[lxc-devel] [PATCH] cgfs: don't mount /sys/fs/cgroup readonly

[Christian Seiler]

Hi,

> On an ubuntu system, mountall wants /sys/fs/cgroup to be mounted rw.
> So on container startup, mountall will see that /sys/fs/cgroup is ro
> and hang startup (waiting for the user to say whether to skip
> or manually fix) because it's not allowed to remount /sys/fs/cgroup
> rw.

Ouch.

Irrespective of what we do in LXC, perhaps mountall should also be fixed
that if inside a container (env var ${container} set) and /sys/fs/cgroup
is mounted ro, then it should just print a warning but not hang?
(Obviously that doesn't absolve LXC from changing something, since this
will never be backported to arbitrarily old versions of mountall, but I
do think it's better for robustness.)

> Hm.  So IIUC lxc would have to (and, you're telling me, now does not)

Currently, it doesn't do that, no, because I saw no reason for it, since
I saw no reason for /sys/fs/cgroup being rw itself until you mentioned
the problem with Ubuntu.

> have the following mounts?
> 
>> /sys/fs/cgroup                  [tmpfs, rw]
>> /sys/fs/cgroup/cpu              [tmpfs, ro]
>> /sys/fs/cgroup/cpu/lxc/c1       [bind-mount of host, rw]

Yes, that would be a good solution for that. I still don't like it
aesthetically that /sys/fs/cgroup is rw, but better a workaround than
containers hanging at boot...

I'll post a patch that does that tomorrow. (I could write it easily just
now in 5 minutes, but I do want to test it properly beforehand.)

Regards,
Christian