[PATCH] mm: Stop background writeback if there is other work queued for the thread

[Jan Kara]

Background writeback and kupdate-style 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.
Fix the problem by interrupting background and kupdate writeback if there
is some other work to do. We can return to them after completing all the
queued work.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/fs-writeback.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d5be169..542471e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -633,6 +633,14 @@ static long wb_writeback(struct bdi_writeback *wb,
 			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
 		 */
-- 
1.6.4.2


--pWyiEgJYm5f9v55/--
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