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[Seth Forshee <seth.forshee+lp@xxxxxxxxxxxxx>]

Test build is at http://kernel.ubuntu.com/~sforshee/for-tych0/,
confirmed to fix the problem by @tycho-s.

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https://bugs.launchpad.net/bugs/1570906

Title:
  sysfs mount failure during stateful lxd snapshots

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Xenial:
  In Progress

Bug description:
  During a stateful lxd snapshot criu tries to mount sysfs for the
  container's network namespace from a different user namespace. This
  fails in xenial because sget() won't allow mounting the same super
  block in different user namespaces.

  With sysfs there's no reason that this needs to use the same super
  block, so kernfs can be updated so that a super block with the same ns
  tag but in a different userns is not matched. The only other kernfs-
  based filesystem mountable from non-init user namespaces is cgroupfs,
  and it's already forcing kernfs to return different super blocks to
  avoid similar problems. In fact we can revert part of the cgroupfs
  changes to make this happen if we push this behavior into kernfs.

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