[RFC][PATCH 2/3] memcg: reduce size of struct page_cgroup.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>]

Now, page_cgroup->flags has only 3bits. Considering alignment of
struct mem_cgroup, which is allocated by kmalloc(), we can encode
pointer to mem_cgroup and flags into a word.

After this patch, pc->flags is encoded as

 63                           2     0
  | pointer to memcg..........|flags|

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 include/linux/page_cgroup.h |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 92768cb..bca5447 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -1,6 +1,10 @@
 #ifndef __LINUX_PAGE_CGROUP_H
 #define __LINUX_PAGE_CGROUP_H
 
+/*
+ * Because these flags are encoded into ->flags with a pointer,
+ * we cannot have too much flags.
+ */
 enum {
 	/* flags for mem_cgroup */
 	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
@@ -9,6 +13,8 @@ enum {
 	__NR_PCG_FLAGS,
 };
 
+#define PCG_FLAGS_MASK	((1 << __NR_PCG_FLAGS) - 1)
+
 #ifndef __GENERATING_BOUNDS_H
 #include <generated/bounds.h>
 
@@ -21,10 +27,12 @@ enum {
  * page_cgroup helps us identify information about the cgroup
  * All page cgroups are allocated at boot or memory hotplug event,
  * then the page cgroup for pfn always exists.
+ *
+ * flags and a pointer to memory cgroup are encoded into ->flags.
+ * Lower 3bits are used for flags and others are used for a pointer to memcg.
  */
 struct page_cgroup {
 	unsigned long flags;
-	struct mem_cgroup *mem_cgroup;
 };
 
 void __meminit pgdat_page_cgroup_init(struct pglist_data *pgdat);
@@ -85,13 +93,14 @@ static inline void unlock_page_cgroup(struct page_cgroup *pc)
 
 static inline struct mem_cgroup* pc_to_mem_cgroup(struct page_cgroup *pc)
 {
-	return pc->mem_cgroup;
+	return (struct mem_cgroup *)(pc->flags & ~PCG_FLAGS_MASK);
 }
 
 static inline void
 pc_set_mem_cgroup(struct page_cgroup *pc, struct mem_cgroup *memcg)
 {
-	pc->mem_cgroup = memcg;
+	unsigned long bits = pc->flags & PCG_FLAGS_MASK;
+	pc->flags = (unsigned long)memcg | bits;
 }
 
 #else /* CONFIG_CGROUP_MEM_RES_CTLR */
-- 
1.7.4.1


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