[PATCH rfc 3/5] mm: memcontrol/slab: use helpers to access slab page's memcg_data

[Roman Gushchin <guro@xxxxxx>]

To gather all direct accesses to struct page's memcg_data field
in one place, let's introduce 4 new helper functions to use in
the slab accounting code:
  struct obj_cgroup **page_obj_cgroups(struct page *page);
  struct obj_cgroup **page_obj_cgroups_check(struct page *page);
  bool set_page_obj_cgroups(struct page *page, struct obj_cgroup **objcgs);
  void clear_page_obj_cgroups(struct page *page);

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@xxxxxx>
---
 include/linux/memcontrol.h | 48 ++++++++++++++++++++++++++++++++++++++
 mm/memcontrol.c            |  4 ++--
 mm/slab.h                  | 27 +++------------------
 3 files changed, 53 insertions(+), 26 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 0997220c84ce..48d4c2c1ce81 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -375,6 +375,54 @@ static inline void clear_page_mem_cgroup(struct page *page)
 	page->memcg_data = 0;
 }
 
+#ifdef CONFIG_MEMCG_KMEM
+static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
+{
+	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
+}
+
+static inline struct obj_cgroup **page_obj_cgroups_check(struct page *page)
+{
+	unsigned long memcg_data = page->memcg_data;
+
+	if (memcg_data && (memcg_data & 0x1UL))
+		return (struct obj_cgroup **)memcg_data;
+
+	return NULL;
+}
+
+static inline bool set_page_obj_cgroups(struct page *page,
+					struct obj_cgroup **objcgs)
+{
+	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
+}
+
+static inline void clear_page_obj_cgroups(struct page *page)
+{
+	page->memcg_data = 0;
+}
+#else
+static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
+{
+	return NULL;
+}
+
+static inline struct obj_cgroup **page_obj_cgroups_check(struct page *page)
+{
+	return NULL;
+}
+
+static inline bool set_page_obj_cgroups(struct page *page,
+					struct obj_cgroup **objcgs)
+{
+	return true;
+}
+
+static inline void clear_page_obj_cgroups(struct page *page)
+{
+}
+#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 19180674e38a..ba9b053b1b88 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2890,7 +2890,7 @@ 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_obj_cgroups(page, vec))
 		kfree(vec);
 	else
 		kmemleak_not_leak(vec);
@@ -2918,7 +2918,7 @@ 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_obj_cgroups_check(page)) {
 		struct obj_cgroup *objcg;
 		unsigned int off;
 
diff --git a/mm/slab.h b/mm/slab.h
index b9787c4d6e78..9a46ab76cb61 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -239,29 +239,13 @@ 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->mem_cgroup and page->obj_cgroups 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));
-	page->memcg_data = 0;
+	clear_page_obj_cgroups(page);
 }
 
 static inline size_t obj_full_size(struct kmem_cache *s)
@@ -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_obj_cgroups(page) &&
 			    memcg_alloc_page_obj_cgroups(page, s, flags)) {
 				obj_cgroup_uncharge(objcg, obj_full_size(s));
 				continue;
@@ -349,7 +333,7 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct page *page,
 	if (!memcg_kmem_enabled())
 		return;
 
-	if (!page_has_obj_cgroups(page))
+	if (!page_obj_cgroups(page))
 		return;
 
 	off = obj_to_index(s, page, p);
@@ -367,11 +351,6 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s, struct page *page,
 }
 
 #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