Referring to this patch:
https://lists.linuxcontainers.org/pipermail/lxc-devel/2014-October/010604.html
Starting lxc with userns in prepare_ramfs_root() I got -1 from:
if (umount2("./proc", MNT_DETACH)) {
Shall this error be rather ignored in case of userns? Thus the same
logic as for other mount points processed by mentioned function?