mm, compaction: abstract compaction feedback to helpers

Compaction can provide a wild variation of feedback to the caller.  Many
of them are implementation specific and the caller of the compaction
(especially the page allocator) shouldn't be bound to specifics of the
current implementation.

This patch abstracts the feedback into three basic types:
	- compaction_made_progress - compaction was active and made some
	  progress.
	- compaction_failed - compaction failed and further attempts to
	  invoke it would most probably fail and therefore it is not
	  worth retrying
	- compaction_withdrawn - compaction wasn't invoked for an
          implementation specific reasons. In the current implementation
          it means that the compaction was deferred, contended or the
          page scanners met too early without any progress. Retrying is
          still worthwhile.

[[email protected]: do not change thp back off behavior]
[[email protected]: fix typo in comment, per Hillf]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Hillf Danton <[email protected]>
Acked-by: Vlastimil Babka <[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

@@ -78,6 +78,70 @@ extern void compaction_defer_reset(struct zone *zone, int order,
 				bool alloc_success);
 extern bool compaction_restarting(struct zone *zone, int order);
 
+/* Compaction has made some progress and retrying makes sense */
+static inline bool compaction_made_progress(enum compact_result result)
+{
+	/*
+	 * Even though this might sound confusing this in fact tells us
+	 * that the compaction successfully isolated and migrated some
+	 * pageblocks.
+	 */
+	if (result == COMPACT_PARTIAL)
+		return true;
+
+	return false;
+}
+
+/* Compaction has failed and it doesn't make much sense to keep retrying. */
+static inline bool compaction_failed(enum compact_result result)
+{
+	/* All zones were scanned completely and still not result. */
+	if (result == COMPACT_COMPLETE)
+		return true;
+
+	return false;
+}
+
+/*
+ * Compaction  has backed off for some reason. It might be throttling or
+ * lock contention. Retrying is still worthwhile.
+ */
+static inline bool compaction_withdrawn(enum compact_result result)
+{
+	/*
+	 * Compaction backed off due to watermark checks for order-0
+	 * so the regular reclaim has to try harder and reclaim something.
+	 */
+	if (result == COMPACT_SKIPPED)
+		return true;
+
+	/*
+	 * If compaction is deferred for high-order allocations, it is
+	 * because sync compaction recently failed. If this is the case
+	 * and the caller requested a THP allocation, we do not want
+	 * to heavily disrupt the system, so we fail the allocation
+	 * instead of entering direct reclaim.
+	 */
+	if (result == COMPACT_DEFERRED)
+		return true;
+
+	/*
+	 * If compaction in async mode encounters contention or blocks higher
+	 * priority task we back off early rather than cause stalls.
+	 */
+	if (result == COMPACT_CONTENDED)
+		return true;
+
+	/*
+	 * Page scanners have met but we haven't scanned full zones so this
+	 * is a back off in fact.
+	 */
+	if (result == COMPACT_PARTIAL_SKIPPED)
+		return true;
+
+	return false;
+}
+
 extern int kcompactd_run(int nid);
 extern void kcompactd_stop(int nid);
 extern void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx);
@@ -114,6 +178,21 @@ static inline bool compaction_deferred(struct zone *zone, int order)
 	return true;
 }
 
+static inline bool compaction_made_progress(enum compact_result result)
+{
+	return false;
+}
+
+static inline bool compaction_failed(enum compact_result result)
+{
+	return false;
+}
+
+static inline bool compaction_withdrawn(enum compact_result result)
+{
+	return true;
+}
+
 static inline int kcompactd_run(int nid)
 {
 	return 0;