From: Jan Kara <jack@xxxxxxx> Background writeback is easily livelockable in a loop in wb_writeback() by a process continuously re-dirtying pages (or continuously appending to a file). This is in fact intended as the target of background writeback is to write dirty pages it can find as long as we are over dirty_background_threshold. But the above behavior gets inconvenient at times because no other work queued in the flusher thread's queue gets processed. In particular, since e.g. sync(1) relies on flusher thread to do all the IO for it, sync(1) can hang forever waiting for flusher thread to do the work. Generally, when a flusher thread has some work queued, someone submitted the work to achieve a goal more specific than what background writeback does. Moreover by working on the specific work, we also reduce amount of dirty pages which is exactly the target of background writeout. So it makes sense to give specific work 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. This may delay the writeback of expired inodes for a while, however the expired inodes will eventually be flushed to disk as long as the other works won't livelock. Signed-off-by: Jan Kara <jack@xxxxxxx> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> --- fs/fs-writeback.c | 10 ++++++++++ 1 file changed, 10 insertions(+) --- linux-next.orig/fs/fs-writeback.c 2010-11-10 07:04:34.000000000 +0800 +++ linux-next/fs/fs-writeback.c 2010-11-10 10:32:09.000000000 +0800 @@ -651,6 +651,16 @@ static long wb_writeback(struct bdi_writ break; /* + * Background writeout and kupdate-style writeback may + * run forever. Stop them if there is other work to do + * so that e.g. sync can proceed. They'll be restarted + * after the other works are all done. + */ + 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 */ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx 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: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>