On Wed, Feb 09, 2011 at 04:46:06PM +0100, Johannes Weiner wrote: > Hi, > > I think this should fix the problem of processes getting stuck in > reclaim that has been reported several times. I don't think it's the only source but I'm basing this on seeing constant looping in balance_pgdat() and calling congestion_wait() a few weeks ago that I haven't rechecked since. However, this looks like a real fix for a real problem. > Kent actually > single-stepped through this code and noted that it was never exiting > shrink_zone(), which really narrowed it down a lot, considering the > tons of nested loops from the allocator down to the list shrinking. > > Hannes > > --- > From: Johannes Weiner <hannes@xxxxxxxxxxx> > Subject: vmscan: fix zone shrinking exit when scan work is done > > '3e7d344 mm: vmscan: reclaim order-0 and use compaction instead of > lumpy reclaim' introduced an indefinite loop in shrink_zone(). > > It meant to break out of this loop when no pages had been reclaimed > and not a single page was even scanned. The way it would detect the > latter is by taking a snapshot of sc->nr_scanned at the beginning of > the function and comparing it against the new sc->nr_scanned after the > scan loop. But it would re-iterate without updating that snapshot, > looping forever if sc->nr_scanned changed at least once since > shrink_zone() was invoked. > > This is not the sole condition that would exit that loop, but it > requires other processes to change the zone state, as the reclaimer > that is stuck obviously can not anymore. > > This is only happening for higher-order allocations, where reclaim is > run back to back with compaction. > > Reported-by: Michal Hocko <mhocko@xxxxxxx> > Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Well spotted. Acked-by: Mel Gorman <mel@xxxxxxxxx> -- Mel Gorman SUSE Labs -- 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 internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>