[Bug 1734410] Re: systemd: handle undelegated cgroup2 hierarchy

[Christian Brauner]

Sorry for the brevity before. I tested this with systemd 23{5,6}
inside xenial and artful containers which is really the only case
where it matters.

A systemd with my patch applied would happily:
1. skip over undelegated /sys/fs/cgroup/unified mountpoints
   (e07aefbd675b651f8d45b5fb458f2747b04d6e04).
2. skip over undelegated pur cgroup2 mountpoints at /sys/fs/cgroup
   (2d56b80a1855836abf1d7458394c345ad9d55382)
3. remove any empty mountpoints created for case 1. and 2.
   (1ff654e28b7b8e7d0a0be33522a84069ac6b07c0)

Thanks for backporting these patches!
Christian

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https://bugs.launchpad.net/bugs/1734410

Title:
  systemd: handle undelegated cgroup2 hierarchy

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  Confirmed
Status in systemd source package in Zesty:
  Won't Fix
Status in systemd source package in Artful:
  Fix Released
Status in systemd source package in Bionic:
  Fix Released

Bug description:
  [Impact]

   * When a container is presented with a unified cgroup hierarchy,
  which is not properly delegated, systemd should not attempt (and fail)
  to use. This improves compatibility of xenial containers running on
  unified cgroup hierarchy hosts.

  [Test Case]

   * Xenial containers should boot, with non-writable unified cgroup
  hierarchy hosts.

  [Regression Potential]

   * unified cgroup hierarchy is not in use by default on xenial hosts,
  thus this is forward compatibility improvment with e.g. bionic hosts
  running xenial containers.

  [Other Info]
   
   * Original bug report

  Hey everyone,

  Current systemd versions all fail when the unified cgroup hierarchy is
  not-writable. This is especially problematic in containers where the
  systemd administrator might decide to not delegate the unified
  hierarchy or when running with a liblxc driver that doesn't yet know
  how to handle the unified cgroup hierarchy. I've pushed patches to
  systemd upstream that let systemd ingnore the non-delegated unified
  hierarchy. The relevant commits are:

  e07aefbd675b651f8d45b5fb458f2747b04d6e04
  2d56b80a1855836abf1d7458394c345ad9d55382
  1ff654e28b7b8e7d0a0be33522a84069ac6b07c0

  These patches will be in 236 but should be backported from xenial
  upwards.

  Christian

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