mm: memcontrol: protect lruvec critical section with the RCU read lock

While acquiring the lruvec lock is sufficient to prevent memcgs and
objcgs from being released [1], calling folio_memcg() within the
critical section can trigger a bogus lockdep splat, as rcu_read_lock()
is not held:

  WARNING: CPU: 23 PID: 186 at include/linux/memcontrol.h:407 lru_add+0x411/0x460
  [...]
  Call Trace:
   <TASK>
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? folio_lruvec_lock_irqsave+0x135/0x2c0
   ? __pfx_lru_add+0x10/0x10
   folio_batch_move_lru+0x6c/0x1d0
   ? lock_acquire+0x107/0x140
   ? __pfx_lru_add+0x10/0x10
   __folio_batch_add_and_move+0x109/0x340
   filemap_add_folio+0xd5/0x100
   [...]
   </TASK>

Altough it is technically safe to not hold the RCU read lock while holding
the lruvec lock, these false warnings make it harder to spot and debug
real issues where the RCU lock is required but missing.

Therefore, explicitly hold the RCU read lock within the critical section
protected by the lruvec lock. This behavior is aligned with the Muchun
Song's latest patch series [2].

Orabug: 37997580

Link: https://lore.kernel.org/linux-mm/[email protected]/ [1]
Link: https://lore.kernel.org/linux-mm/[email protected]/ [2]
Reviewed-by: Imran Khan <[email protected]>
Signed-off-by: Harry Yoo <[email protected]>

Author: Harry Yoo

include/linux/memcontrol.h

@@ -1587,17 +1587,20 @@ static inline struct lruvec *parent_lruvec(struct lruvec *lruvec)
 static inline void lruvec_unlock(struct lruvec *lruvec)
 {
 	spin_unlock(&lruvec->lru_lock);
+	rcu_read_unlock();
 }
 
 static inline void lruvec_unlock_irq(struct lruvec *lruvec)
 {
 	spin_unlock_irq(&lruvec->lru_lock);
+	rcu_read_unlock();
 }
 
 static inline void lruvec_unlock_irqrestore(struct lruvec *lruvec,
 		unsigned long flags)
 {
 	spin_unlock_irqrestore(&lruvec->lru_lock, flags);
+	rcu_read_unlock();
 }
 
 /* Test requires a stable folio->memcg binding, see folio_memcg() */

mm/compaction.c

@@ -564,7 +564,6 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
 		spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
 		goto retry;
 	}
-	rcu_read_unlock();
 
 	return lruvec;
 }

mm/memcontrol.c

@@ -1347,7 +1347,6 @@ struct lruvec *folio_lruvec_lock(struct folio *folio)
 		spin_unlock(&lruvec->lru_lock);
 		goto retry;
 	}
-	rcu_read_unlock();
 
 	return lruvec;
 }
@@ -1378,7 +1377,6 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio)
 		spin_unlock_irq(&lruvec->lru_lock);
 		goto retry;
 	}
-	rcu_read_unlock();
 
 	return lruvec;
 }
@@ -1411,7 +1409,6 @@ struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
 		spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
 		goto retry;
 	}
-	rcu_read_unlock();
 
 	return lruvec;
 }