linux-kernel - Re: [PATCH] balance_dirty_pages - exit loop when no more pages available

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Date:	Thu, 02 Aug 2007 15:54:14 -0400
From:	Chuck Ebbert <cebbert@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	richard kennedy <richard@....demon.co.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] balance_dirty_pages - exit loop when no more pages	available

On 07/31/2007 01:11 PM, richard kennedy wrote:

Peter, did you see this?

> exit loop in balance_dirty_pages when no more pages available to write
> 
> On a bdi that has very little traffic balance_dirty_pages can loop
> needlessly waiting until do_writepages has written enough pages.
> 
> do_writepages will return encountered_congestion==0 && nr_to_write > 0 
> when it has completed a pass but did not find enough pages available to
> write. balance_dirty_pages ignores this and keeps looping until a total
> of chunk pages was written. 
> 
> this patch adds an extra exit condition to break out of the loop in this
> case. 
> 
> I've tested this on my amd64 desktop, and I also have a version of this
> patch that includes a printk and a test case that occasionally does
> trigger this condition.  
> 
> Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> 
> ------
> --- linux-2.6.22.1/mm/page-writeback.c.orig	2007-07-30 16:36:09.000000000 +0100
> +++ linux-2.6.22.1/mm/page-writeback.c	2007-07-31 16:26:43.000000000 +0100
> @@ -250,6 +250,8 @@ static void balance_dirty_pages(struct a
>  			pages_written += write_chunk - wbc.nr_to_write;
>  			if (pages_written >= write_chunk)
>  				break;		/* We've done our duty */
> +			if (!wbc.encountered_congestion && wbc.nr_to_write > 0)
> +				break;	/* didn't find enough to do */
>  		}
>  		congestion_wait(WRITE, HZ/10);
>  	}
> 

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