While using memory control cgroup, page-migration under it works as following.
==
 1. uncharge all refs at try to unmap.
 2. charge regs again remove_migration_ptes()
==
This is simple but has following problems.
==
 The page is uncharged and chaged back again if *mapped*.
    - This means that cgroup before migraion can be different from one after
      migraion
    - If page is not mapped but charged as page cache, charge is just ignored
      (because not mapped, it will not be uncharged before migration)
      This is memory leak.
==
This patch tries to keep memory cgroup at page migration by increasing
one refcnt during it. 3 functions are added.

 mem_cgroup_prepare_migration() --- increase refcnt of page->page_cgroup
 mem_cgroup_end_migration()     --- decrease refcnt of page->page_cgroup
 mem_cgroup_page_migration() --- copy page->page_cgroup from old page to
                                 new page.

During migration
  - old page is under PG_locked.
  - new page is under PG_locked, too.
  - both old page and new page are not on LRU.

These 3 facts guarantees page_cgroup() migration has no race, I think.

Tested and worked well in x86_64/fake-NUMA box.

Changelog v1 -> v2:
  - reflected comments.
  - divided a patche to !PageLRU patch and migration patch.



Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

 include/linux/memcontrol.h |   19 +++++++++++++++++++
 mm/memcontrol.c            |   43 +++++++++++++++++++++++++++++++++++++++++++
 mm/migrate.c               |   14 +++++++++++---
 3 files changed, 73 insertions(+), 3 deletions(-)

Index: devel-2.6.23-rc8-mm2/mm/migrate.c
===================================================================
--- devel-2.6.23-rc8-mm2.orig/mm/migrate.c
+++ devel-2.6.23-rc8-mm2/mm/migrate.c
@@ -598,9 +598,10 @@ static int move_to_new_page(struct page 
 	else
 		rc = fallback_migrate_page(mapping, newpage, page);
 
-	if (!rc)
+	if (!rc) {
+		mem_cgroup_page_migration(page, newpage);
 		remove_migration_ptes(page, newpage);
-	else
+	} else
 		newpage->mapping = NULL;
 
 	unlock_page(newpage);
@@ -619,6 +620,7 @@ static int unmap_and_move(new_page_t get
 	int *result = NULL;
 	struct page *newpage = get_new_page(page, private, &result);
 	int rcu_locked = 0;
+	int charge = 0;
 
 	if (!newpage)
 		return -ENOMEM;
@@ -660,14 +662,20 @@ static int unmap_and_move(new_page_t get
 	 */
 	if (!page->mapping)
 		goto rcu_unlock;
+
+	charge = mem_cgroup_prepare_migration(page);
 	/* Establish migration ptes or remove ptes */
 	try_to_unmap(page, 1);
 
 	if (!page_mapped(page))
 		rc = move_to_new_page(newpage, page);
 
-	if (rc)
+	if (rc) {
 		remove_migration_ptes(page, page);
+		if (charge)
+			mem_cgroup_end_migration(page);
+	} else if (charge)
+		mem_cgroup_end_migration(newpage);
 rcu_unlock:
 	if (rcu_locked)
 		rcu_read_unlock();
Index: devel-2.6.23-rc8-mm2/include/linux/memcontrol.h
===================================================================
--- devel-2.6.23-rc8-mm2.orig/include/linux/memcontrol.h
+++ devel-2.6.23-rc8-mm2/include/linux/memcontrol.h
@@ -56,6 +56,10 @@ static inline void mem_cgroup_uncharge_p
 	mem_cgroup_uncharge(page_get_page_cgroup(page));
 }
 
+extern int mem_cgroup_prepare_migration(struct page *page);
+extern void mem_cgroup_end_migration(struct page *page);
+extern void mem_cgroup_page_migration(struct page *page, struct page *newpage);
+
 #else /* CONFIG_CGROUP_MEM_CONT */
 static inline void mm_init_cgroup(struct mm_struct *mm,
 					struct task_struct *p)
@@ -107,6 +111,21 @@ static inline struct mem_cgroup *mm_cgro
 	return NULL;
 }
 
+static inline int mem_cgroup_prepare_migration(struct page *page)
+{
+	return 0;
+}
+
+static inline void mem_cgroup_end_migration(struct page *page)
+{
+}
+
+static inline void
+mem_cgroup_page_migration(struct page *page, struct page *newpage);
+{
+}
+
+
 #endif /* CONFIG_CGROUP_MEM_CONT */
 
 #endif /* _LINUX_MEMCONTROL_H */
Index: devel-2.6.23-rc8-mm2/mm/memcontrol.c
===================================================================
--- devel-2.6.23-rc8-mm2.orig/mm/memcontrol.c
+++ devel-2.6.23-rc8-mm2/mm/memcontrol.c
@@ -463,6 +463,49 @@ void mem_cgroup_uncharge(struct page_cgr
 		}
 	}
 }
+/*
+ * Returns non-zero if a page (under migration) has valid page_cgroup member.
+ * Refcnt of page_cgroup is incremented.
+ */
+
+int mem_cgroup_prepare_migration(struct page *page)
+{
+	struct page_cgroup *pc;
+	int ret = 0;
+	lock_page_cgroup(page);
+	pc = page_get_page_cgroup(page);
+	if (pc && atomic_inc_not_zero(&pc->ref_cnt))
+		ret = 1;
+	unlock_page_cgroup(page);
+	return ret;
+}
+
+void mem_cgroup_end_migration(struct page *page)
+{
+	struct page_cgroup *pc = page_get_page_cgroup(page);
+	mem_cgroup_uncharge(pc);
+}
+/*
+ * We know both *page* and *newpage* are now not-on-LRU and Pg_locked.
+ * And no rece with uncharge() routines becasue page_cgroup for *page*
+ * has extra one reference by mem_cgroup_prepare_migration.
+ */
+
+void mem_cgroup_page_migration(struct page *page, struct page *newpage)
+{
+	struct page_cgroup *pc;
+retry:
+	pc = page_get_page_cgroup(page);
+	if (!pc)
+		return;
+	if (clear_page_cgroup(page, pc) != pc)
+		goto retry;
+	pc->page = newpage;
+	lock_page_cgroup(newpage);
+	page_assign_page_cgroup(newpage, pc);
+	unlock_page_cgroup(newpage);
+	return;
+}
 
 int mem_cgroup_write_strategy(char *buf, unsigned long long *tmp)
 {

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