From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wu Fengguang Subject: Re: [PATCH 01/15] writeback: introduce .tagged_writepages for the WB_SYNC_NONE sync stage Date: Wed, 8 Jun 2011 07:24:56 +0800 Message-ID: <20110607232456.GA19547@localhost> References: <20110607213236.634026193@intel.com> <20110607213853.513366237@intel.com> <20110607160234.53de9f93.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: <20110607160234.53de9f93.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, Jun 08, 2011 at 07:02:34AM +0800, Andrew Morton wrote: > On Wed, 08 Jun 2011 05:32:37 +0800 > Wu Fengguang 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. Problem is: the WB_SYNC_NONE phase has no livelock prevention _at all_. Jan's commit f446daaea9 ("mm: implement writeback livelock avoidance using page tagging") is a partial fix in that it only fixed the WB_SYNC_ALL phase livelock. Although ext4 is tested to no longer livelock with commit f446daaea9, it may due to some "redirty_tail() after pages_skipped" effect which is by no means a guarantee for _all_ the file systems. > Suggest that the changelog be fleshed out to cover all of this. OK, I'll add the above two paragraphs to the changelog. Thanks, Fengguang