[PATCH v7 10/10] mm: reparent memcg kmem_caches on cgroup removal

[Roman Gushchin <guro@fb.com>]

Let's reparent non-root kmem_caches on memcg offlining. This allows us
to release the memory cgroup without waiting for the last outstanding
kernel object (e.g. dentry used by another application).

Since the parent cgroup is already charged, everything we need to do
is to splice the list of kmem_caches to the parent's kmem_caches list,
swap the memcg pointer, drop the css refcounter for each kmem_cache
and adjust the parent's css refcounter.

Please, note that kmem_cache->memcg_params.memcg isn't a stable
pointer anymore. It's safe to read it under rcu_read_lock(),
cgroup_mutex held, or any other way that protects the memory cgroup
from being released.

We can race with the slab allocation and deallocation paths. It's not
a big problem: parent's charge and slab global stats are always
correct, and we don't care anymore about the child usage and global
stats. The child cgroup is already offline, so we don't use or show it
anywhere.

Local slab stats (NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE)
aren't used anywhere except count_shadow_nodes(). But even there it
won't break anything: after reparenting "nodes" will be 0 on child
level (because we're already reparenting shrinker lists), and on
parent level page stats always were 0, and this patch won't change
anything.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/slab.h |  2 +-
 mm/memcontrol.c      | 14 ++++++++------
 mm/slab.h            | 26 ++++++++++++++++++++------
 mm/slab_common.c     | 19 +++++++++++++++++--
 4 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 1b54e5f83342..fecf40b7be69 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -152,7 +152,7 @@ void kmem_cache_destroy(struct kmem_cache *);
 int kmem_cache_shrink(struct kmem_cache *);
 
 void memcg_create_kmem_cache(struct mem_cgroup *, struct kmem_cache *);
-void memcg_deactivate_kmem_caches(struct mem_cgroup *);
+void memcg_deactivate_kmem_caches(struct mem_cgroup *, struct mem_cgroup *);
 
 /*
  * Please use this macro to create slab caches. Simply specify the
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 25e72779fd33..db46a9dc37ab 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3289,15 +3289,15 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 	 */
 	memcg->kmem_state = KMEM_ALLOCATED;
 
-	memcg_deactivate_kmem_caches(memcg);
-
-	kmemcg_id = memcg->kmemcg_id;
-	BUG_ON(kmemcg_id < 0);
-
 	parent = parent_mem_cgroup(memcg);
 	if (!parent)
 		parent = root_mem_cgroup;
 
+	memcg_deactivate_kmem_caches(memcg, parent);
+
+	kmemcg_id = memcg->kmemcg_id;
+	BUG_ON(kmemcg_id < 0);
+
 	/*
 	 * Change kmemcg_id of this cgroup and all its descendants to the
 	 * parent's id, and then move all entries from this cgroup's list_lrus
@@ -3330,7 +3330,6 @@ static void memcg_free_kmem(struct mem_cgroup *memcg)
 	if (memcg->kmem_state == KMEM_ALLOCATED) {
 		WARN_ON(!list_empty(&memcg->kmem_caches));
 		static_branch_dec(&memcg_kmem_enabled_key);
-		WARN_ON(page_counter_read(&memcg->kmem));
 	}
 }
 #else
@@ -4777,6 +4776,9 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 
 	/* The following stuff does not apply to the root */
 	if (!parent) {
+#ifdef CONFIG_MEMCG_KMEM
+		INIT_LIST_HEAD(&memcg->kmem_caches);
+#endif
 		root_mem_cgroup = memcg;
 		return &memcg->css;
 	}
diff --git a/mm/slab.h b/mm/slab.h
index 7ead47cb9338..a4c9b9d042de 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -261,6 +261,9 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
  * which do not have slab_cache pointer set.
  * So this function assumes that the page can pass PageHead() and PageSlab()
  * checks.
+ *
+ * The kmem_cache can be reparented asynchronously. The caller must ensure
+ * the memcg lifetime, e.g. by taking rcu_read_lock() or cgroup_mutex.
  */
 static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
 {
@@ -268,7 +271,7 @@ static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
 
 	s = READ_ONCE(page->slab_cache);
 	if (s && !is_root_cache(s))
-		return s->memcg_params.memcg;
+		return READ_ONCE(s->memcg_params.memcg);
 
 	return NULL;
 }
@@ -285,10 +288,18 @@ static __always_inline int memcg_charge_slab(struct page *page,
 	struct lruvec *lruvec;
 	int ret;
 
