linux-kernel - Re: [PATCH 2/5] writeback: stop periodic/background work on seeing sync works

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Message-ID: <20100803134431.GA32173@localhost>
Date:	Tue, 3 Aug 2010 21:44:31 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 2/5] writeback: stop periodic/background work on seeing
 sync works

On Tue, Aug 03, 2010 at 09:22:16PM +0800, Wu Fengguang wrote:
> > >   Fengguang, how about merging also the attached simple patch together with
> > > my fix? With these two patches, I'm not able to trigger any sync livelock
> > > while without one of them I hit them quite easily...
> > 
> > This looks OK. However note that redirty_tail() can modify
> > dirtied_when unexpectedly. So the more we rely on wb_start, the more
> > possibility an inode is (wrongly) skipped by sync. I have a bunch of
> > patches to remove redirty_tail(). However they may not be good
> > candidates for 2.6.36..
> 
> It looks that setting wb_start at the beginning of
> writeback_inodes_wb() won't be easily affected by redirty_tail().

Except for this redirty_tail(), which may mess up the dirtied_when
ordering in b_dirty and later on break the assumption of
inode_dirtied_after(inode, wbc->wb_start).

It can be replaced by a requeue_io() for now.  Christoph mentioned a
patchset to introduce sb->s_wb, which should be a better solution.

Thanks,
Fengguang

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a178828..e56e68b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -457,12 +457,7 @@ int generic_writeback_sb_inodes(struct super_block *sb,
 
 		if (inode->i_sb != sb) {
 			if (only_this_sb) {
-				/*
-				 * We only want to write back data for this
-				 * superblock, move all inodes not belonging
-				 * to it back onto the dirty list.
-				 */
-				redirty_tail(inode);
+				requeue_io(inode);
 				continue;
 			}
 
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