[PATCH v3 3/4] mm: introduce page memcg flags

[Roman Gushchin <guro@xxxxxx>]

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 enum
page_memcg_data_flags.

Additional flags might be added later.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
---
 include/linux/memcontrol.h | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index d4d15c04bbaf..35f846c6b89b 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -343,6 +343,15 @@ struct mem_cgroup {
 
 extern struct mem_cgroup *root_mem_cgroup;
 
+enum page_memcg_data_flags {
+	/* page->memcg_data is a pointer to an objcgs vector */
+	MEMCG_DATA_OBJCGS,
+	/* the next bit after the last actual flag */
+	__NR_MEMCG_DATA_FLAGS,
+};
+
+#define MEMCG_DATA_FLAGS_MASK ((0x1UL << __NR_MEMCG_DATA_FLAGS) - 1)
+
 /*
  * page_memcg - get the memory cgroup associated with a page
  * @page: a pointer to the page struct
@@ -404,13 +413,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
 	 */
 	unsigned long memcg_data = READ_ONCE(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 (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS))
 		return NULL;
 
 	return (struct mem_cgroup *)memcg_data;
@@ -459,7 +462,12 @@ static inline void clear_page_memcg(struct page *page)
  */
 static inline struct obj_cgroup **page_objcgs(struct page *page)
 {
-	return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
+	unsigned long memcg_data = READ_ONCE(page->memcg_data);
+
+	VM_BUG_ON_PAGE(memcg_data &&
+		       !(memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)), page);
+
+	return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
 
 /*
@@ -474,8 +482,9 @@ 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);
+	if (memcg_data && (memcg_data & (0x1UL << MEMCG_DATA_OBJCGS)))
+		return (struct obj_cgroup **)
+			(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 
 	return NULL;
 }
@@ -490,7 +499,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
 static inline bool set_page_objcgs(struct page *page,
 					struct obj_cgroup **objcgs)
 {
-	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
+	unsigned long memcg_data = (unsigned long)objcgs;
+
+	memcg_data |= (0x1UL << MEMCG_DATA_OBJCGS);
+
+	return !cmpxchg(&page->memcg_data, 0, memcg_data);
 }
 
 /*
-- 
2.26.2