[RFC][BUGFIX][PATCH 1/2] memcg: fix charge bypass route of migration

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

I'd like to wait until next mmotm comes out. (So, [RFC]) I'll rebase
This patch is based on
 mmotm-2010/04/05
 +
 mm-migration-take-a-reference-to-the-anon_vma-before-migrating.patch
 mm-migration-do-not-try-to-migrate-unmapped-anonymous-pages.patch
 mm-share-the-anon_vma-ref-counts-between-ksm-and-page-migration.patch
 mm-migration-allow-the-migration-of-pageswapcache-pages.patch
 memcg-fix-prepare-migration.patch

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

This is an additonal fix to memcg-fix-prepare-migration.patch

Now, try_charge can bypass charge if TIF_MEMDIE at el are marked on the caller.
In this case, the charge is bypassed. This makes accounts corrupted.
(PageCgroup will be marked as PCG_USED even if bypassed, and css->refcnt
 can leak.)

This patch clears passed "*memcg" in bypass route.

Because usual page allocater passes memcg=NULL, this patch only affects
some special case as
  - move account
  - migration
  - swapin.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Index: mmotm-temp/mm/memcontrol.c
===================================================================
--- mmotm-temp.orig/mm/memcontrol.c
+++ mmotm-temp/mm/memcontrol.c
@@ -1606,8 +1606,12 @@ static int __mem_cgroup_try_charge(struc
 	 * MEMDIE process.
 	 */
 	if (unlikely(test_thread_flag(TIF_MEMDIE)
-		     || fatal_signal_pending(current)))
+		     || fatal_signal_pending(current))) {
+		/* Showing we skipped charge */
+		if (memcg)
+			*memcg = NULL;
 		goto bypass;
+	}
 
 	/*
 	 * We always charge the cgroup the mm_struct belongs to.
@@ -2523,7 +2527,6 @@ int mem_cgroup_prepare_migration(struct 
 		ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false);
 		css_put(&mem->css);
 	}
-	*ptr = mem;
 	return ret;
 }