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: <20150422105845.GA11059@quack.suse.cz>
Date:	Wed, 22 Apr 2015 12:58:45 +0200
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>, 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 Tue 21-04-15 16:49:13, 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.
  So IMHO the only proper protection against division by zero is
	if (unlikely(divisor == 0))
		return some_value;
	else
		do division

  But we took a shortcut with +1 or |1 in those calculations since that
should be OK given the *expected* numbers we deal with. Once numbers get
out of expected range, both shortcuts have issues, just different ones
(sure you avoid division by zero with |1 but the absurd values you'll be
getting will have interesting effect on the system anyway). So I'm OK with
changing +1 to |1 to avoid oops and make code consistent, I just wanted to
comment on the fact that the change is for avoiding oops, not about getting
better values.

Otherwise feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 5daf556..eb59f7e 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -580,7 +580,7 @@ static long long pos_ratio_polynom(unsigned long setpoint,
>  	long x;
>  
>  	x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
> -		    limit - setpoint + 1);
> +		      (limit - setpoint) | 1);
>  	pos_ratio = x;
>  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
>  	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
> @@ -807,7 +807,7 @@ 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
> @@ -822,7 +822,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
>  
>  	if (bdi_dirty < x_intercept - span / 4) {
>  		pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
> -				    x_intercept - bdi_setpoint + 1);
> +				      (x_intercept - bdi_setpoint) | 1);
>  	} else
>  		pos_ratio /= 4;
>  
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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