memcg: make page->mapping NULL before uncharge

This patch tries to make page->mapping to be NULL before
mem_cgroup_uncharge_cache_page() is called.

"page->mapping == NULL" is a good check for "whether the page is still
radix-tree or not".  This patch also adds BUG_ON() to
mem_cgroup_uncharge_cache_page();

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e93a4db93fbe..6f8b5b3b38b2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -734,6 +734,7 @@ void mem_cgroup_uncharge_page(struct page *page)
 void mem_cgroup_uncharge_cache_page(struct page *page)
 {
         VM_BUG_ON(page_mapped(page));
+        VM_BUG_ON(page->mapping);
         __mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
 }