Re: [PATCH block/for-4.1-fixes] writeback: use |1 instead of +1 to protect against div by zero

[Jens Axboe]

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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