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 <redacted>
---
 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.