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

[Brian Murray]

** Package changed: ubuntu => nfs-utils (Ubuntu)

** Tags added: focal

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Title:
  "File system loop detected" with btrfs over NFS

Status in nfs-utils package in Ubuntu:
  New

Bug description:
  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

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