linux-kernel - Re: [PATCH 01/15] writeback: introduce .tagged_writepages for the WB_SYNC

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Message-Id: <20110607160234.53de9f93.akpm@linux-foundation.org>
Date:	Tue, 7 Jun 2011 16:02:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	<linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/15] writeback: introduce .tagged_writepages for the
 WB_SYNC_NONE sync stage

On Wed, 08 Jun 2011 05:32:37 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
> WB_SYNC_ALL sync. Identify the first stage with .tagged_writepages and
> do livelock prevention for it, too.
> 
> Note that writeback_inodes_sb() is called by not only sync(), they are
> treated the same because the other callers also need livelock prevention.
> 
> Impact:  It changes the order in which pages/inodes are synced to disk.
> Now in the WB_SYNC_NONE stage, it won't proceed to write the next inode
> until finished with the current inode.

What problem is this patch actually fixing?  It sounds like there's
some livelock scenario in the WB_SYNC_NONE phase.  otoh the final
paragraph implies that the WB_SYNC_NONE phase is failing to write some
pages under some situations.

Suggest that the changelog be fleshed out to cover all of this.
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