Bug 209399

Summary: Can't unmount bind-mounted NFS mounts with "Stale file handle"
Product: File System Reporter: Vadim Rutkovsky (vrutkovs)
Component: NFSAssignee: Trond Myklebust (trondmy)
Status: NEW ---    
Severity: normal    
Priority: P1    
Hardware: All   
OS: Linux   
Kernel Version: 5.7 Subsystem:
Regression: Yes Bisected commit-id:

Description Vadim Rutkovsky 2020-09-26 08:03:28 UTC 
Steps to reproduce (basically, get "Stale file handle" error on bind-mounted nfs dir):

1. Use this dummy /etc/exports:
/var/tmp 127.0.0.1(rw,sync,all_squash,anonuid=1000)

2. Mount it to /mnt/test:
$ mkdir /mnt/test
$ mount localhost:/var/tmp /mnt/test

3. Bind-mount a subdirectory of it to /mnt/test2:
$ mkdir /mnt/test/reproduce
$ mkdir /mnt/test2
$ mount --bind /mnt/test/reproduce /mnt/test2

4. Remove the bind-mounted dir
$ rmdir /mnt/test/reproduce

5. Check that /mnt/test2 is not happy about that
$ ls /mnt/test2
ls: cannot access '/mnt/test2': Stale file handle

This is expected.

6. Try to unmount /mnt/test2
$ umount /mnt/test2
umount.nfs4: /mnt/test2: Stale file handle

This is not expected! There is no way how to unmount the directory. It's mounted forever. Even reboot gets stuck.

Reported in Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=1854379
This seems to work as expected in 5.6.x kernels, started happening in 5.7 and affects 5.8 too