Description of problem:

Unable to remove lvmcache device

Version-Release number of selected component (if applicable):

Debian Buster, fully upgraded
Kernel 5.7.0-0.bpo.2-amd64
LVM version as reported by lvs --version

# lvs --version
  LVM version:     2.03.02(2) (2018-12-18)
  Library version: 1.02.155 (2018-12-18)
  Driver version:  4.42.0
  Configuration:   ./configure --build=x86_64-linux-gnu --prefix=/usr --includedir=${prefix}/include --mandir=${prefix}/share/man --infodir=${prefix}/share/info --sysconfdir=/etc --localstatedir=/var --disable-silent-rules --libdir=${prefix}/lib/x86_64-linux-gnu --libexecdir=${prefix}/lib/x86_64-linux-gnu --runstatedir=/run --disable-maintainer-mode --disable-dependency-tracking --exec-prefix= --bindir=/bin --libdir=/lib/x86_64-linux-gnu --sbindir=/sbin --with-usrlibdir=/usr/lib/x86_64-linux-gnu --with-optimisation=-O2 --with-cache=internal --with-device-uid=0 --with-device-gid=6 --with-device-mode=0660 --with-default-pid-dir=/run --with-default-run-dir=/run/lvm --with-default-locking-dir=/run/lock/lvm --with-thin=internal --with-thin-check=/usr/sbin/thin_check --with-thin-dump=/usr/sbin/thin_dump --with-thin-repair=/usr/sbin/thin_repair --enable-applib --enable-blkid_wiping --enable-cmdlib --enable-dmeventd --enable-dbus-service --enable-lvmlockd-dlm --enable-lvmlockd-sanlock --enable-lvmpolld --enable-notify-dbus --enable-pkgconfig --enable-readline --enable-udev_rules --enable-udev_sync
root@smilla:~# uname -r
5.7.0-0.bpo.2-amd64

How reproducible:

Always

Steps to Reproduce:
1. lvconvert --uncache data/data

Actual results:

Doesn't fail, but loops forever with output

  Unknown feature in status: 8 2484/262144 128 819199/819200 57472609 35325443 19996086 9940614 209 210 1 3 metadata2 writethrough no_discard_passdown 2 migration_threshold 2048 smq 0 rw -
  Flushing 1 blocks for cache data/data.

Expected results:

Cache is removed

Additional info:

I first tried with kernel 5.4, didn't work. I upgraded to 5.7, didn't work. I tried with --uncache --force, same result as before.

# lvs -o+cache_mode data/data
  Unknown feature in status: 8 2488/262144 128 819200/819200 57472726 35325517 19996091 9940616 7 8 1 3 metadata2 writethrough no_discard_passdown 2 migration_threshold 2048 smq 0 rw -
  LV   VG   Attr       LSize  Pool     Origin       Data%  Meta%  Move Log Cpy%Sync Convert CacheMode
  data data Cwi-aoC--- 13,67t [_cache] [data_corig] 100,00 0,95            0,01             writethrough

So it shows writethrough cache, which is what I wanted. It also shows some unknown feature that I don't know.

Zdenek Kabelac asked me on the mailing list to list relevant packages installed, so here you go

linux-image-5.7.0-0.bpo.2-amd64 <-- running now
linux-image-5.4.0-0.bpo.3-amd64 <-- was running before that
(some older kernel was probably running before that, the server has been around for some time)