[PATCH v5 7/7] mm: fix /proc/kpagecgroup interface for slab pages

[Roman Gushchin <guro@fb.com>]

Switching to an indirect scheme of getting mem_cgroup pointer for
!root slab pages broke /proc/kpagecgroup interface for them.

Let's fix it by learning page_cgroup_ino() how to get memcg
pointer for slab pages.

Reported-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c  |  5 ++++-
 mm/slab.h        | 25 +++++++++++++++++++++++++
 mm/slab_common.c |  1 +
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index de664ff1e310..f58454f5cedc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -494,7 +494,10 @@ ino_t page_cgroup_ino(struct page *page)
 	unsigned long ino = 0;
 
 	rcu_read_lock();
-	memcg = READ_ONCE(page->mem_cgroup);
+	if (PageHead(page) && PageSlab(page))
+		memcg = memcg_from_slab_page(page);
+	else
+		memcg = READ_ONCE(page->mem_cgroup);
 	while (memcg && !(memcg->css.flags & CSS_ONLINE))
 		memcg = parent_mem_cgroup(memcg);
 	if (memcg)
diff --git a/mm/slab.h b/mm/slab.h
index 7ba50e526d82..50fa534c0fc0 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -256,6 +256,26 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
 	return s->memcg_params.root_cache;
 }
 
+/*
+ * Expects a pointer to a slab page. Please note, that PageSlab() check
+ * isn't sufficient, as it returns true also for tail compound slab pages,
+ * which do not have slab_cache pointer set.
+ * So this function assumes that the page can pass PageHead() and PageSlab()
+ * checks.
+ */
+static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
+{
+	struct kmem_cache *s;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	s = READ_ONCE(page->slab_cache);
+	if (s && !is_root_cache(s))
+		return rcu_dereference(s->memcg_params.memcg);
+
+	return NULL;
+}
+
 /*
  * Charge the slab page belonging to the non-root kmem_cache.
  * Can be called for non-root kmem_caches only.
@@ -353,6 +373,11 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
 	return s;
 }
 
+static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
+{
+	return NULL;
+}
+
 static inline int memcg_charge_slab(struct page *page, gfp_t gfp, int order,
 				    struct kmem_cache *s)
 {
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 7607a40772aa..e818609c8209 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -254,6 +254,7 @@ static void memcg_unlink_cache(struct kmem_cache *s)
 		list_del(&s->memcg_params.kmem_caches_node);
 		mem_cgroup_put(rcu_dereference_protected(s->memcg_params.memcg,
 			lockdep_is_held(&slab_mutex)));
+		rcu_assign_pointer(s->memcg_params.memcg, NULL);
 	}
 }
 #else
-- 
2.20.1