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Date: Mon, 12 Jul 2010 12:01:09 +1000
From: Dave Chinner <david@fromorbit.com>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>, Jan Kara <jack@suse.cz>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-fsdevel@vger.kernel.org,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/6] writeback: dont redirty tail an inode with dirty
 pages
Message-ID: <20100712020109.GB25335@dastard>
References: <20100711020656.340075560@intel.com>
 <20100711021749.021449821@intel.com>
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On Sun, Jul 11, 2010 at 10:07:00AM +0800, Wu Fengguang wrote:
> This avoids delaying writeback for an expired (XFS) inode with lots of
> dirty pages, but no active dirtier at the moment. Previously we only do
> that for the kupdate case.
> 
> CC: Dave Chinner <david@fromorbit.com>
> CC: Christoph Hellwig <hch@infradead.org>
> Acked-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  fs/fs-writeback.c |   20 +++++++-------------
>  1 file changed, 7 insertions(+), 13 deletions(-)
> 
> --- linux-next.orig/fs/fs-writeback.c	2010-07-11 08:53:44.000000000 +0800
> +++ linux-next/fs/fs-writeback.c	2010-07-11 08:57:35.000000000 +0800
> @@ -367,18 +367,7 @@ writeback_single_inode(struct inode *ino
>  	spin_lock(&inode_lock);
>  	inode->i_state &= ~I_SYNC;
>  	if (!(inode->i_state & I_FREEING)) {
> -		if ((inode->i_state & I_DIRTY_PAGES) && wbc->for_kupdate) {
> -			/*
> -			 * More pages get dirtied by a fast dirtier.
> -			 */
> -			goto select_queue;
> -		} else if (inode->i_state & I_DIRTY) {
> -			/*
> -			 * At least XFS will redirty the inode during the
> -			 * writeback (delalloc) and on io completion (isize).
> -			 */
> -			redirty_tail(inode);
> -		} else if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
> +		if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
>  			/*
>  			 * We didn't write back all the pages.  nfs_writepages()
>  			 * sometimes bales out without doing anything. Redirty
> @@ -400,7 +389,6 @@ writeback_single_inode(struct inode *ino
>  				 * soon as the queue becomes uncongested.
>  				 */
>  				inode->i_state |= I_DIRTY_PAGES;
> -select_queue:
>  				if (wbc->nr_to_write <= 0) {
>  					/*
>  					 * slice used up: queue for next turn
> @@ -423,6 +411,12 @@ select_queue:
>  				inode->i_state |= I_DIRTY_PAGES;
>  				redirty_tail(inode);
>  			}
> +		} else if (inode->i_state & I_DIRTY) {
> +			/*
> +			 * At least XFS will redirty the inode during the
> +			 * writeback (delalloc) and on io completion (isize).
> +			 */
> +			redirty_tail(inode);

I'd drop the mention of XFS here - any filesystem that does delayed
allocation or unwritten extent conversion after Io completion will
cause this. Perhaps make the comment:

	/*
	 * Filesystems can dirty the inode during writeback
	 * operations, such as delayed allocation during submission
	 * or metadata updates after data IO completion.
	 */

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com