[BUGFIX][PATCH(resend) mmotm] memcg: try_get_mem_cgroup_from_swapcache fix (Re: 2.6.30 -mm merge plans)

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

>     memcg-use-css-id.patch
>     memcg-hierarchical-stat.patch
>     memcg-fix-shrinking-memory-to-return-ebusy-by-fixing-retry-algorithm.patch
>     memcg-fix-oom-killer-under-memcg.patch
>     memcg-fix-oom-killer-under-memcg-fix2.patch
>     memcg-fix-oom-killer-under-memcg-fix.patch
>     memcg-show-memcg-information-during-oom.patch
>     memcg-show-memcg-information-during-oom-fix2.patch
>     memcg-show-memcg-information-during-oom-fix.patch
>     memcg-show-memcg-information-during-oom-fix-fix.patch
>     memcg-show-memcg-information-during-oom-fix-fix-checkpatch-fixes.patch
>     memcg-remove-mem_cgroup_calc_mapped_ratio-take2.patch
>     memcg-remove-mem_cgroup_reclaim_imbalance-remnants.patch
>     memcg-charge-swapcache-to-proper-memcg.patch
>     cgroups-use-css-id-in-swap-cgroup-for-saving-memory-v5.patch
>     memcg-remove-redundant-message-at-swapon.patch
> 
>  Merge
> 
This patch is fix for cgroups-use-css-id-in-swap-cgroup-for-saving-memory-v5.patch.

I think it would be better to fold them.

===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

css_tryget can be called twice in !PageCgroupUsed case.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujtsu.com>
---

 mm/memcontrol.c |   10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5de6be9..55dea59 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1027,9 +1027,11 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
 	/*
 	 * Used bit of swapcache is solid under page lock.
 	 */
-	if (PageCgroupUsed(pc))
+	if (PageCgroupUsed(pc)) {
 		mem = pc->mem_cgroup;
-	else {
+		if (mem && !css_tryget(&mem->css))
+			mem = NULL;
+	} else {
 		ent.val = page_private(page);
 		id = lookup_swap_cgroup(ent);
 		rcu_read_lock();
@@ -1038,10 +1040,6 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
 			mem = NULL;
 		rcu_read_unlock();
 	}
-	if (!mem)
-		return NULL;
-	if (!css_tryget(&mem->css))
-		return NULL;
 	return mem;
 }