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Subject[PATCH 3/5] writeback: stop background/kupdate works from livelocking other works
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 2010-11-07 21:56:42.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2010-11-07 22:00:51.000000000 +0800
@@ -651,6 +651,15 @@ 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
*/



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