).

Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
mm/page-writeback.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 50055d2..2caf780 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -201,11 +201,11 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
if (this_bw < tot_bw) {
if (min) {
min *= this_bw;
- do_div(min, tot_bw);
+ min = div64_ul(min, tot_bw);
}
if (max < 100) {
max *= this_bw;
- do_div(max, tot_bw);
+ max = div64_ul(max, tot_bw);
}
}

@@ -766,7 +766,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
struct wb_domain *dom = dtc_dom(dtc);
unsigned long thresh = dtc->thresh;
u64 wb_thresh;
- long numerator, denominator;
+ unsigned long numerator, denominator;
unsigned long wb_min_ratio, wb_max_ratio;

/*
@@ -777,7 +777,7 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)

wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
wb_thresh *= numerator;
- do_div(wb_thresh, denominator);
+ wb_thresh = div64_ul(wb_thresh, denominator);

wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);

@@ -1102,7 +1102,7 @@ static void wb_update_write_bandwidth(struct bdi_writeback *wb,
bw = written - min(written, wb->written_stamp);
bw *= HZ;
if (unlikely(elapsed > period)) {
- do_div(bw, elapsed);
+ bw = div64_ul(bw, elapsed);
avg = bw;
goto out;
}
--
1.8.3.1
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