Messages in this thread Patch in this message | | | From | lizf@kernel ... | Subject | [PATCH 3.4 170/172] writeback: use |1 instead of +1 to protect against div by zero | Date | Tue, 16 Jun 2015 16:37:04 +0800 |
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From: Tejun Heo <tj@kernel.org>
3.4.108-rc1 review patch. If anyone has any objections, please let me know.
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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> --- mm/page-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 25aaa31..ecd1c4c 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 -- 1.9.1
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