[PATCH RFC v3 13/17] mm/mglru: make folio_inc_lru_refs lruvec lockless

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

The lruvec spinlock in folio_inc_lru_refs was only taken to keep
concurrent aging from corrupting the size counters.  This is no
longer needed: the per-generation counters are atomic, and the
active/inactive ABI counters are updated per folio and linearized by
the CAS on the folio flags, so concurrent aging can no longer corrupt
them.  The same argument already allowed folio_reset_lru_refs() to
drop the lock.

Use folio_lruvec_live_get()/folio_lruvec_live_put() to hold the RCU
read lock around the lruvec lookup, and drop the spinlock entirely.

Without the lock, aging can advance max_seq while a promotion is in
flight.  Generations are indexed by seq % MAX_NR_GENS, so the folio
flags can return to the value read earlier and the cmpxchg succeeds
with a target gen computed from a stale max_seq.  Recheck max_seq after
a gen-changing cmpxchg: if it moved, account the committed update and
retry with LRU_REF_FORCE, which promotes the folio to the newest gen.
The retry may count the access twice, which is acceptable and avoids
folio_activate(), whose LRU lock causes latency jitter.  smp_rmb()
orders the flags read, including the one returned by a failed
cmpxchg(), before the max_seq read.

Also simplify the post-CAS accounting guards: gen is only assigned
non-negative values after the old_gen < 0 early exit, so gen != old_gen
implies gen >= 0, and lru_gen_update_size() needs no explicit guard.
The active/inactive ABI update keeps its gen >= 0 check because an
off-LRU folio has no lruvec to update.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 include/linux/mm_inline.h | 10 +++++-----
 include/linux/mmzone.h    |  3 ++-
 mm/vmscan.c               | 47 ++++++++++++++++++++++++++++-------------------
 3 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index ee7fcbe2366b..b68d68101248 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -293,10 +293,9 @@ static inline int folio_lru_gen(const struct folio *folio)
 	return lru_get_gen_flags(READ_ONCE(*const_folio_flags(folio, 0)));
 }
 
-static inline void lru_gen_update_size(struct lruvec *lruvec, struct folio *folio,
-				       int old_gen, int new_gen)
+static inline void lru_gen_update_size(struct lruvec *lruvec, int type,
+		struct folio *folio, int old_gen, int new_gen)
 {
-	int type = folio_is_file_lru(folio);
 	int zone = folio_zonenum(folio);
 	int delta = folio_nr_pages(folio);
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
@@ -371,7 +370,7 @@ static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
 	/* use the refs from the atomic snapshot to avoid raced update */
 	refs = lru_get_refs_flags(flags);
 
-	lru_gen_update_size(lruvec, folio, -1, gen);
+	lru_gen_update_size(lruvec, type, folio, -1, gen);
 	if (lru_refs_is_active(refs))
 		lru += LRU_ACTIVE;
 	__update_lru_size(lruvec, lru, zone, delta);
@@ -389,6 +388,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
 {
 	unsigned long flags;
 	int gen, refs;
+	int type = folio_is_file_lru(folio);
 	int zone = folio_zonenum(folio);
 	int delta = folio_nr_pages(folio);
 	enum lru_list lru = folio_is_file_lru(folio) * LRU_INACTIVE_FILE;
@@ -411,7 +411,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
 	if (!reclaiming && ((max_seq - gen) % MAX_NR_GENS) < MIN_NR_GENS)
 		folio_set_active(folio);
 
-	lru_gen_update_size(lruvec, folio, gen, -1);
+	lru_gen_update_size(lruvec, type, folio, gen, -1);
 	if (lru_refs_is_active(refs))
 		lru += LRU_ACTIVE;
 	__update_lru_size(lruvec, lru, zone, -delta);
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index e096fd0a5047..d78e3f97423e 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -558,9 +558,10 @@ enum lruvec_flags {
 #define LRU_TIER_MIN		0U
 #define LRU_TIER_MAX		(MAX_NR_TIERS - 1)
 
-/* Access source flags for folio_inc_lru_refs() */
+/* Flags for folio_inc_lru_refs() */
 #define LRU_REF_MAPPED		0x1U
 #define LRU_REF_EXEC		0x2U
+#define LRU_REF_FORCE		0x4U
 
