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authorTejun Heo <tj@kernel.org>2015-04-21 16:49:13 -0400
committerJens Axboe <axboe@fb.com>2015-04-23 10:36:33 -0600
commit464d1387acb94dc43ba772b35242345e3d2ead1b (patch)
treed331377c5d30736d8538dafcdb91709e593bf239 /fs
parent2a34c0872adf252f23a6fef2d051a169ac796cef (diff)
writeback: use |1 instead of +1 to protect against div by zero
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> Cc: stable@vger.kernel.org Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Jens Axboe <axboe@fb.com>
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