Messages in this thread | | | Date | Thu, 23 Apr 2015 10:37:58 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH block/for-4.1-fixes] writeback: use |1 instead of +1 to protect against div by zero |
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On 04/21/2015 02:49 PM, Tejun Heo wrote: > 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.
The |1 is a litle iffy imho, but I guess it gets the job done. Applied for 4.1.
-- Jens Axboe
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