From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 02/15] writeback: update dirtied_when for synced inode to prevent livelock Date: Wed, 8 Jun 2011 07:51:28 +0800 Message-ID: <20110607235127.GB19547@localhost> References: <20110607213236.634026193@intel.com> <20110607213853.635444678@intel.com> <20110607160245.9270aa27.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Dave Chinner , Christoph Hellwig , "linux-fsdevel@vger.kernel.org" , LKML To: Andrew Morton Return-path: Content-Disposition: inline In-Reply-To: <20110607160245.9270aa27.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, Jun 08, 2011 at 07:02:45AM +0800, Andrew Morton wrote: > On Wed, 08 Jun 2011 05:32:38 +0800 > Wu Fengguang 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. The exact livelock condition is, during sync(1): (1) no new inodes are dirtied (2) an inode being actively dirtied On (2), the inode will be tagged and synced with .nr_to_write=LONG_MAX. When finished, it will be redirty_tail()ed because it's still dirty and (.nr_to_write > 0). redirty_tail() won't update its ->dirtied_when on condition (1). The sync work will then revisit it on the next queue_io() and find it eligible again because its old ->dirtied_when predates the sync work start time. I'll add the above to the changelog. > > 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? There are no reported real cases for "touch f" style livelock. It's merely a possibility in theory and the more concurrent meta data dirties, the more likelihood it will happen. > Why is this? Because the inode keeps on getting rotated to head-of-list? Yes, when the inode is always redirty_tail()ed without updating its ->dirtied_when. Thanks, Fengguang