[PATCH][mmotm] memcg use css_tryget fix

[KAMEZAWA Hiroyuki]

I'm sorry for that I'm still generating new bugs...sigh.
-Kame
=
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

root_mem->last_scanned_child can be NULL here.
This may cause NULL pointer access when hierarchy is used.
This is a fix for memcg-use-css-tryget-in-memcg.patch

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/memcontrol.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: mmotm-2.6.28-Dec19/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Dec19.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Dec19/mm/memcontrol.c
@@ -656,7 +656,7 @@ mem_cgroup_get_first_node(struct mem_cgr
 
 	if (!root_mem->last_scanned_child || obsolete) {
 
-		if (obsolete)
+		if (obsolete && root_mem->last_scanned_child)
 			mem_cgroup_put(root_mem->last_scanned_child);
 
 		cgroup = list_first_entry(&root_mem->css.cgroup->children,

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