[RFC PATCH 14/26] mm: compaction: simplify should_compact_retry()

[Johannes Weiner <hannes@cmpxchg.org>]

The different branches for retry are unnecessarily complicated. There
is really only three outcomes: progress, skipped, failed. Also, the
retry counter only applies to loops that made progress, move it there.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.c | 60 +++++++++++++++++--------------------------------
 1 file changed, 20 insertions(+), 40 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c3b7dc479936..18fa2bbba44b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4608,7 +4608,6 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 		     enum compact_priority *compact_priority,
 		     int *compaction_retries)
 {
-	int max_retries = MAX_COMPACT_RETRIES;
 	int min_priority;
 	bool ret = false;
 	int retries = *compaction_retries;
@@ -4621,19 +4620,27 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 		return false;
 
 	/*
-	 * Compaction managed to coalesce some page blocks, but the
-	 * allocation failed presumably due to a race. Retry some.
+	 * Compaction coalesced some page blocks, but the allocation
+	 * failed, presumably due to a race. Retry a few times.
 	 */
-	if (compact_result == COMPACT_SUCCESS)
-		(*compaction_retries)++;
+	if (compact_result == COMPACT_SUCCESS) {
+		int max_retries = MAX_COMPACT_RETRIES;
 
-	/*
-	 * All zones were scanned completely and still no result. It
-	 * doesn't really make much sense to retry except when the
-	 * failure could be caused by insufficient priority
-	 */
-	if (compact_result == COMPACT_COMPLETE)
-		goto check_priority;
+		/*
+		 * !costly requests are much more important than
+		 * __GFP_RETRY_MAYFAIL costly ones because they are de
+		 * facto nofail and invoke OOM killer to move on while
+		 * costly can fail and users are ready to cope with
+		 * that. 1/4 retries is rather arbitrary but we would
+		 * need much more detailed feedback from compaction to
+		 * make a better decision.
+		 */
+		if (order > PAGE_ALLOC_COSTLY_ORDER)
+			max_retries /= 4;
+
+		ret = ++(*compaction_retries) <= MAX_COMPACT_RETRIES;
+		goto out;
+	}
 
 	/*
 	 * Compaction was skipped due to a lack of free order-0
@@ -4645,35 +4652,8 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	}
 
 	/*
-	 * If compaction backed due to being deferred, due to
-	 * contended locks in async mode, or due to scanners meeting
-	 * after a partial scan, retry with increased priority.
-	 */
-	if (compact_result == COMPACT_DEFERRED ||
-	    compact_result == COMPACT_CONTENDED ||
-	    compact_result == COMPACT_PARTIAL_SKIPPED)
-		goto check_priority;
-
-	/*
-	 * !costly requests are much more important than __GFP_RETRY_MAYFAIL
-	 * costly ones because they are de facto nofail and invoke OOM
-	 * killer to move on while costly can fail and users are ready
-	 * to cope with that. 1/4 retries is rather arbitrary but we
-	 * would need much more detailed feedback from compaction to
-	 * make a better decision.
-	 */
-	if (order > PAGE_ALLOC_COSTLY_ORDER)
-		max_retries /= 4;
-	if (*compaction_retries <= max_retries) {
-		ret = true;
-		goto out;
-	}
-
-	/*
-	 * Make sure there are attempts at the highest priority if we exhausted
-	 * all retries or failed at the lower priorities.
+	 * Compaction failed. Retry with increasing priority.
 	 */
-check_priority:
 	min_priority = (order > PAGE_ALLOC_COSTLY_ORDER) ?
 			MIN_COMPACT_COSTLY_PRIORITY : MIN_COMPACT_PRIORITY;
 
-- 
2.39.2