Re: [PATCH] writeback: prevent bandwidth calculation overflow

[Michal Hocko <mhocko@suse.cz>]

On Fri 19-11-10 00:13:56, Wu Fengguang wrote:
> On Fri, Nov 19, 2010 at 12:02:01AM +0800, Peter Zijlstra wrote:
> > On Thu, 2010-11-18 at 23:44 +0800, Wu Fengguang wrote:
> > > +               pause = HZ * pages_dirtied / (bw + 1);
> > 
> > Shouldn't that be using something like div64_u64 ?
> 
> Thanks for review. Here is the updated patch using div64_u64().
> 
> ---
> Subject: writeback: prevent bandwidth calculation overflow
> Date: Thu Nov 18 12:55:42 CST 2010
> 
> On 32bit kernel, bdi->write_bandwidth can express at most 4GB/s.
> 
> However the current calculation code can overflow when disk bandwidth
> reaches 800MB/s.  Fix it by using "long long" and div64_u64() in the
> calculations.
> 
> And further change its unit from bytes/second to pages/second.
> That allows up to 16TB/s bandwidth in 32bit kernel.
> 
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Acked-by: Rik van Riel <riel@redhat.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/backing-dev.c    |    4 ++--
>  mm/page-writeback.c |   11 +++++------
>  2 files changed, 7 insertions(+), 8 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2010-11-18 12:42:58.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-11-19 00:08:23.000000000 +0800
> @@ -494,7 +494,7 @@ void bdi_update_write_bandwidth(struct b
>  	unsigned long written;
>  	unsigned long elapsed;
>  	unsigned long bw;
> -	unsigned long w;
> +	unsigned long long w;
>  
>  	if (*bw_written == 0)
>  		goto snapshot;
> @@ -513,7 +513,7 @@ void bdi_update_write_bandwidth(struct b
>  		goto snapshot;
>  
>  	written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
> -	bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
> +	bw = (HZ * written + elapsed/2) / elapsed;

Sorry for a dumb question, but where did PAGE_CACHE_SIZE part go?

>  	w = min(elapsed / unit_time, 128UL);
>  	bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
>  	bdi->write_bandwidth_update_time = jiffies;

-- 
Michal Hocko
L3 team 
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic