[PATCH 2/2] writeback: stop background/kupdate works from livelocking other works

[Wu Fengguang <fengguang.wu@intel.com>]

From: Jan Kara <jack@suse.cz>

Background writeback are easily livelockable (from a definition of their
target). This is inconvenient because it can make sync(1) stall forever waiting
on its queued work to be finished. Generally, when a flusher thread has
some work queued, someone submitted the work to achieve a goal more specific
than what background writeback does. So it makes sense to give it a priority
over a generic page cleaning.

Thus we interrupt background writeback if there is some other work to do. We
return to the background writeback after completing all the queued work.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- linux-next.orig/fs/fs-writeback.c	2010-11-01 19:50:22.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2010-11-01 19:56:54.000000000 +0800
@@ -664,6 +664,15 @@ static long wb_writeback(struct bdi_writ
 			break;
 
 		/*
+		 * Background writeout and kupdate-style writeback are
+		 * easily livelockable. Stop them if there is other work
+		 * to do so that e.g. sync can proceed.
+		 */
+		if ((work->for_background || work->for_kupdate) &&
+		    !list_empty(&wb->bdi->work_list))
+			break;
+
+		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */