linux-kernel - Re: [PATCH 02/15] writeback: update dirtied_when for synced inode to prevent livelock

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Message-Id: <20110607160245.9270aa27.akpm@linux-foundation.org>
Date:	Tue, 7 Jun 2011 16:02:45 -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 02/15] writeback: update dirtied_when for synced inode
 to prevent livelock

On Wed, 08 Jun 2011 05:32:38 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> Explicitly update .dirtied_when on synced inodes, so that they are no
> longer considered for writeback in the next round.

It sounds like this somewhat answers my questions for [1/15].

But I'm not seeing a description of exactly what caused the livelock.

> We'll do more aggressive "keep writeback as long as we wrote something"
> logic in wb_writeback(). The "use LONG_MAX .nr_to_write" trick in commit
> b9543dac5bbc ("writeback: avoid livelocking WB_SYNC_ALL writeback") will
> no longer be enough to stop sync livelock.
> 
> It can prevent both of the following livelock schemes:
> 
> - while true; do echo data >> f; done
> - while true; do touch f;        done

You're kidding.  This livelocks sync(1)?  When did we break this?

Why is this?  Because the inode keeps on getting rotated to head-of-list?
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