mm: compaction: encapsulate defer reset logic

Currently there are several functions to manipulate the deferred
compaction state variables. The remaining case where the variables are
touched directly is when a successful allocation occurs in direct
compaction, or is expected to be successful in the future by kswapd.
Here, the lowest order that is expected to fail is updated, and in the
case of successful allocation, the deferred status and counter is reset
completely.

Create a new function compaction_defer_reset() to encapsulate this
functionality and make it easier to understand the code. No functional
change.

Signed-off-by: Vlastimil Babka <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Reviewed-by: Rik van Riel <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/compaction.h

···
 	return zone->compact_considered < defer_limit;
 }
 
+/*
+ * Update defer tracking counters after successful compaction of given order,
+ * which means an allocation either succeeded (alloc_success == true) or is
+ * expected to succeed.
+ */
+static inline void compaction_defer_reset(struct zone *zone, int order,
+		bool alloc_success)
+{
+	if (alloc_success) {
+		zone->compact_considered = 0;
+		zone->compact_defer_shift = 0;
+	}
+	if (order >= zone->compact_order_failed)
+		zone->compact_order_failed = order + 1;
+}
+
 /* Returns true if restarting compaction after many failures */
 static inline bool compaction_restarting(struct zone *zone, int order)
 {

mm/compaction.c

···
 			compact_zone(zone, cc);
 
 		if (cc->order > 0) {
-			int ok = zone_watermark_ok(zone, cc->order,
-						low_wmark_pages(zone), 0, 0);
-			if (ok && cc->order >= zone->compact_order_failed)
-				zone->compact_order_failed = cc->order + 1;
+			if (zone_watermark_ok(zone, cc->order,
+						low_wmark_pages(zone), 0, 0))
+				compaction_defer_reset(zone, cc->order, false);
 			/* Currently async compaction is never deferred. */
-			else if (!ok && cc->sync)
+			else if (cc->sync)
 				defer_compaction(zone, cc->order);
 		}
 

mm/page_alloc.c

···
 				preferred_zone, migratetype);
 		if (page) {
 			preferred_zone->compact_blockskip_flush = false;
-			preferred_zone->compact_considered = 0;
-			preferred_zone->compact_defer_shift = 0;
-			if (order >= preferred_zone->compact_order_failed)
-				preferred_zone->compact_order_failed = order + 1;
+			compaction_defer_reset(preferred_zone, order, true);
 			count_vm_event(COMPACTSUCCESS);
 			return page;
 		}