mm, compaction: distinguish COMPACT_DEFERRED from COMPACT_SKIPPED

try_to_compact_pages() can currently return COMPACT_SKIPPED even when
the compaction is defered for some zone just because zone DMA is skipped
in 99% of cases due to watermark checks.  This makes COMPACT_DEFERRED
basically unusable for the page allocator as a feedback mechanism.

Make sure we distinguish those two states properly and switch their
ordering in the enum.  This would mean that the COMPACT_SKIPPED will be
returned only when all eligible zones are skipped.

As a result COMPACT_DEFERRED handling for THP in __alloc_pages_slowpath
will be more precise and we would bail out rather than reclaim.

Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Michal Hocko

include/linux/compaction.h

@@ -4,13 +4,16 @@
 /* Return values for compact_zone() and try_to_compact_pages() */
 /* When adding new states, please adjust include/trace/events/compaction.h */
 enum compact_result {
-	/* compaction didn't start as it was deferred due to past failures */
-	COMPACT_DEFERRED,
 	/*
 	 * compaction didn't start as it was not possible or direct reclaim
 	 * was more suitable
 	 */
 	COMPACT_SKIPPED,
+	/* compaction didn't start as it was deferred due to past failures */
+	COMPACT_DEFERRED,
+	/* compaction not active last round */
+	COMPACT_INACTIVE = COMPACT_DEFERRED,
+
 	/* compaction should continue to another pageblock */
 	COMPACT_CONTINUE,
 	/*

include/trace/events/compaction.h

@@ -10,8 +10,8 @@
 #include <trace/events/mmflags.h>
 
 #define COMPACTION_STATUS					\
-	EM( COMPACT_DEFERRED,		"deferred")		\
 	EM( COMPACT_SKIPPED,		"skipped")		\
+	EM( COMPACT_DEFERRED,		"deferred")		\
 	EM( COMPACT_CONTINUE,		"continue")		\
 	EM( COMPACT_PARTIAL,		"partial")		\
 	EM( COMPACT_COMPLETE,		"complete")		\

mm/compaction.c

@@ -1578,7 +1578,7 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 	int may_perform_io = gfp_mask & __GFP_IO;
 	struct zoneref *z;
 	struct zone *zone;
-	enum compact_result rc = COMPACT_DEFERRED;
+	enum compact_result rc = COMPACT_SKIPPED;
 	int all_zones_contended = COMPACT_CONTENDED_LOCK; /* init for &= op */
 
 	*contended = COMPACT_CONTENDED_NONE;
@@ -1595,8 +1595,10 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 		enum compact_result status;
 		int zone_contended;
 
-		if (compaction_deferred(zone, order))
+		if (compaction_deferred(zone, order)) {
+			rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
 			continue;
+		}
 
 		status = compact_zone_order(zone, order, gfp_mask, mode,
 				&zone_contended, alloc_flags,
@@ -1667,7 +1669,7 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 	 * If at least one zone wasn't deferred or skipped, we report if all
 	 * zones that were tried were lock contended.
 	 */
-	if (rc > COMPACT_SKIPPED && all_zones_contended)
+	if (rc > COMPACT_INACTIVE && all_zones_contended)
 		*contended = COMPACT_CONTENDED_LOCK;
 
 	return rc;