[Bug 1918599] [NEW] "File system loop detected" with btrfs over NFS

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tsmsrvi and tsmsrvj both run Ubuntu 20.04 with all updates applied.
When I try to access a btrfs filesystem via nfs, I get the error:

root at tsmsrvi:~# mount tsmsrvj:/data/fex /nfs/tsmsrvj/fex

root at tsmsrvi:~# find /nfs/tsmsrvj/fex
/nfs/tsmsrvj/fex
find: File system loop detected; '/nfs/tsmsrvj/fex/spool' is part of the same file system loop as '/nfs/tsmsrvj/fex'.

root at tsmsrvi:~# du -s /nfs/tsmsrvj/fex
du: WARNING: Circular directory structure.
This almost certainly means that you have a corrupted file system.
NOTIFY YOUR SYSTEM MANAGER.
The following directory is part of the cycle:
  /nfs/tsmsrvj/fex/spool

0       /nfs/tsmsrvj/fex

root at tsmsrvi:~# ls -l /nfs/tsmsrvj/fex/
total 0
drwxr-xr-x 1 fex fex 261964 Mar  7 14:53 spool

On tsmsrvj I have in /etc/exports:

/data/fex       localhost(rw,async,no_subtree_check,no_root_squash)
/data/fex       tsmsrvi(rw,async,no_subtree_check,no_root_squash)

/data/fex is a btrfs filesystem with subvolumes:

root at tsmsrvj:~# mount | grep /data
/dev/sdb1 on /data type btrfs (rw,relatime,space_cache,user_subvol_rm_allowed,subvolid=5,subvol=/)

root at tsmsrvj:~# btrfs subvolume list /data
ID 257 gen 35 top level 5 path fex
ID 270 gen 36 top level 257 path fex/spool
ID 271 gen 21 top level 270 path fex/spool/.snapshot/2021-03-07_1453.test
ID 272 gen 23 top level 270 path fex/spool/.snapshot/2021-03-07_1531.test
ID 273 gen 25 top level 270 path fex/spool/.snapshot/2021-03-07_1532.test
ID 274 gen 27 top level 270 path fex/spool/.snapshot/2021-03-07_1718.test


Accessing btrfs directly without nfs has no problems:

root at tsmsrvj:~# find /data/fex | wc -l
489887

root at tsmsrvj:~# lsb_release -rd
Description:    Ubuntu 20.04.2 LTS
Release:        20.04

root at tsmsrvj:# uname -a
Linux tsmsrvj 5.4.0-66-generic #74-Ubuntu SMP Wed Jan 27 22:54:38 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

root at tsmsrvj:# btrfs version
btrfs-progs v5.4.1

root at tsmsrvj:# dpkg -l | grep nfs-
ii  nfs-common                             1:1.3.4-2.5ubuntu3.3              amd64        NFS support files common to client and server
ii  nfs-kernel-server                      1:1.3.4-2.5ubuntu3.3              amd64        support for NFS kernel server

** Affects: nfs-utils (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: bot-comment
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"File system loop detected" with btrfs over NFS
https://bugs.launchpad.net/bugs/1918599
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