linux-kernel - Re: [PATCH 03/10] writeback: stop background writeback when below background threshold

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Message-Id: <1253804615.18939.36.camel@laptop>
Date:	Thu, 24 Sep 2009 17:03:35 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, fengguang.wu@...el.com,
	shaohua.li@...el.com, chris.mason@...cle.com, jack@...e.cz
Subject: Re: [PATCH 03/10] writeback: stop background writeback when below
 background threshold

On Thu, 2009-09-24 at 16:40 +0200, Jens Axboe wrote:
> From: Wu Fengguang <fengguang.wu@...el.com>
> 
> Treat bdi_start_writeback(0) as a special request to do background write,
> and stop such work when we are below the background dirty threshold.
> 
> Also simplify the (nr_pages <= 0) checks. Since we already pass in
> nr_pages=LONG_MAX for WB_SYNC_ALL and background writes, we don't
> need to worry about it being decreased to zero.
> 
> Reported-by: Richard Kennedy <richard@....demon.co.uk>
> CC: Jan Kara <jack@...e.cz>
> CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> ---

> @@ -720,20 +730,16 @@ static long wb_writeback(struct bdi_writeback *wb,
>  
>  	for (;;) {
>  		/*
> +		 * Stop writeback when nr_pages has been consumed
>  		 */
> +		if (args->nr_pages <= 0)
>  			break;
>  
>  		/*
> +		 * For background writeout, stop when we are below the
> +		 * background dirty threshold
>  		 */
> +		if (args->for_background && !over_bground_thresh())
>  			break;


What I'm not getting is why this is conditional on for_background(),
shouldn't we always stop writeback when below the background threshold?

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