linux-kernel - Re: [PATCH 5/6] writeback: try more writeback as long as something was written

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Message-ID: <20110421071426.GA24790@infradead.org>
Date:	Thu, 21 Apr 2011 03:14:26 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...ux.vnet.ibm.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Itaru Kitayama <kitayama@...bb4u.ne.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] writeback: try more writeback as long as something
 was written

On Thu, Apr 21, 2011 at 05:09:47PM +1000, Dave Chinner wrote:
> Likely just timing. When IO completes and updates the inode IO size,
> XFS calls mark_inode_dirty() again to ensure that the metadata that
> was changed gets written out at a later point in time.
> Hence every single file that is created by the test will be marked
> dirty again after the first write has returned and disappeared.
> 
> Why you see different numbers? it's timing dependent based on Io
> completion rates - if you have a fast disk the IO completion can
> occur before write_inode() is called and so the inode can be written
> and the dirty page state removed in the one writeback_single_inode()
> call...
> 
> That's my initial guess without looking at it in any real detail,
> anyway.

We shouldn't have I_DIRTY_PAGES set for that case, as we only redirty
metadata.  But we're actually doing a xfs_mark_inode_dirty, which
dirties all of I_DIRTY, which includes I_DIRTY_PAGES.  I guess it
should change to

	__mark_inode_dirty(inode, I_DIRTY_SYNC | I_DIRTY_DATASYNC);

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