mm: compaction: remove compaction result helpers

Patch series "mm: compaction: cleanups & simplifications".

These compaction cleanups are split out from the huge page allocator
series[1], as requested by reviewer feedback.

[1] https://lore.kernel.org/linux-mm/20230418191313.268131-1-hannes@cmpxchg.org/


This patch (of 5):

The compaction result helpers encode quirks that are specific to the
allocator's retry logic.  E.g.  COMPACT_SUCCESS and COMPACT_COMPLETE
actually represent failures that should be retried upon, and so on.  I
frequently found myself pulling up the helper implementation in order to
understand and work on the retry logic.  They're not quite clean
abstractions; rather they split the retry logic into two locations.

Remove the helpers and inline the checks.  Then comment on the result
interpretations directly where the decision making happens.

Link: https://lkml.kernel.org/r/20230519123959.77335-1-hannes@cmpxchg.org
Link: https://lkml.kernel.org/r/20230519123959.77335-2-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/compaction.h

@@ -95,78 +95,6 @@ extern enum compact_result compaction_suitable(struct zone *zone, int order,
 extern void compaction_defer_reset(struct zone *zone, int order,
 				bool alloc_success);
 
-/* 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_SUCCESS)
-		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 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
-	 * so the regular reclaim has to try harder and reclaim something.
-	 */
-	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
-	 * 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;
-}
-
-
 bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
 					int alloc_flags);
 
@@ -185,26 +113,6 @@ static inline enum compact_result compaction_suitable(struct zone *zone, int ord
 	return COMPACT_SKIPPED;
 }
 
-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_needs_reclaim(enum compact_result result)
-{
-	return false;
-}
-
-static inline bool compaction_withdrawn(enum compact_result result)
-{
-	return true;
-}
-
 static inline void kcompactd_run(int nid)
 {
 }

include/trace/events/mmflags.h

@@ -223,8 +223,8 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,	"softdirty"	)		\
 #define compact_result_to_feedback(result)	\
 ({						\
 	enum compact_result __result = result;	\
-	(compaction_failed(__result)) ? COMPACTION_FAILED : \
-		(compaction_withdrawn(__result)) ? COMPACTION_WITHDRAWN : COMPACTION_PROGRESS; \
+	(__result == COMPACT_COMPLETE) ? COMPACTION_FAILED : \
+		(__result == COMPACT_SUCCESS) ? COMPACTION_PROGRESS : COMPACTION_WITHDRAWN; \
 })
 
 #define COMPACTION_FEEDBACK		\

mm/page_alloc.c

@@ -3469,35 +3469,39 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	if (fatal_signal_pending(current))
 		return false;
 
-	if (compaction_made_progress(compact_result))
+	/*
+	 * Compaction managed to coalesce some page blocks, but the
+	 * allocation failed presumably due to a race. Retry some.
+	 */
+	if (compact_result == COMPACT_SUCCESS)
 		(*compaction_retries)++;
 
 	/*
-	 * compaction considers all the zone as desperately out of memory
-	 * so it doesn't really make much sense to retry except when the
+	 * 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 (compaction_failed(compact_result))
+	if (compact_result == COMPACT_COMPLETE)
 		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.
+	 * Compaction was skipped due to a lack of free order-0
+	 * migration targets. Continue if reclaim can help.
 	 */
-	if (compaction_needs_reclaim(compact_result)) {
+	if (compact_result == COMPACT_SKIPPED) {
 		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 the next retry should use a higher priority if allowed, so
-	 * we don't just keep bailing out endlessly.
+	 * 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 (compaction_withdrawn(compact_result)) {
+	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