[PATCH 2/3] mm: Make list_lru_node::memcg_lrus RCU protected

[Kirill Tkhai]

The array list_lru_node::memcg_lrus::list_lru_one[] only grows,
and it never shrinks. The growths happens in memcg_update_list_lru_node(),
and old array's members remain the same after it.

So, the access to the array's members may become RCU protected,
and it's possible to avoid using list_lru_node::lock to dereference it.
This will be used to get list's nr_items in next patch lockless.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 include/linux/list_lru.h |    2 +
 mm/list_lru.c            |   70 +++++++++++++++++++++++++++++++---------------
 2 files changed, 48 insertions(+), 24 deletions(-)

diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index b65505b32a3d..a55258100e40 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -43,7 +43,7 @@ struct list_lru_node {
 	struct list_lru_one	lru;
 #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
 	/* for cgroup aware lrus points to per cgroup lists, otherwise NULL */
-	struct list_lru_memcg	*memcg_lrus;
+	struct list_lru_memcg	__rcu *memcg_lrus;
 #endif
 	long nr_items;
 } ____cacheline_aligned_in_smp;
diff --git a/mm/list_lru.c b/mm/list_lru.c
index a726e321bf3e..2db3cdadb577 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -42,24 +42,30 @@ static void list_lru_unregister(struct list_lru *lru)
 #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
 static inline bool list_lru_memcg_aware(struct list_lru *lru)
 {
+	struct list_lru_memcg *memcg_lrus;
 	/*
 	 * This needs node 0 to be always present, even
 	 * in the systems supporting sparse numa ids.
+	 *
+	 * Here we only check the pointer is not NULL,
+	 * so RCU lock is not need.
 	 */
-	return !!lru->node[0].memcg_lrus;
+	memcg_lrus = rcu_dereference_check(lru->node[0].memcg_lrus, true);
+	return !!memcg_lrus;
 }
 
 static inline struct list_lru_one *
 list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
 {
+	struct list_lru_memcg *memcg_lrus;
 	/*
-	 * The lock protects the array of per cgroup lists from relocation
-	 * (see memcg_update_list_lru_node).
+	 * Either lock and RCU protects the array of per cgroup lists
+	 * from relocation (see memcg_update_list_lru_node).
 	 */
-	lockdep_assert_held(&nlru->lock);
-	if (nlru->memcg_lrus && idx >= 0)
-		return nlru->memcg_lrus->lru[idx];
-
+	memcg_lrus = rcu_dereference_check(nlru->memcg_lrus,
+					   lockdep_is_held(&nlru->lock));
+	if (memcg_lrus && idx >= 0)
+		return memcg_lrus->lru[idx];
 	return &nlru->lru;
 }
 
@@ -76,9 +82,12 @@ static __always_inline struct mem_cgroup *mem_cgroup_from_kmem(void *ptr)
 static inline struct list_lru_one *
 list_lru_from_kmem(struct list_lru_node *nlru, void *ptr)
 {
+	struct list_lru_memcg *memcg_lrus;
 	struct mem_cgroup *memcg;
 
-	if (!nlru->memcg_lrus)
+	/* Here we only check the pointer is not NULL, so RCU lock isn't need */
+	memcg_lrus = rcu_dereference_check(nlru->memcg_lrus, true);
+	if (!memcg_lrus)
 		return &nlru->lru;
 
 	memcg = mem_cgroup_from_kmem(ptr);
@@ -323,25 +332,33 @@ static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
 
 static int memcg_init_list_lru_node(struct list_lru_node *nlru)
 {
+	struct list_lru_memcg *memcg_lrus;
 	int size = memcg_nr_cache_ids;
 
-	nlru->memcg_lrus = kmalloc(sizeof(struct list_lru_memcg) +
-				   size * sizeof(void *), GFP_KERNEL);
-	if (!nlru->memcg_lrus)
+	memcg_lrus = kmalloc(sizeof(*memcg_lrus) +
+			     size * sizeof(void *), GFP_KERNEL);
+	if (!memcg_lrus)
 		return -ENOMEM;
 
