btrfs wrongly reports 0 space available in 5.4.15

[Marc Lehmann <schmorp@schmorp.de>]

Hi!

I upgraded one machine from 5.2.21 to 5.4.15, and one (freshly-created)
btrfs filesystem wrongly shows 0 avail in df after writing a bit of data:

   Filesystem               Size  Used Avail Use% Mounted on
   /dev/mapper/vg_ssd-data  6.5G  160M     0 100% /ssd

When at the same time, the filesystem should claim gobs of free space
(the df output corresponds to a time a bit earlier when not all data was
written yet):

   Overall:
       Device size:                           6.00GiB
       Device allocated:                      0.63GiB
       Device unallocated:                    5.37GiB
       Device missing:                        0.00GiB
       Used:                                  0.22GiB
       Free (estimated):                      5.77GiB      (min: 5.77GiB)
       Data ratio:                               1.00
       Metadata ratio:                           1.00
       Global reserve:                        0.00GiB      (used: 0.00GiB)

   Data,single: Size:0.62GiB, Used:0.22GiB
      /dev/mapper/vg_ssd-data         0.62GiB

   Metadata,single: Size:0.01GiB, Used:0.00GiB
      /dev/mapper/vg_ssd-data         0.01GiB

   System,single: Size:0.00GiB, Used:0.00GiB
      /dev/mapper/vg_ssd-data         0.00GiB

   Unallocated:
      /dev/mapper/vg_ssd-data         5.37GiB

This didn't happen under 5.2.21 for me, obviously.

Deleting some files makes the remaining space appear again, creating
them again changes to 0 free. I retried with a 76G partition but had
essentially the same results.

Only df output seems affected, the fs is still writable (I only realized
because apt told me the disk was full - the fs only stores apt package
lists and the apt cache).

When writing, this switch to 0 free happens suddenly (example with bigger
partition):

   /dev/mapper/vg_ssd-data   76G  3.9M   76G   1% /ssd
   /dev/mapper/vg_ssd-data   76G  3.9M   76G   1% /ssd
   /dev/mapper/vg_ssd-data   76G  122M   76G   1% /ssd
   /dev/mapper/vg_ssd-data   76G  201M     0 100% /ssd
   /dev/mapper/vg_ssd-data   76G  244M     0 100% /ssd

Mount options are noatime,nossd,discard.

I see there were multiple reports for this and some discussion in december
(https://www.spinics.net/lists/linux-btrfs/msg95694.html), but apparently
nothing came of it and the bug still persists.

-- 
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