memcg: fix memory migration of shmem swapcache

In the current implementation mem_cgroup_end_migration() decides whether
the page migration has succeeded or not by checking "oldpage->mapping".

But if we are tring to migrate a shmem swapcache, the page->mapping of it
is NULL from the begining, so the check would be invalid. As a result,
mem_cgroup_end_migration() assumes the migration has succeeded even if
it's not, so "newpage" would be freed while it's not uncharged.

This patch fixes it by passing mem_cgroup_end_migration() the result of
the page migration.

Signed-off-by: Daisuke Nishimura <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Acked-by: Balbir Singh <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Reviewed-by: Johannes Weiner <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/memcontrol.h

···
 mem_cgroup_prepare_migration(struct page *page,
 	struct page *newpage, struct mem_cgroup **ptr);
 extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
-	struct page *oldpage, struct page *newpage);
+	struct page *oldpage, struct page *newpage, bool migration_ok);
 
 /*
  * For memory reclaim.
···
 }
 
 static inline void mem_cgroup_end_migration(struct mem_cgroup *mem,
-					struct page *oldpage,
-					struct page *newpage)
+		struct page *oldpage, struct page *newpage, bool migration_ok)
 {
 }
 

mm/memcontrol.c

···
 
 /* remove redundant charge if migration failed*/
 void mem_cgroup_end_migration(struct mem_cgroup *mem,
-	struct page *oldpage, struct page *newpage)
+	struct page *oldpage, struct page *newpage, bool migration_ok)
 {
 	struct page *used, *unused;
 	struct page_cgroup *pc;
···
 		return;
 	/* blocks rmdir() */
 	cgroup_exclude_rmdir(&mem->css);
-	/* at migration success, oldpage->mapping is NULL. */
-	if (oldpage->mapping) {
+	if (!migration_ok) {
 		used = oldpage;
 		unused = newpage;
 	} else {

mm/migrate.c

···
 
 uncharge:
 	if (!charge)
-		mem_cgroup_end_migration(mem, page, newpage);
+		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
 unlock:
 	unlock_page(page);