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[Tycho Andersen <tycho.andersen@xxxxxxxxxxxxx>]

I can confirm that the kernels at
http://kernel.ubuntu.com/~sforshee/lp1570906/ work for me. Thanks, Seth!

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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:
  SRU Justification:

  Impact: Stateful lxd container snapshotting fails due to a failure to
  mount the container's sysfs in the host's user namespace. This is a
  regression.

  Fix: Force kernfs to use a new super block for mounts in different
  user namespaces.

  Test Case: "lxc snapshot --stateful <container>" fails in the current
  xenial kernel without the fix. It succeeds with the fix applied.

  ---

  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.

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