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From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: [PATCH 1/5] mm: compaction: remove compaction result helpers
Date:   Fri, 19 May 2023 14:39:55 +0200
Message-Id: <20230519123959.77335-2-hannes@cmpxchg.org>
In-Reply-To: <20230519123959.77335-1-hannes@cmpxchg.org>
References: <20230519123959.77335-1-hannes@cmpxchg.org>
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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.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/compaction.h     | 92 ----------------------------------
 include/trace/events/mmflags.h |  4 +-
 mm/page_alloc.c                | 30 ++++++-----
 3 files changed, 19 insertions(+), 107 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index a6e512cfb670..1f0328a2ba48 100644
--- a/include/linux/compaction.h
+++ b/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)
 {
 }
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index b63e7c0fbbe5..1478b9dd05fa 100644
--- a/include/trace/events/mmflags.h
+++ b/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		\
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 47421bedc12b..5a84a0bebc37 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3768,35 +3768,39 @@ 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
-- 
2.40.0