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Subject[PATCH 2/3] mm: Make list_lru_node::memcg_lrus RCU protected
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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@virtuozzo.com>
---
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],
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