From d3132e4b83e6bd383c74d716f7281d7c3136089c Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:08 -0800 Subject: [PATCH] mm: compaction: reset cached scanner pfn's before reading them Compaction caches pfn's for its migrate and free scanners to avoid scanning the whole zone each time. In compact_zone(), the cached values are read to set up initial values for the scanners. There are several situations when these cached pfn's are reset to the first and last pfn of the zone, respectively. One of these situations is when a compaction has been deferred for a zone and is now being restarted during a direct compaction, which is also done in compact_zone(). However, compact_zone() currently reads the cached pfn's *before* resetting them. This means the reset doesn't affect the compaction that performs it, and with good chance also subsequent compactions, as update_pageblock_skip() is likely to be called and update the cached pfn's to those being processed. Another chance for a successful reset is when a direct compaction detects that migration and free scanners meet (which has its own problems addressed by another patch) and sets update_pageblock_skip flag which kswapd uses to do the reset because it goes to sleep. This is clearly a bug that results in non-deterministic behavior, so this patch moves the cached pfn reset to be performed *before* the values are read. Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman Acked-by: Rik van Riel Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/mm/compaction.c b/mm/compaction.c index 927de97cab8d..f4e2c166880b 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -946,6 +946,14 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) ; } + /* + * Clear pageblock skip if there were failures recently and compaction + * is about to be retried after being deferred. kswapd does not do + * this reset as it'll reset the cached information when going to sleep. + */ + if (compaction_restarting(zone, cc->order) && !current_is_kswapd()) + __reset_isolation_suitable(zone); + /* * Setup to move all movable pages to the end of the zone. Used cached * information on where the scanners should start but check that it @@ -962,14 +970,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) zone->compact_cached_migrate_pfn = cc->migrate_pfn; } - /* - * Clear pageblock skip if there were failures recently and compaction - * is about to be retried after being deferred. kswapd does not do - * this reset as it'll reset the cached information when going to sleep. - */ - if (compaction_restarting(zone, cc->order) && !current_is_kswapd()) - __reset_isolation_suitable(zone); - trace_mm_compaction_begin(start_pfn, cc->migrate_pfn, cc->free_pfn, end_pfn); migrate_prep_local(); -- 2.20.1