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| author | Tejun Heo <tj@kernel.org> | 2015-04-21 16:49:13 -0400 |
|---|---|---|
| committer | Ben Hutchings <ben@decadent.org.uk> | 2015-08-07 00:32:06 +0100 |
| commit | 57279928b028b49bb26bb5af62aa455759521d32 (patch) | |
| tree | 3f3888e6dc71078a1ccb996f0907e4978f238327 /mm | |
| parent | 330a80fdcee47ec8eb0a1a37edeaebdcf3720960 (diff) | |
| download | linux-stable-57279928b028b49bb26bb5af62aa455759521d32.tar.gz linux-stable-57279928b028b49bb26bb5af62aa455759521d32.tar.bz2 linux-stable-57279928b028b49bb26bb5af62aa455759521d32.zip | |
writeback: use |1 instead of +1 to protect against div by zero
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 <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/page-writeback.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 1bf1f74b4945..62bfbd9ee105 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -560,7 +560,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, */ 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(struct backing_dev_info *bdi, * 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(struct backing_dev_info *bdi, 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; |