I have waited some time and some Ubuntu updates, but the bug is still there:

On Thu 2021-03-11 (08:46), Ulli Horlacher wrote:

On Wed 2021-03-10 (08:46), Ulli Horlacher wrote:

quoted

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:~# time find /nfs/tsmsrvj/fex | wc -l
find: File system loop detected; '/nfs/tsmsrvj/fex/spool' is part of the same file system loop as '/nfs/tsmsrvj/fex'.

It is even worse:

root@tsmsrvj:# grep localhost /etc/exports
/data/fex       localhost(rw,async,no_subtree_check,no_root_squash)

root@tsmsrvj:# mount localhost:/data/fex /nfs/localhost/fex

root@tsmsrvj:# du -s /data/fex
64282240        /data/fex

root@tsmsrvj:# du -s /nfs/localhost/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/localhost/fex/spool

0       /nfs/localhost/fex

root@tsmsrvj:# btrfs subvolume list /data
ID 257 gen 42 top level 5 path fex
ID 270 gen 42 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

root@tsmsrvj:~# uname -a
Linux tsmsrvj 5.4.0-77-generic #86-Ubuntu SMP Thu Jun 17 02:35:03 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                             1:1.3.4-2.5ubuntu3.4              amd64        NFS support files common to client and server
ii  nfs-kernel-server                      1:1.3.4-2.5ubuntu3.4              amd64        support for NFS kernel server

This makes btrfs with snapshots unusable as a nfs server :-(

How/where can I escalate it further?

My Ubuntu bug report has been ignored :-(

https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1918599

-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum TIK         
Universitaet Stuttgart         E-Mail: horlacher@tik.uni-stuttgart.de
Allmandring 30a                Tel:    ++49-711-68565868
70569 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/
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