[Devel] Re: [PATCH 0/3][V2] remove the ns_cgroup

[Eric W. Biederman]

"Serge E. Hallyn" <serge.hallyn at canonical.com> writes:

> Quoting Andrew Morton (akpm at linux-foundation.org):
>> On Mon, 27 Sep 2010 12:14:10 +0200
>> Daniel Lezcano <daniel.lezcano at free.fr> wrote:
>> 
>> > The ns_cgroup is a control group interacting with the namespaces.
>> > When a new namespace is created, a corresponding cgroup is 
>> > automatically created too. The cgroup name is the pid of the process
>> > who did 'unshare' or the child of 'clone'.
>> > 
>> > This cgroup is tied with the namespace because it prevents a
>> > process to escape the control group and use the post_clone callback,
>> > so the child cgroup inherits the values of the parent cgroup.
>> > 
>> > Unfortunately, the more we use this cgroup and the more we are facing
>> > problems with it:
>> > 
>> >  (1) when a process unshares, the cgroup name may conflict with a previous
>> >  cgroup with the same pid, so unshare or clone return -EEXIST
>> > 
>> >  (2) the cgroup creation is out of control because there may have an
>> >  application creating several namespaces where the system will automatically
>> >  create several cgroups in his back and let them on the cgroupfs (eg. a vrf
>> >  based on the network namespace).
>> > 
>> >  (3) the mix of (1) and (2) force an administrator to regularly check and
>> >  clean these cgroups.
>> > 
>> > This patchset removes the ns_cgroup by adding a new flag to the cgroup
>> > and the cgroupfs mount option. It enables the copy of the parent cgroup
>> > when a child cgroup is created. We can then safely remove the ns_cgroup as
>> > this flag brings a compatibility. We have now to manually create and add the
>> > task to a cgroup, which is consistent with the cgroup framework.
>> 
>> So this is a non-backward-compatible userspace-visible change?
>
> Yes, it is.
>

We have always been able to compile out the ns cgroup right?

In which case this is not a backwards incompatible change so much
as the permanent removal of a borked kernel feature.

Eric
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