[BUGFIX][PATCH] memcg: force_emtpy fix to confirm cgroup has no tasks.

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

This is a fix to memcg-new-force_empty-to-free-pages-under-group.patch

Thanks,
-Kame
==
Now, force_empty can be called and do reclaim pages
while there are tasks. fix it.
(reclaim occurs but move doesn't occur in this case.)

And, we have interface to count # of tasks under cgroup.
Using it is better.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
 mm/memcontrol.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: mmotm-2.6.28-Nov24/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Nov24.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Nov24/mm/memcontrol.c
@@ -1481,6 +1481,7 @@ static int mem_cgroup_force_empty(struct
 	int ret;
 	int node, zid, shrink;
 	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
+	struct cgroup *cgrp = mem->css.cgroup;
 
 	css_get(&mem->css);
 
@@ -1491,7 +1492,7 @@ static int mem_cgroup_force_empty(struct
 move_account:
 	while (mem->res.usage > 0) {
 		ret = -EBUSY;
-		if (atomic_read(&mem->css.cgroup->count) > 0)
+		if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
 			goto out;
 		ret = -EINTR;
 		if (signal_pending(current))
@@ -1523,8 +1524,10 @@ out:
 	return ret;
 
 try_to_free:
-	/* returns EBUSY if we come here twice. */
-	if (shrink) {
+	/* returns EBUSY if there is a task or if we come here twice. */
+	if (cgroup_task_count(cgrp)
+	    || !list_empty(&cgrp->children)
+	    || shrink) {
 		ret = -EBUSY;
 		goto out;
 	}

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