Re: CGroup Namespaces (v6)

From: Alban Crequy
Date: Tue Dec 08 2015 - 05:10:36 EST


Thanks for the patches!

On 8 December 2015 at 00:06, <serge.hallyn@xxxxxxxxxx> wrote:
> Hi,
> following is a revised set of the CGroup Namespace patchset which Aditya
> Kali has previously sent. The code can also be found in the cgroupns.v6
> branch of
> To summarize the semantics:
> 1. CLONE_NEWCGROUP re-uses 0x02000000, which was previously CLONE_STOPPED
> 2. unsharing a cgroup namespace makes all your current cgroups your new
> cgroup root.
> 3. /proc/pid/cgroup always shows cgroup paths relative to the reader's
> cgroup namespce root. A task outside of your cgroup looks like
> 8:memory:/../../..
> 4. when a task mounts a cgroupfs, the cgroup which shows up as root depends
> on the mounting task's cgroup namespace.
> 5. setns to a cgroup namespace switches your cgroup namespace but not
> your cgroups.
> With this, using #2015-11-09/cgns (and
> #2015-11-10/cgns) we can start a container in a full
> proper cgroup namespace, avoiding either cgmanager or lxcfs cgroup bind mounts.

I tested cgroupns.v6 with systemd-nspawn + patches from using
unshare(CLONE_NEWCGROUP) booted with
systemd.unified_cgroup_hierarchy=1 in Fedora22. Tested with and
without userns. It worked for me :)

Do you need people to run more tests, with other scenarios?

Do you have patches already for /usr/bin/unshare and /usr/bin/nsenter?

> This is completely backward compatible and will be completely invisible
> to any existing cgroup users (except for those running inside a cgroup
> namespace and looking at /proc/pid/cgroup of tasks outside their
> namespace.)
> Changes from V5:
> 1. To get a root dentry for cgroup namespace mount, walk the path from the
> kernfs root dentry.
> Changes from V4:
> 1. Move the FS_USERNS_MOUNT flag to last patch
> 2. Rebase onto cgroup/for-4.5
> 3. Don't non-init user namespaces to bind new subsystems when mounting.
> 4. Address feedback from Tejun (thanks). Specificaly, not addressed:
> . kernfs_obtain_root - walking dentry from kernfs root.
> (I think that's the only piece)
> 5. Dropped unused get_task_cgroup fn/patch.
> 6. Reworked kernfs_path_from_node_locked() to try to simplify the logic.
> It now finds a common ancestor, walks from the source to it, then back
> up to the target.
> Changes from V3:
> 1. Rebased onto latest cgroup changes. In particular switch to
> css_set_lock and ns_common.
> 2. Support all hierarchies.
> Changes from V2:
> 1. Added documentation in Documentation/cgroups/namespace.txt
> 2. Fixed a bug that caused crash
> 3. Incorporated some other suggestions from last patchset:
> - removed use of threadgroup_lock() while creating new cgroupns
> - use task_lock() instead of rcu_read_lock() while accessing
> task->nsproxy
> - optimized setns() to own cgroupns
> - simplified code around sane-behavior mount option parsing
> 4. Restored ACKs from Serge Hallyn from v1 on few patches that have
> not changed since then.
> Changes from V1:
> 1. No pinning of processes within cgroupns. Tasks can be freely moved
> across cgroups even outside of their cgroupns-root. Usual DAC/MAC policies
> apply as before.
> 2. Path in /proc/<pid>/cgroup is now always shown and is relative to
> cgroupns-root. So path can contain '/..' strings depending on cgroupns-root
> of the reader and cgroup of <pid>.
> 3. setns() does not require the process to first move under target
> cgroupns-root.
> Changes form RFC (V0):
> 1. setns support for cgroupns
> 2. 'mount -t cgroup cgroup <mntpt>' from inside a cgroupns now
> mounts the cgroup hierarcy with cgroupns-root as the filesystem root.
> 3. writes to cgroup files outside of cgroupns-root are not allowed
> 4. visibility of /proc/<pid>/cgroup is further restricted by not showing
> anything if the <pid> is in a sibling cgroupns and its cgroup falls outside
> your cgroupns-root.
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