On Tue, 09 Nov 2010 07:09:19 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > I find the description to be somewhat incomplete... > From: Jan Kara <jack@xxxxxxx> > > Background writeback are easily livelockable (from a definition of their > target). *why* is background writeback easily livelockable? Under which circumstances does this happen and how does it come about? > This is inconvenient because it can make sync(1) stall forever waiting > on its queued work to be finished. Again, why? Because there are works queued from the flusher thread, but that thread is stuck in a livelocked state in <unspecified code location> so it is unable to service the other works? But the pocess which called sync() will as a last resort itself perform all the required IO, will it not? If so, how can it livelock? > 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@xxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > --- > 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 > */ So... what prevents higher priority works (eg, sync(1)) from livelocking or seriously retarding background or kudate writeout? -- 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>