RHEL7: NFS client stale submount remains in /proc/mounts due to periodic path_resolution operation even if the export has been removed on server

Issue

• We use RHEL7 along with NetApp filers that provide NFS storage. These mount utilize the NetApp snapshot backup technology to do backups of the NAS mounts. We have been getting a lot of reports recently where some hosts with NAS have the hidden ".snapshot" directory mounted on the host. Over time, these snapshot backups are removed, and the mounts then go stale due to them no longer existing. Such mounts are never removed on the NFS client but remain in /proc/mounts and visible.
  Example:

  df: '/mnt/netapp/.snapshot/hourly.2018-01-15_2200': Stale file handle
  

• Basically you cd into the ".snapshot" directory on the NAS mount and do something like a ls and the ".snapshot" directory automounts and doesn’t go away. So temp fix is to just umount the stale mounts and they go away.