memcg: update document to mention that swapoff should be tested

Considering the recently found problem "memcg: fix refcnt handling at
swapoff", it's better to mention swapoff behavior in the memcg_test
document.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memcg_test.txt

@@ -1,6 +1,6 @@
 Memory Resource Controller(Memcg)  Implementation Memo.
-Last Updated: 2008/12/15
-Base Kernel Version: based on 2.6.28-rc8-mm.
+Last Updated: 2009/1/19
+Base Kernel Version: based on 2.6.29-rc2.
 
 Because VM is getting complex (one of reasons is memcg...), memcg's behavior
 is complex. This is a document for memcg's internal behavior.
@@ -340,3 +340,23 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
 	# mount -t cgroup none /cgroup -t cpuset,memory,cpu,devices
 
 	and do task move, mkdir, rmdir etc...under this.
+
+ 9.7 swapoff.
+	Besides management of swap is one of complicated parts of memcg,
+	call path of swap-in at swapoff is not same as usual swap-in path..
+	It's worth to be tested explicitly.
+
+	For example, test like following is good.
+	(Shell-A)
+	# mount -t cgroup none /cgroup -t memory
+	# mkdir /cgroup/test
+	# echo 40M > /cgroup/test/memory.limit_in_bytes
+	# echo 0 > /cgroup/test/tasks
+	Run malloc(100M) program under this. You'll see 60M of swaps.
+	(Shell-B)
+	# move all tasks in /cgroup/test to /cgroup
+	# /sbin/swapoff -a
+	# rmdir /test/cgroup
+	# kill malloc task.
+
+	Of course, tmpfs v.s. swapoff test should be tested, too.