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

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Message-ID: <55391FE6.2050102@kernel.dk>
Date:	Thu, 23 Apr 2015 10:37:58 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH block/for-4.1-fixes] writeback: use |1 instead of +1 to
 protect against div by zero

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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