Public bug reported:
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@tsmsrvi:~# mount tsmsrvj:/data/fex /nfs/tsmsrvj/fex
root@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@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@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@tsmsrvj:~# mount | grep /data
/dev/sdb1 on /data type btrfs
(rw,relatime,space_cache,user_subvol_rm_allowed,subvolid=5,subvol=/)
root@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@tsmsrvj:~# find /data/fex | wc -l
489887
root@tsmsrvj:~# lsb_release -rd
Description: Ubuntu 20.04.2 LTS
Release: 20.04
root@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@tsmsrvj:# btrfs version
btrfs-progs v5.4.1
root@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: ubuntu
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1918599
Title:
"File system loop detected" with btrfs over NFS
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