df shows no available space in 5.4.1

[Martin Raiber <martin@xxxxxxxxxxxx>]

Hi,

with kernel 5.4.1 I have the problem that df shows 100% space used. I
can still write to the btrfs volume, but my software looks at the
available space and starts deleting stuff if statfs() says there is a
low amount of available space.

# df -h
Filesystem                                            Size  Used Avail
Use% Mounted on
...
/dev/loop0                                            7.4T  623G     0
100% /media/backup
...

statfs("/media/backup", {f_type=BTRFS_SUPER_MAGIC, f_bsize=4096,
f_blocks=1985810876, f_bfree=1822074245, f_bavail=0, f_files=0,
f_ffree=0, f_fsid={val=[3667078581, 2813298474]}, f_namelen=255,
f_frsize=4096, f_flags=ST_VALID|ST_NOATIME}) = 0

# btrfs fi usage /media/backup
Overall:
    Device size:                   7.40TiB
    Device allocated:            671.02GiB
    Device unallocated:            6.74TiB
    Device missing:                  0.00B
    Used:                        622.49GiB
    Free (estimated):              6.79TiB      (min: 6.79TiB)
    Data ratio:                       1.00
    Metadata ratio:                   1.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,single: Size:666.01GiB, Used:617.95GiB
   /dev/loop0    666.01GiB

Metadata,single: Size:5.01GiB, Used:4.54GiB
   /dev/loop0      5.01GiB

System,single: Size:4.00MiB, Used:96.00KiB
   /dev/loop0      4.00MiB

Unallocated:
   /dev/loop0      6.74TiB

# btrfs fi df /media/backup
Data, single: total=666.01GiB, used=617.95GiB
System, single: total=4.00MiB, used=96.00KiB
Metadata, single: total=5.01GiB, used=4.54GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

# mount

...
/dev/loop0 on /media/backup type btrfs
(rw,noatime,nossd,discard,space_cache=v2,enospc_debug,skip_balance,commit=86400,subvolid=5,subvol=/)
...

(I remounted with enospc_debug and the available space did not change...)

Regards,
Martin Raiber