diff options
| author | Tejun Heo <tj@kernel.org> | 2015-04-21 16:49:13 -0400 |
|---|---|---|
| committer | Zefan Li <lizefan@huawei.com> | 2015-06-19 11:40:34 +0800 |
| commit | c905f0af23c68732315840412e3cf0f180a63d0c (patch) | |
| tree | de6dad171f176b6edb6fbdb554e6dbf508f82dce | |
| parent | 397c6496535ee3f1de27c46e904e3a8f95ce60f6 (diff) | |
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>
[lizf: Backported to 3.4: drop other two changes as there's only one
such statment in 3.4]
Signed-off-by: Zefan Li <lizefan@huawei.com>
| -rw-r--r-- | mm/page-writeback.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 25aaa311cd55..ecd1c4c4207c 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -716,7 +716,7 @@ 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 |