-	if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) {
-		kfree(nlru->memcg_lrus);
+	if (__memcg_init_list_lru_node(memcg_lrus, 0, size)) {
+		kfree(memcg_lrus);
 		return -ENOMEM;
 	}
+	rcu_assign_pointer(nlru->memcg_lrus, memcg_lrus);
 
 	return 0;
 }
 
 static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
 {
-	__memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids);
-	kfree(nlru->memcg_lrus);
+	struct list_lru_memcg *memcg_lrus;
+	/*
+	 * This is called when shrinker has already been unregistered,
+	 * and nobody can use it. So, it's not need to use kfree_rcu().
+	 */
+	memcg_lrus = rcu_dereference_check(nlru->memcg_lrus, true);
+	__memcg_destroy_list_lru_node(memcg_lrus, 0, memcg_nr_cache_ids);
+	kfree(memcg_lrus);
 }
 
 static int memcg_update_list_lru_node(struct list_lru_node *nlru,
@@ -350,8 +367,10 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
 	struct list_lru_memcg *old, *new;
 
 	BUG_ON(old_size > new_size);
+	lockdep_assert_held(&list_lrus_mutex);
 
-	old = nlru->memcg_lrus;
+	/* list_lrus_mutex is held, nobody can change memcg_lrus. Silence RCU */
+	old = rcu_dereference_check(nlru->memcg_lrus, true);
 	new = kmalloc(sizeof(*new) + new_size * sizeof(void *), GFP_KERNEL);
 	if (!new)
 		return -ENOMEM;
@@ -364,26 +383,31 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
 	memcpy(&new->lru, &old->lru, old_size * sizeof(void *));
 
 	/*
-	 * The lock guarantees that we won't race with a reader
-	 * (see list_lru_from_memcg_idx).
+	 * The locking below allows the readers, that already take nlru->lock,
+	 * not to use additional rcu_read_lock()/rcu_read_unlock() pair.
 	 *
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */
 	spin_lock_irq(&nlru->lock);
-	nlru->memcg_lrus = new;
+	rcu_assign_pointer(nlru->memcg_lrus, new);
 	spin_unlock_irq(&nlru->lock);
 
-	kfree(old);
+	kfree_rcu(old, rcu);
 	return 0;
 }
 
 static void memcg_cancel_update_list_lru_node(struct list_lru_node *nlru,
 					      int old_size, int new_size)
 {
+	struct list_lru_memcg *memcg_lrus;
+
+	lockdep_assert_held(&list_lrus_mutex);
+	memcg_lrus = rcu_dereference_check(nlru->memcg_lrus, true);
+
 	/* do not bother shrinking the array back to the old size, because we
 	 * cannot handle allocation failures here */
-	__memcg_destroy_list_lru_node(nlru->memcg_lrus, old_size, new_size);
+	__memcg_destroy_list_lru_node(memcg_lrus, old_size, new_size);
 }
 
 static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
@@ -400,7 +424,7 @@ static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
 	return 0;
 fail:
 	for (i = i - 1; i >= 0; i--) {
-		if (!lru->node[i].memcg_lrus)
+		if (!rcu_dereference_check(lru->node[i].memcg_lrus, true))
 			continue;
 		memcg_destroy_list_lru_node(&lru->node[i]);
 	}
@@ -434,7 +458,7 @@ static int memcg_update_list_lru(struct list_lru *lru,
 	return 0;
 fail:
 	for (i = i - 1; i >= 0; i--) {
-		if (!lru->node[i].memcg_lrus)
+		if (!rcu_dereference_check(lru->node[i].memcg_lrus, true))
 			continue;
 
 		memcg_cancel_update_list_lru_node(&lru->node[i],