From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 1F0986B00CB for ; Fri, 5 Nov 2010 22:55:52 -0400 (EDT) Date: Sat, 6 Nov 2010 10:55:48 +0800 From: Wu Fengguang Subject: Re: [PATCH] mm: Avoid livelocking of WB_SYNC_ALL writeback Message-ID: <20101106025548.GA16378@localhost> References: <1288992383-25475-1-git-send-email-jack@suse.cz> <20101105223038.GA16666@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101105223038.GA16666@lst.de> Sender: owner-linux-mm@kvack.org To: Christoph Hellwig Cc: Jan Kara , "linux-fsdevel@vger.kernel.org" , Andrew Morton , linux-mm@kvack.org, Johannes Weiner List-ID: [add CC to linux-mm list] On Sat, Nov 06, 2010 at 06:30:38AM +0800, Christoph Hellwig wrote: > > + /* > > + * In WB_SYNC_ALL mode, we just want to ignore nr_to_write as > > + * we need to write everything and livelock avoidance is implemented > > + * differently. > > + */ > > + if (wbc.sync_mode == WB_SYNC_NONE) > > + write_chunk = MAX_WRITEBACK_PAGES; > > + else > > + write_chunk = LONG_MAX; Good catch! > > I think it would be useful to elaborate here on how livelock avoidance > is supposed to work. It's supposed to sync files in a big loop for each dirty inode write_cache_pages() (quickly) tag currently dirty pages (maybe slowly) sync all tagged pages Ideally the loop should call write_cache_pages() _once_ for each inode. At least this is the assumption made by commit f446daaea (mm: implement writeback livelock avoidance using page tagging). Setting wbc.nr_to_write to LONG_MAX ensures that writeback_inodes_wb() will complete the above loop before returning to wb_writeback(), and to prevent wb_writeback() from looping (thus re-syncing extra data) in the below range of code. 643 wbc.nr_to_write = MAX_WRITEBACK_PAGES; 644 wbc.pages_skipped = 0; 645 646 trace_wbc_writeback_start(&wbc, wb->bdi); 647 if (work->sb) 648 __writeback_inodes_sb(work->sb, wb, &wbc); 649 else 650 writeback_inodes_wb(wb, &wbc); 651 trace_wbc_writeback_written(&wbc, wb->bdi); 652 653 work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; 654 wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write; 655 656 /* 657 * If we consumed everything, see if we have more 658 */ 659 if (wbc.nr_to_write <= 0) 660 continue; Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org