memcg Can't context between v1 and v2 because css->refcnt not released

[喻望 <yuwang.yuwang-gPhfCIXyaqCqndwCJWfcng@public.gmane.org>]

Hello Johannes ,Michal,and Tejun:

  i using memcg v1,  but some reason  i want to context to  memcg v2,
but i can't, here is my step:
#cat /proc/cgroups
#subsys_name hierarchy num_cgroups enabled
 memory 5 1 1
#cd /sys/fs/cgroup/memory
#mkdir a
#echo 0 > a/cgroup.procs
#sleep 1
#echo 0 > cgroup.procs
#cat /proc/cgroups
#subsys_name hierarchy num_cgroups enabled
 memory 5 2 1  
the num_cgroups not go to "1"
so it will lead to can't context to memcg 2
#cd ..
#umount memory
umount: /sys/fs/cgroup/memory: target is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

  and i have tracked  the root cause, i found that "b2052564e66d mm:
memcontrol: continue cache reclaim from offlined groups"from Johannes
Weiner, remove mem_cgroup_reparent_charges when mem_cgroup_css_offline, so
the css->refcount  not go to "0", so the css_release not call when rmdir
cgroup, and nr_cgroups not released.
  so i want to ask does it reasonable can't context between v1 and v2