[PATCH rfc 4/5] mm: introduce page memcg flags

[Roman Gushchin <guro@fb.com>]

The lowest bit in page->memcg_data is used to distinguish between
struct memory_cgroup pointer and a pointer to a objcgs array.
All checks and modifications of this bit are open-coded.

Let's formalize it using page memcg flags, defined in page_memcg_flags
enum and replace all open-coded accesses with test_bit()/__set_bit().

Few additional flags might be added later. Flags are intended to be
mutually exclusive.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/memcontrol.h | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 48d4c2c1ce81..7ab5f92bb686 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -340,23 +340,25 @@ struct mem_cgroup {
 
 extern struct mem_cgroup *root_mem_cgroup;
 
+enum page_memcg_flags {
+	/* page->memcg_data is a pointer to a objcgs vector */
+	PG_MEMCG_OBJ_CGROUPS,
+};
+
 static inline struct mem_cgroup *page_mem_cgroup(struct page *page)
 {
+	unsigned long memcg_data = page->memcg_data;
+
 	VM_BUG_ON_PAGE(PageSlab(page), page);
-	return (struct mem_cgroup *)page->memcg_data;
+
+	return (struct mem_cgroup *)memcg_data;
 }
 
 static inline struct mem_cgroup *page_mem_cgroup_check(struct page *page)
 {
 	unsigned long memcg_data = page->memcg_data;
 
-	/*
-	 * The lowest bit set means that memcg isn't a valid
-	 * memcg pointer, but a obj_cgroups pointer.
-	 * In this case the page is shared and doesn't belong
-	 * to any specific memory cgroup.
-	 */
-	if (memcg_data & 0x1UL)
+	if (test_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data))
 		return NULL;
 
 	return (struct mem_cgroup *)memcg_data;
@@ -378,14 +380,20 @@ static inline void clear_page_mem_cgroup(struct page *page)
 #ifdef CONFIG_MEMCG_KMEM
 static inline struct obj_cgroup **page_obj_cgroups(struct page *page)
 {
-	return (struct obj_cgroup **)(page->memcg_data & ~0x1UL);
+	unsigned long memcg_data = page->memcg_data;
+
+	VM_BUG_ON_PAGE(memcg_data && !test_bit(PG_MEMCG_OBJ_CGROUPS,
+					       &memcg_data), page);
+	__clear_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data);
+
+	return (struct obj_cgroup **)memcg_data;
 }
 
 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))
+	if (memcg_data && test_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data))
 		return (struct obj_cgroup **)memcg_data;
 
 	return NULL;
@@ -394,7 +402,11 @@ static inline struct obj_cgroup **page_obj_cgroups_check(struct page *page)
 static inline bool set_page_obj_cgroups(struct page *page,
 					struct obj_cgroup **objcgs)
 {
-	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
+	unsigned long memcg_data = (unsigned long)objcgs;
+
+	__set_bit(PG_MEMCG_OBJ_CGROUPS, &memcg_data);
+
+	return !cmpxchg(&page->memcg_data, 0, memcg_data);
 }
 
 static inline void clear_page_obj_cgroups(struct page *page)
-- 
2.26.2