NFS mount points got unmounted unexpectedly

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Environment

SUSE Linux Enterprise Server 12 SP2

SUSE Linux Enterprise Server 12 SP1

Situation

Occasionally, some NFS mount points may get un-mounted unexpectedly.

Resolution

This bug is fixed in the following kernel versions:
For SLES12 SP2 : 4.4.90-92.45.1 or newer
For SLES12 SP1 LTSS : 3.12.74-60.64.82.1 or newer

Cause

This issue is caused by a bug in the NFS implementation.

Additional Information

If NFS debug log is enabled at the NFS Client side with the following setting:
# sysctl -w sunrpc.nfs_debug=1023
# sysctl -w sunrpc.rpc_debug=1023

The following type of NFS debug messages are printed when the issue occurred:
NFS call getattr
RPC: 45631 call_start nfs3 proc GETATTR (sync)
NFS reply getattr: -512
nfs_revalidate_inode: (0:75/2) getattr failed, error=-512
NFS: nfs_weak_revalidate: inode 2 is invalid
....
NFS reply getattr: -512
nfs_revalidate_inode: (0:76/36928) getattr failed, error=-512
NFS: nfs_lookup_revalidate(/hr_interfaces) is invalid

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