On Fri, May 17, 2013 at 10:48:05AM +0100, Mel Gorman wrote: > If a page is on a pagevec then it is !PageLRU and mark_page_accessed() > may fail to move a page to the active list as expected. Now that the LRU > is selected at LRU drain time, mark pages PageActive if they are on the > local pagevec so it gets moved to the correct list at LRU drain time. > Using a debugging patch it was found that for a simple git checkout based > workload that pages were never added to the active file list in practice > but with this patch applied they are. > > before after > LRU Add Active File 0 750583 > LRU Add Active Anon 2640587 2702818 > LRU Add Inactive File 8833662 8068353 > LRU Add Inactive Anon 207 200 > > Note that only pages on the local pagevec are considered on purpose. A > !PageLRU page could be in the process of being released, reclaimed, migrated > or on a remote pagevec that is currently being drained. Marking it PageActive > is vunerable to races where PageLRU and Active bits are checked at the > wrong time. Page reclaim will trigger VM_BUG_ONs but depending on when the > race hits, it could also free a PageActive page to the page allocator and > trigger a bad_page warning. Similarly a potential race exists between a > per-cpu drain on a pagevec list and an activation on a remote CPU. > > lru_add_drain_cpu > __pagevec_lru_add > lru = page_lru(page); > mark_page_accessed > if (PageLRU(page)) > activate_page > else > SetPageActive > SetPageLRU(page); > add_page_to_lru_list(page, lruvec, lru); > > In this case a PageActive page is added to the inactivate list and later the > inactive/active stats will get skewed. While the PageActive checks in vmscan > could be removed and potentially dealt with, a skew in the statistics would > be very difficult to detect. Hence this patch deals just with the common case > where a page being marked accessed has just been added to the local pagevec. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>