 #define LRU_REFS_REFERENCED	0x1
 #define LRU_REFS_WORKINGSET	0x2
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ae4a0522a8d5..9cc06a95d931 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -982,38 +982,33 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
 	long nr_pages = folio_nr_pages(folio);
 	struct lru_gen_folio *lrugen;
 	struct lruvec *lruvec = NULL;
+	bool aged = false;
 
+retry:
 	old_flags = READ_ONCE(*folio_flags(folio, 0));
 	do {
 		new_flags = old_flags;
 		old_gen = lru_get_gen_flags(old_flags);
 		old_refs = lru_get_refs_flags(old_flags);
-		file = folio_flags_is_file_lru(&old_flags);
 		refs = old_refs + 1;
 		gen = old_gen;
+		file = folio_flags_is_file_lru(&old_flags);
 		if (old_gen < 0)
 			goto out;
-		/*
-		 * Lock the lruvec if the folio is on-list. We are already
-		 * doing lazy promotion so in theory we don't need this,
-		 * but for now, concurrent aging would still corrupt the
-		 * size counters.  This is a temporary limitation and
-		 * will be lifted very soon, so the lock here is not a
-		 * performance concern.
-		 */
 		if (!lruvec) {
-			lruvec = lruvec_live_lock_irq(folio_lruvec(folio));
+			lruvec = folio_lruvec_live_get(folio);
 			lrugen = &lruvec->lrugen;
 		}
+		/* Failed cmpxchg() does not order old_flags before max_seq. */
+		smp_rmb();
 		max_seq = READ_ONCE(lrugen->max_seq);
 		max_gen = lru_gen_from_seq(max_seq);
 		min_gen = lru_gen_from_seq(READ_ONCE(lrugen->min_seq[file]));
 		if (old_gen == max_gen)
 			goto out;
-
-		if (flags & (LRU_REF_MAPPED | LRU_REF_EXEC)) {
-			/* Promote second page table access or executable */
-			if (refs > LRU_REFS_REFERENCED || flags & LRU_REF_EXEC)
+		if (flags & (LRU_REF_MAPPED | LRU_REF_EXEC | LRU_REF_FORCE)) {
+			if (refs > LRU_REFS_REFERENCED ||
+			    flags & (LRU_REF_EXEC | LRU_REF_FORCE))
 				gen = max_gen;
 			/* First access only defers eviction from the oldest gen */
 			else if (old_gen == min_gen)
@@ -1037,9 +1032,19 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
 			break;
 	} while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
 
-	if (gen != old_gen)
-		lru_gen_update_size(lruvec, folio, old_gen, gen);
-	if (lru_refs_is_active(old_refs) != lru_refs_is_active(refs) && old_gen >= 0) {
+	if (gen != old_gen) {
+		/*
+		 * Gen-index reuse can fool cmpxchg(). Retry locklessly, accepting
+		 * an extra reference to avoid folio_activate() latency.
+		 */
+		if (unlikely(READ_ONCE(lrugen->max_seq) != max_seq)) {
+			flags = LRU_REF_FORCE;
+			aged = true;
+		}
+		lru_gen_update_size(lruvec, file, folio, old_gen, gen);
+	}
+
+	if (lru_refs_is_active(old_refs) != lru_refs_is_active(refs) && gen >= 0) {
 		enum lru_list lru = file * LRU_INACTIVE_FILE;
 
 		__update_lru_size(lruvec, lru + lru_refs_is_active(old_refs),
@@ -1047,8 +1052,12 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
 		__update_lru_size(lruvec, lru + lru_refs_is_active(refs),
 				  folio_zonenum(folio), nr_pages);
 	}
+	if (aged) {
+		aged = false;
+		goto retry;
+	}
 	if (lruvec)
-		lruvec_unlock_irq(lruvec);
+		folio_lruvec_live_put(lruvec);
 }
 
 /*
@@ -3648,7 +3657,7 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
 
 	new_gen = __folio_inc_gen(lruvec, folio, old_gen, &gen_increased);
 	if (gen_increased)
-		lru_gen_update_size(lruvec, folio, old_gen, new_gen);
+		lru_gen_update_size(lruvec, type, folio, old_gen, new_gen);
 
 	return new_gen;
 }

-- 
2.55.0