[PATCH v6 2/4] mm: memcontrol/slab: use helpers to access slab page's memcg_data

[Roman Gushchin <guro@fb.com>]

To gather all direct accesses to struct page's memcg_data field
in one place, let's introduce 3 new helpers to use in the slab
accounting code:
  struct obj_cgroup **page_objcgs(struct page *page);
  struct obj_cgroup **page_objcgs_check(struct page *page);
  bool set_page_objcgs(struct page *page, struct obj_cgroup **objcgs);

They are similar to the corresponding API for generic pages, except
that the setter can return false, indicating that the value has been
already set from a different thread.

Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 include/linux/memcontrol.h | 64 ++++++++++++++++++++++++++++++++++++++
 mm/memcontrol.c            |  6 ++--
 mm/slab.h                  | 35 +++++----------------
 3 files changed, 75 insertions(+), 30 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index f95c1433461c..c7ac0a5b8989 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -416,6 +416,70 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
 	return (struct mem_cgroup *)memcg_data;
 }
 
+#ifdef CONFIG_MEMCG_KMEM
+/*
+ * page_objcgs - get the object cgroups vector associated with a page
+ * @page: a pointer to the page struct
+ *
+ * Returns a pointer to the object cgroups vector associated with the page,
+ * or NULL. This function assumes that the page is known to have an
+ * associated object cgroups vector. It's not safe to call this function
+ * against pages, which might have an associated memory cgroup: e.g.
+ * kernel stack pages.
+ */
+static inline struct obj_cgroup **page_objcgs(struct page *page)
+{
+	return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
+}
+
+/*
+ * page_objcgs_check - get the object cgroups vector associated with a page
+ * @page: a pointer to the page struct
+ *
+ * Returns a pointer to the object cgroups vector associated with the page,
+ * or NULL. This function is safe to use if the page can be directly associated
+ * with a memory cgroup.
+ */
+static inline struct obj_cgroup **page_objcgs_check(struct page *page)
+{
+	unsigned long memcg_data = READ_ONCE(page->memcg_data);
+
+	if (memcg_data && (memcg_data & 0x1UL))
+		return (struct obj_cgroup **)(memcg_data & ~0x1UL);
+
+	return NULL;
+}
+
+/*
+ * set_page_objcgs - associate a page with a object cgroups vector
+ * @page: a pointer to the page struct
+ * @objcgs: a pointer to the object cgroups vector
+ *
+ * Atomically associates a page with a vector of object cgroups.
+ */
+static inline bool set_page_objcgs(struct page *page,
+					struct obj_cgroup **objcgs)
+{
+	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
+}
+#else
+static inline struct obj_cgroup **page_objcgs(struct page *page)
+{
+	return NULL;
+}
+
+static inline struct obj_cgroup **page_objcgs_check(struct page *page)
+{
+	return NULL;
+}
+
+static inline bool set_page_objcgs(struct page *page,
+					struct obj_cgroup **objcgs)
+{
+	return true;
+}
+#endif
+
 static __always_inline bool memcg_stat_item_in_bytes(int idx)
 {
 	if (idx == MEMCG_PERCPU_B)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 78c8a724a732..94cfde1e4632 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2899,7 +2899,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 	if (!vec)
 		return -ENOMEM;
 
-	if (cmpxchg(&page->memcg_data, 0, (unsigned long)vec | 0x1UL))
+	if (!set_page_objcgs(page, vec))
 		kfree(vec);
 	else
 		kmemleak_not_leak(vec);
@@ -2933,12 +2933,12 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 	 * Memcg membership data for each individual object is saved in
 	 * the page->obj_cgroups.
 	 */
-	if (page_has_obj_cgroups(page)) {
+	if (page_objcgs_check(page)) {
 		struct obj_cgroup *objcg;
 		unsigned int off;
 
 		off = obj_to_index(page->slab_cache, page, p);
-		objcg = page_obj_cgroups(page)[off];
+		objcg = page_objcgs(page)[off];
 		if (objcg)
 			return obj_cgroup_memcg(objcg);
 
diff --git a/mm/slab.h b/mm/slab.h
index e2535cee0d33..9a54a0cb5cca 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -239,28 +239,12 @@ static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t fla
 }
 
 #ifdef CONFIG_MEMCG_KMEM
-static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
-{
-	/*
-	 * Page's memory cgroup and obj_cgroups vector are sharing the same
-	 * space. To distinguish between them in case we don't know for sure
-	 * that the page is a slab page (e.g. page_cgroup_ino()), let's
-	 * always set the lowest bit of obj_cgroups.
-	 */
-	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
-}
-
-static inline bool page_has_obj_cgroups(struct page *page)
-{
-	return page->memcg_data & 0x1UL;
-}
-
 int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 				 gfp_t gfp);
 
 static inline void memcg_free_page_obj_cgroups(struct page *page)
 {
-	kfree(page_obj_cgroups(page));
+	kfree(page_objcgs(page));
 	page->memcg_data = 0;
 }
 
@@ -322,7 +306,7 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 		if (likely(p[i])) {
 			page = virt_to_head_page(p[i]);
 
-			if (!page_has_obj_cgroups(page) &&
+			if (!page_objcgs(page) &&
 			    memcg_alloc_page_obj_cgroups(page, s, flags)) {
 				obj_cgroup_uncharge(objcg, obj_full_size(s));
 				continue;
@@ -330,7 +314,7 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 
 			off = obj_to_index(s, page, p[i]);
 			obj_cgroup_get(objcg);
-			page_obj_cgroups(page)[off] = objcg;
+			page_objcgs(page)[off] = objcg;
 			mod_objcg_state(objcg, page_pgdat(page),
 					cache_vmstat_idx(s), obj_full_size(s));
 		} else {
@@ -344,6 +328,7 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
 					void **p, int objects)
 {
 	struct kmem_cache *s;
+	struct obj_cgroup **objcgs;
 	struct obj_cgroup *objcg;
 	struct page *page;
 	unsigned int off;
@@ -357,7 +342,8 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
 			continue;
 
 		page = virt_to_head_page(p[i]);
-		if (!page_has_obj_cgroups(page))
+		objcgs = page_objcgs(page);
+		if (!objcgs)
 			continue;
 
 		if (!s_orig)
@@ -366,11 +352,11 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
 			s = s_orig;
 
 		off = obj_to_index(s, page, p[i]);
-		objcg = page_obj_cgroups(page)[off];
+		objcg = objcgs[off];
 		if (!objcg)
 			continue;
 
-		page_obj_cgroups(page)[off] = NULL;
+		objcgs[off] = NULL;
 		obj_cgroup_uncharge(objcg, obj_full_size(s));
 		mod_objcg_state(objcg, page_pgdat(page), cache_vmstat_idx(s),
 				-obj_full_size(s));
@@ -379,11 +365,6 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
 }
 
 #else /* CONFIG_MEMCG_KMEM */
-static inline bool page_has_obj_cgroups(struct page *page)
-{
-	return false;
-}
-
 static inline struct mem_cgroup *memcg_from_slab_obj(void *ptr)
 {
 	return NULL;
-- 
2.26.2