[BUGFIX][PATCH] memcg: fix try_get_mem_cgroup_from_swapcache()

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

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

memcg: fix try_get_mem_cgroup_from_swapcache()

This is a bugfix for commit 3c776e64660028236313f0e54f3a9945764422df(included 2.6.30-rc1).
Used bit of swapcache is solid under page lock, but considering move_account,
pc->mem_cgroup is not.

We need lock_page_cgroup() anyway.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ccc69b4..84f856c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1024,9 +1024,7 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
 		return NULL;
 
 	pc = lookup_page_cgroup(page);
-	/*
-	 * Used bit of swapcache is solid under page lock.
-	 */
+	lock_page_cgroup(pc);
 	if (PageCgroupUsed(pc)) {
 		mem = pc->mem_cgroup;
 		if (mem && !css_tryget(&mem->css))
@@ -1040,6 +1038,7 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
 			mem = NULL;
 		rcu_read_unlock();
 	}
+	unlock_page_cgroup(pc);
 	return mem;
 }
 

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