[CRIU] Restore into different cgroups

[Pavel Emelyanov]

On 08/13/2014 06:03 PM, Tycho Andersen wrote:
> Hi Pavel, Serge,
> 
> On Tue, Aug 12, 2014 at 05:11:29PM +0000, Serge Hallyn wrote:
>> Quoting Pavel Emelyanov (xemul at parallels.com):
>>> On 08/12/2014 07:13 PM, Tycho Andersen wrote:
>>>
>>>>> Doing regexp substitution in criu code would be confusing... I was 
>>>>> thinking about treating all the paths as being relative to the init 
>>>>> task's cgset and keep this task's full paths in the image. Then
>>>>> introduce the --cgroup-root option that would override the init task's
>>>>> path from the image.
>>>>>
>>>>> Would that work for you?
>>>>
>>>> I don't understand exactly how that would work. It would just add a
>>>> prefix, so --cgroup-root=/foo would rewrite /lxc/u1 to /foo/lxc/u1?
>>>> What about other cgroups unrelated to the container? Can we avoid
>>>> rewriting those?
>>>
>>> OK, let me explain in more details what I mean.
>>>
>>> Let's imagine we have init task living in /lxc/ct, some other task
>>> in /lxc/ct/sub and there's also an empty subgroup /lxc/ct/empty.
>>>
>>> When we dump this, in the image we'd get:
>>>
>>> * root task lives in /lxc/ct
>>> * "some other task" lives in /sub
>>> * there's a subgroup called /sub
>>> * there's a subgroup called /empty
>>>
>>> On restore criu would prepend the /lxc/ct (path from root task)
>>> to whatever path it meets in the cgroup image.
>>>
>>> If we want to restore tasks into /lxc/ct2/lxc/ct, then we'd say on
>>> restore --cgroup-root /lxc/ct2/lxc/ct and criu would prepend _this_
>>> to all the other paths (except for the root task's, i.e. /lxc/ct
>>> from the image would be ignored).
>>>
>>> This will be so for every controller. Do we need per-controller
>>> control? If can look like
>>>
>>> --cgroup-root $path -- prepend $path for every controller
>>> --cgroup-root $ctl:$path -- only for controller $ctl
>>>
>>> and the option can be specified multiple times.
>>
>> Again, sounds good to me.  The code shouldn't be bad, just
>> grab /proc/1/cgroup contents at start, and then index into
>> each tasks' cgroup (ensuring that it lives under the init
>> tasks' cgroup, which i think you're already doing) to get
>> the sub-cgroup to checkpoint.
> 
> I see, combined with the patch Pavel sent this morning and a good
> sleep this makes more sense now. Pavel, are you implementing this or
> would you like me to?

I would greatly appreciate if you do on top of what's now on the
mailing list.

> The only other question I have is what to do about cgroups where we
> don't want to reset the root (e.g. the user.slice systemd groups,
> presumably).

I think we can do it like

    --cgroup-root /new-root --cgroup-root name=systemd:/$orig_root

The first option would set the root for all, the 2nd would then
override it for name=systemd one. $orig_root should be the proper
path, but maybe for convenience we can reserve some keyword, like
ORIG.

Does this make sense?

Thanks,
Pavel