[PATCH RFC 08/15] mm/memcg: add folio-based lruvec live helper

Add a helper that resolves a stable lruvec for a folio under RCU
without taking the lruvec lock.  It takes a folio directly so the
lruvec lookup happens inside the RCU read-side critical section,
which a lruvec-based interface cannot guarantee.

The lock-taking variant now inlines the ancestor walk instead of
calling a separate helper.

No functional change.

Signed-off-by: Kairui Song <[email protected]>
---
 include/linux/memcontrol.h | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 68f363000d7f..ea0111392b9b 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1506,6 +1506,44 @@ static inline void lruvec_lock_irq(struct lruvec *lruvec)
 	spin_lock_irq(&lruvec->lru_lock);
 }
 
+/**
+ * folio_lruvec_live_get - get a live lruvec for a folio under RCU
+ * @folio: the folio
+ *
+ * Computes @folio's lruvec and walks up to the nearest live ancestor
+ * if the folio's memcg is dying.  Must be paired with
+ * folio_lruvec_live_put().
+ *
+ * Return: the live lruvec, with rcu_read_lock held.
+ */
+static inline struct lruvec *folio_lruvec_live_get(struct folio *folio)
+{
+#ifdef CONFIG_MEMCG
+	struct lruvec *lruvec;
+	struct pglist_data *pgdat;
+	struct mem_cgroup *memcg;
+
+	rcu_read_lock();
+	lruvec = folio_lruvec(folio);
+	pgdat = lruvec_pgdat(lruvec);
+	memcg = lruvec_memcg(lruvec);
+	while (unlikely(memcg && css_is_dying(&memcg->css))) {
+		memcg = parent_mem_cgroup(memcg);
+		lruvec = mem_cgroup_lruvec(memcg, pgdat);
+	}
+	return lruvec;
+#else
+	return folio_lruvec(folio);
+#endif
+}
+
+static inline void folio_lruvec_live_put(struct lruvec *lruvec)
+{
+#ifdef CONFIG_MEMCG
+	rcu_read_unlock();
+#endif
+}
+
 static inline struct lruvec *lruvec_live_lock_irq(struct lruvec *lruvec)
 {
 #ifdef CONFIG_MEMCG

-- 
2.55.0