From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: __fsync_super and inode dirty pages Date: Wed, 6 Aug 2008 11:36:33 +1000 Message-ID: <20080806013633.GM21635@disturbed> References: <20080805175439.GA17772@skywalker> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christoph Hellwig , Alexander Viro , Andrew Morton , "linux-fsdevel@vger.kernel.org" To: "Aneesh Kumar K.V" Return-path: Received: from ipmail04.adl2.internode.on.net ([203.16.214.57]:59169 "EHLO ipmail04.adl2.internode.on.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759346AbYHFBgj (ORCPT ); Tue, 5 Aug 2008 21:36:39 -0400 Content-Disposition: inline In-Reply-To: <20080805175439.GA17772@skywalker> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, Aug 05, 2008 at 11:24:39PM +0530, Aneesh Kumar K.V wrote: > Hi, > > I am trying to understand the expectation on filesystem > writepages by __fsync_super -> generic_sync_sb_inodes call path. > __fsync_super make two calls to generic_sync_sb_inodes and the > second call is made with WB_SYNC_ALL. The problem is, i am finding > multiple ways we may 'break' from the generic_sync_sb_inodes > while (!list_empty(&sb->s_io)) loop. For ex: > > a) When wbc->nr_to_write <=0 > This should normally indicate that we wrote more than we requested > That means we have more dirty pages . So why break from loop ? > This may be ok for nr_to_write limited writeback. But is it ok > for __fsync_super ? Should never happen - nr_to_write is set to 150% of the number of dirty pages in the system in sync_inodes_sb(). If this is not enough, then something is skipping lots of dirty pages during a data integrity sync and that sounds wrong.... > b) pages_skipped != wbc->pages_skipped. > That means writepages was not able to write some of the pages > and it redirtied the same. We add such inode to s_dirty list. But > we don't loop through the s_dirty list later trying to write the > dirty pages. We actually call generic_sync_sb_inodes again. That > can actually cause s_dirty list content to move to s_io once > via queue_io. But then we can have pages skipped again in the second > call to generic_sync_sb_inodes ? Again, should never happen if WB_SYNC_ALL is set. We should block waiting for locks, etc (rather than behaving in a non-blocking manner and redirtying the page instead of blocking) if WB_SYNC_ALL is set. i.e. if WB_SYNC_ALL is set we should never skip pages...... > Shouldn't we have a variant that actually loop trough s_dirty and s_more_io > list MULTIPLE times and make sure we force all the dirty pages to disk > before __fsync_super returns ? Or is there an expectation from file > system writepages that would make sure calling generic_sync_sb_inodes > TWICE would guarantee that ? The two calls are for performance reasons. The first is non-blocking, async I/O to get the dirty data and inodes out to disk as fast and efficiently as possible. The second pass is a synchronous pass that provides the data integrity part of sync. Cheers, Dave. -- Dave Chinner david@fromorbit.com