linux-kernel - Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty

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Message-Id: <20140711152732.de78603744cd861497eca5dc@linux-foundation.org>
Date:	Fri, 11 Jul 2014 15:27:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Maxim Patlasov <MPatlasov@...allels.com>
Cc:	riel@...hat.com, linux-kernel@...r.kernel.org, mhocko@...e.cz,
	linux-mm@...ck.org, kosaki.motohiro@...fujitsu.com,
	fengguang.wu@...el.com, jweiner@...hat.com
Subject: Re: [PATCH] mm/page-writeback.c: fix divide by zero in
 bdi_dirty_limits

On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov <MPatlasov@...allels.com> wrote:

> Under memory pressure, it is possible for dirty_thresh, calculated by
> global_dirty_limits() in balance_dirty_pages(), to equal zero.

Under what circumstances?  Really small values of vm_dirty_bytes?

> Then, if
> strictlimit is true, bdi_dirty_limits() tries to resolve the proportion:
> 
>   bdi_bg_thresh : bdi_thresh = background_thresh : dirty_thresh
> 
> by dividing by zero.
> 
> ...
>
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1306,9 +1306,9 @@ static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
>  	*bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
>  
>  	if (bdi_bg_thresh)
> -		*bdi_bg_thresh = div_u64((u64)*bdi_thresh *
> -					 background_thresh,
> -					 dirty_thresh);
> +		*bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
> +							background_thresh,
> +							dirty_thresh) : 0;

This introduces a peculiar discontinuity:

if dirty_thresh==3, treat it as 3
if dirty_thresh==2, treat it as 2
if dirty_thresh==1, treat it as 1
if dirty_thresh==0, treat it as infinity

Would it not make more sense to change global_dirty_limits() to convert
0 to 1?  With an appropriate comment, obviously.


Or maybe the fix lies elsewhere.  Please do tell us how this zero comes
about.

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