linux-kernel - Re: cgroup, kernfs: make mountinfo show properly scoped path for cgroup namespaces

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Date:	Wed, 11 May 2016 17:10:16 -0700
From:	Andrei Vagin <avagin@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>
Subject: Re: cgroup, kernfs: make mountinfo show properly scoped path for
 cgroup namespaces

Hello,

https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=4f41fc59620fcedaa97cbdf3d7d2956d80fcd922

This commit breaks criu:

[root@...2-vm criu]# ./criu/criu check
Error (mount.c:403): mnt: No root found for mountpoint 35 (@./sys/kernel/debug)

The problem is that mountinfo doesn't contain a /sys mount, so criu
can't build a tree of mounts.

[root@...2-vm criu]# cat /proc/self/mountinfo  | grep 17
20 17 0:18 / /sys/kernel/security rw,nosuid,nodev,noexec,relatime
shared:7 - securityfs securityfs rw
24 17 0:21 / /sys/fs/cgroup ro,nosuid,nodev,noexec shared:8 - tmpfs
tmpfs ro,mode=755
26 17 0:23 / /sys/fs/pstore rw,nosuid,nodev,noexec,relatime shared:18
- pstore pstore rw
34 24 0:31 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime
shared:17 - cgroup cgroup rw,freezer
54 17 0:32 / /sys/kernel/config rw,relatime shared:19 - configfs configfs rw
35 17 0:7 / /sys/kernel/debug rw,relatime shared:22 - debugfs debugfs rw

[root@...2-vm criu]# python
>>> import os
>>> os.open("/sys", os.O_RDONLY)
3
>>>
[1]+  Stopped                 python
[root@...2-vm criu]# ps
  PID TTY          TIME CMD
  760 pts/0    00:00:00 bash
  913 pts/0    00:00:00 python
  914 pts/0    00:00:00 ps
[root@...2-vm criu]# cat /proc/913/fdinfo/3
pos: 0
flags: 0100000
mnt_id: 17

Thanks,
Andrew