Re: [PATCH 2/3] cgroup: make the default root invisible when it's umounted

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2014/6/3 21:01, Tejun Heo wrote:
> On Tue, Jun 03, 2014 at 12:05:22PM +0800, Li Zefan wrote:
>> Before this patch (in a fresh system):
>>
>>    # cat /proc/$$/cgroup
>>    # mount -t cgroup -o __DEVEL__sane_behavior xxx /cgroup
>>    # umount /cgroup
>>    # cat /proc/$$/cgroup
>>    0:cpuset,cpu,cpuacct,memory,devices,freezer,net_cls,blkio,perf_event,net_prio,hugetlb:/
>>
>> After this patch (in a fresh system):
>>
>>    # cat ...
>>    # mount ...
>>    # umount ...
>>    # cat /proc/$$/cgroup
>>    #
>>
>> You won't see the default root after it's umounted.
> 
> Hmmmm... I intentionally left it visible tho.  The only reason we gate
> its visibility is avoid disturbing userland which doesn't know about
> and won't use the unified hierarchy.  If the userland starts making
> use of it, there's no reason to hide it again especially as that's
> consistent with how other hierarchies behave too - they keep showing
> up if they have lingering refs.
> 

The example I gave is the same result if sane_behavior is not specified,
so this is a behavioural change for the old interface?

Do we need a fix like this?

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3f46165..6f10cff 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1789,6 +1789,9 @@ static void cgroup_kill_sb(struct super_block *sb)
 	else
 		percpu_ref_kill(&root->cgrp.self.refcnt);

+	if (root == &cgrp_dfl_root && !cgroup_sane_behavior(&root->cgrp))
+		cgrp_dfl_root_visible = false;
+
 	kernfs_kill_sb(sb);