Badari, I think you used SLUB. If so, page_cgroup's notifier callback was not
called and newly allocated page's page_cgroup wasn't allocated.
This is a fix. (notifier saw STOP_HERE flag added by slub's notifier.)

I'm now testing modified kernel, which does alloc/free page_cgroup by notifier.
(Usually, all page_cgroups are from bootmem and not freed.
 so, modified a bit for test)

And I cannot reproduce panic. I think you do "real" memory hotplug other than
online/offline and saw panic caused by this. 

Is this slub's behavior intentional ? page_cgroup's notifier has lower priority
than slub, now.

Thanks,
-Kame
==
notifier callback's notifier_from_errno() just works well in error
route. (It adds mask for "stop here")

Hanlder should return NOTIFY_OK in explict way.

Signed-off-by:KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/page_cgroup.c |    5 ++++-
 mm/slub.c        |    6 ++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

Index: mmotm-2.6.28-Nov10/mm/slub.c
===================================================================
--- mmotm-2.6.28-Nov10.orig/mm/slub.c
+++ mmotm-2.6.28-Nov10/mm/slub.c
@@ -3220,8 +3220,10 @@ static int slab_memory_callback(struct n
 	case MEM_CANCEL_OFFLINE:
 		break;
 	}
-
-	ret = notifier_from_errno(ret);
+	if (ret)
+		ret = notifier_from_errno(ret);
+	else
+		ret = NOTIFY_OK;
 	return ret;
 }
 
Index: mmotm-2.6.28-Nov10/mm/page_cgroup.c
===================================================================
--- mmotm-2.6.28-Nov10.orig/mm/page_cgroup.c
+++ mmotm-2.6.28-Nov10/mm/page_cgroup.c
@@ -216,7 +216,10 @@ static int page_cgroup_callback(struct n
 		break;
 	}
 
-	ret = notifier_from_errno(ret);
+	if (ret)
+		ret = notifier_from_errno(ret);
+	else
+		ret = NOTIFY_OK;
 
 	return ret;
 }


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