Commit d3132e4b authored Jan 21, 2014 by Vlastimil Babka Committed by Linus Torvalds Jan 21, 2014

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 <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent de6c60a6

mm/compaction.c

+8 −8

Original line number	Diff line number	Diff line
		@@ -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();

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