linux-kernel - Re: [PATCH 13/17] writeback: remove writeback_control.more

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Message-ID: <20110512230432.GL19446@dastard>
Date:	Fri, 13 May 2011 09:04:32 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/17] writeback: remove writeback_control.more_io

On Thu, May 12, 2011 at 09:57:19PM +0800, Wu Fengguang wrote:
> When wbc.more_io was first introduced, it indicates whether there are
> at least one superblock whose s_more_io contains more IO work. Now with
> the per-bdi writeback, it can be replaced with a simple b_more_io test.
> 
> Acked-by: Jan Kara <jack@...e.cz>
> Acked-by: Mel Gorman <mel@....ul.ie>
> CC: Minchan Kim <minchan.kim@...il.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  fs/fs-writeback.c                |    9 ++-------
>  include/linux/writeback.h        |    1 -
>  include/trace/events/ext4.h      |    6 ++----
>  include/trace/events/writeback.h |    5 +----
>  4 files changed, 5 insertions(+), 16 deletions(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2011-05-05 23:30:30.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2011-05-05 23:30:33.000000000 +0800
> @@ -560,12 +560,8 @@ static int writeback_sb_inodes(struct su
>  		iput(inode);
>  		cond_resched();
>  		spin_lock(&wb->list_lock);
> -		if (wbc->nr_to_write <= 0) {
> -			wbc->more_io = 1;
> +		if (wbc->nr_to_write <= 0)
>  			return 1;
> -		}
> -		if (!list_empty(&wb->b_more_io))
> -			wbc->more_io = 1;
>  	}
>  	/* b_io is empty */
>  	return 1;
> @@ -707,7 +703,6 @@ static long wb_writeback(struct bdi_writ
>  			wbc.older_than_this = &oldest_jif;
>  		}
>  
> -		wbc.more_io = 0;
>  		wbc.nr_to_write = write_chunk;
>  		wbc.pages_skipped = 0;
>  		wbc.inodes_cleaned = 0;
> @@ -755,7 +750,7 @@ retry:
>  		/*
>  		 * No more inodes for IO, bail
>  		 */
> -		if (!wbc.more_io)
> +		if (list_empty(&wb->b_more_io))
>  			break;

We're not holding the wb->list_lock here, so we need to be careful
here. I think this is safe given that there shuold only be one
flusher thread operating on the list, but when we expand to multiple
flusher threads per-bdi, this coul dbe a nasty landmine. A comment
is probably in order explaining why this is safe to check unlocked
right now...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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