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authorTejun Heo <tj@kernel.org>2015-04-21 16:49:13 -0400
committerZefan Li <lizefan@huawei.com>2015-06-19 11:40:34 +0800
commitc905f0af23c68732315840412e3cf0f180a63d0c (patch)
parent397c6496535ee3f1de27c46e904e3a8f95ce60f6 (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>
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