3.2.70-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Tejun Heo <t...@kernel.org>
commit 464d1387acb94dc43ba772b35242345e3d2ead1b upstream.
mm/page-writeback.c has several places where 1 is added to the divisor
to prevent division by zero exceptions; however, if the original
divisor is equivalent to -1, adding 1 leads to division by zero.
There are three places where +1 is used for this purpose - one in
pos_ratio_polynom() and two in bdi_position_ratio(). The second one
in bdi_position_ratio() actually triggered div-by-zero oops on a
machine running a 3.10 kernel. The divisor is
x_intercept - bdi_setpoint + 1 == span + 1
span is confirmed to be (u32)-1. It isn't clear how it ended up that
but it could be from write bandwidth calculation underflow fixed by
c72efb658f7c ("writeback: fix possible underflow in write bandwidth
calculation").
At any rate, +1 isn't a proper protection against div-by-zero. This
patch converts all +1 protections to |1. Note that
bdi_update_dirty_ratelimit() was already using |1 before this patch.
Signed-off-by: Tejun Heo <t...@kernel.org>
Reviewed-by: Jan Kara <j...@suse.cz>
Signed-off-by: Jens Axboe <ax...@fb.com>
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
mm/page-writeback.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -560,7 +560,7 @@ static unsigned long bdi_position_ratio(
*/
setpoint = (freerun + limit) / 2;
x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
- limit - setpoint + 1);
+ (limit - setpoint) | 1);
pos_ratio = x;
pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
@@ -611,7 +611,7 @@ static unsigned long bdi_position_ratio(
* scale global setpoint to bdi's:
* bdi_setpoint = setpoint * bdi_thresh / thresh
*/
- x = div_u64((u64)bdi_thresh << 16, thresh + 1);
+ x = div_u64((u64)bdi_thresh << 16, thresh | 1);
bdi_setpoint = setpoint * (u64)x >> 16;
/*
* Use span=(8*write_bw) in single bdi case as indicated by
@@ -626,7 +626,7 @@ static unsigned long bdi_position_ratio(
if (bdi_dirty < x_intercept - span / 4) {
pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
- x_intercept - bdi_setpoint + 1);
+ (x_intercept - bdi_setpoint) | 1);
} else
pos_ratio /= 4;
--
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