mm, compaction: raise compaction priority after it withdrawns

Mike Kravetz reports that "hugetlb allocations could stall for minutes or
hours when should_compact_retry() would return true more often then it
should.  Specifically, this was in the case where compact_result was
COMPACT_DEFERRED and COMPACT_PARTIAL_SKIPPED and no progress was being
made."

The problem is that the compaction_withdrawn() test in
should_compact_retry() includes compaction outcomes that are only possible
on low compaction priority, and results in a retry without increasing the
priority.  This may result in furter reclaim, and more incomplete
compaction attempts.

With this patch, compaction priority is raised when possible, or
should_compact_retry() returns false.

The COMPACT_SKIPPED result doesn't really fit together with the other
outcomes in compaction_withdrawn(), as that's a result caused by
insufficient order-0 pages, not due to low compaction priority.  With this
patch, it is moved to a new compaction_needs_reclaim() function, and for
that outcome we keep the current logic of retrying if it looks like
reclaim will be able to help.

Link: http://lkml.kernel.org/r/20190806014744.15446-4-mike.kravetz@oracle.comReported-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Tested-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compaction.h

@@ -129,11 +129,8 @@ static inline bool compaction_failed(enum compact_result result)
 	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 needs reclaim to be performed first, so it can continue. */
+static inline bool compaction_needs_reclaim(enum compact_result result)
 {
 	/*
 	 * Compaction backed off due to watermark checks for order-0
@@ -142,6 +139,16 @@ static inline bool compaction_withdrawn(enum compact_result result)
 	if (result == COMPACT_SKIPPED)
 		return true;
 
+	return false;
+}
+
+/*
+ * Compaction has backed off for some reason after doing some work or none
+ * at all. It might be throttling or lock contention. Retrying might be still
+ * worthwhile, but with a higher priority if allowed.
+ */
+static inline bool compaction_withdrawn(enum compact_result result)
+{
 	/*
 	 * If compaction is deferred for high-order allocations, it is
 	 * because sync compaction recently failed. If this is the case
@@ -207,6 +214,11 @@ static inline bool compaction_failed(enum compact_result result)
 	return false;
 }
 
+static inline bool compaction_needs_reclaim(enum compact_result result)
+{
+	return false;
+}
+
 static inline bool compaction_withdrawn(enum compact_result result)
 {
 	return true;

mm/page_alloc.c

@@ -3954,15 +3954,23 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	if (compaction_failed(compact_result))
 		goto check_priority;
 
+	/*
+	 * compaction was skipped because there are not enough order-0 pages
+	 * to work with, so we retry only if it looks like reclaim can help.
+	 */
+	if (compaction_needs_reclaim(compact_result)) {
+		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
+		goto out;
+	}
+
 	/*
 	 * make sure the compaction wasn't deferred or didn't bail out early
 	 * due to locks contention before we declare that we should give up.
-	 * But do not retry if the given zonelist is not suitable for
-	 * compaction.
+	 * But the next retry should use a higher priority if allowed, so
+	 * we don't just keep bailing out endlessly.
 	 */
 	if (compaction_withdrawn(compact_result)) {
-		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
-		goto out;
+		goto check_priority;
 	}
 
 	/*