-	memcg = s->memcg_params.memcg;
+	rcu_read_lock();
+	memcg = READ_ONCE(s->memcg_params.memcg);
+	while (memcg && !css_tryget_online(&memcg->css))
+		memcg = parent_mem_cgroup(memcg);
+	rcu_read_unlock();
+
+	if (unlikely(!memcg))
+		return true;
+
 	ret = memcg_kmem_charge_memcg(page, gfp, order, memcg);
 	if (ret)
-		return ret;
+		goto out;
 
 	lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
 	mod_lruvec_state(lruvec, cache_vmstat_idx(s), 1 << order);
@@ -296,8 +307,9 @@ static __always_inline int memcg_charge_slab(struct page *page,
 	/* transer try_charge() page references to kmem_cache */
 	percpu_ref_get_many(&s->memcg_params.refcnt, 1 << order);
 	css_put_many(&memcg->css, 1 << order);
-
-	return 0;
+out:
+	css_put(&memcg->css);
+	return ret;
 }
 
 /*
@@ -310,10 +322,12 @@ static __always_inline void memcg_uncharge_slab(struct page *page, int order,
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 
-	memcg = s->memcg_params.memcg;
+	rcu_read_lock();
+	memcg = READ_ONCE(s->memcg_params.memcg);
 	lruvec = mem_cgroup_lruvec(page_pgdat(page), memcg);
 	mod_lruvec_state(lruvec, cache_vmstat_idx(s), -(1 << order));
 	memcg_kmem_uncharge_memcg(page, order, memcg);
+	rcu_read_unlock();
 
 	percpu_ref_put_many(&s->memcg_params.refcnt, 1 << order);
 }
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 6b7750f7ea33..91e8c739dc97 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -252,7 +252,8 @@ static void memcg_unlink_cache(struct kmem_cache *s)
 	} else {
 		list_del(&s->memcg_params.children_node);
 		list_del(&s->memcg_params.kmem_caches_node);
-		css_put(&s->memcg_params.memcg->css);
+		mem_cgroup_put(s->memcg_params.memcg);
+		WRITE_ONCE(s->memcg_params.memcg, NULL);
 	}
 }
 #else
@@ -790,11 +791,13 @@ static void kmemcg_cache_deactivate(struct kmem_cache *s)
 	spin_unlock_irq(&memcg_kmem_wq_lock);
 }
 
-void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
+void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg,
+				  struct mem_cgroup *parent)
 {
 	int idx;
 	struct memcg_cache_array *arr;
 	struct kmem_cache *s, *c;
+	unsigned int nr_reparented;
 
 	idx = memcg_cache_id(memcg);
 
@@ -812,6 +815,18 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
 		kmemcg_cache_deactivate(c);
 		arr->entries[idx] = NULL;
 	}
+	nr_reparented = 0;
+	list_for_each_entry(s, &memcg->kmem_caches,
+			    memcg_params.kmem_caches_node) {
+		WRITE_ONCE(s->memcg_params.memcg, parent);
+		css_put(&memcg->css);
+		nr_reparented++;
+	}
+	if (nr_reparented) {
+		list_splice_init(&memcg->kmem_caches,
+				 &parent->kmem_caches);
+		css_get_many(&parent->css, nr_reparented);
+	}
 	mutex_unlock(&slab_mutex);
 
 	put_online_mems();
-- 
2.21.0