I was experimenting with LVM cache and somehow managed to trigger a
bug.

Here is the command that failed:

# lvconvert --type cache --cachevol giant-cache --cachemode writeback
--chunksize 192 system/giant
Erase all existing data on system/giant-cache? [y/n]: y
  device-mapper: resume ioctl on  (254:3) failed: No space left on device
  Unable to resume system-giant (254:3).
  Problem reactivating logical volume system/giant.
  Releasing activation in critical section.
  libdevmapper exiting with 1 device(s) still suspended

In dmesg this triggered:

[1387212.915349] device-mapper: space map metadata: unable to allocate new
metadata block
[1387212.915352] device-mapper: cache: 254:3: could not resize cache metadata
[1387212.915354] device-mapper: cache: 254:3: metadata operation
'dm_cache_resize' failed: error = -28
[1387212.915356] device-mapper: cache: 254:3: aborting current metadata
transaction
[1387212.940222] device-mapper: cache: 254:3: switching cache to read-only mode
[1387212.940227] device-mapper: table: 254:3: cache: preresume failed, error =
-28
[1387243.006881] device-mapper: cache: 254:3: unable to switch cache to write
mode until repaired.
[1387243.006883] device-mapper: cache: 254:3: switching cache to read-only mode
[1387243.021274] device-mapper: table: 254:3: cache: Unable to get write access
to metadata, please check/repair metadata.
[1387243.021277] device-mapper: ioctl: error adding target to table

Now lvs reports:

# lvs -a
  LV            VG     Attr       LSize   Pool          Origin        Data% 
Meta%  Move Log Cpy%Sync Convert
  abuild        system -wi-ao----  50.00g                                       
  arvin         system -wi-ao---- 100.00g                                       
  giant         system Cwi-CoC-M- 512.00g [giant-cache] [giant_corig] 0.00  
0.68            0.00            
  [giant-cache] system Cwi-aoC--- 128.00g                                       
  [giant_corig] system owi-aoC--- 512.00g                                       
  root          system -wi-ao----  50.00g                                       
  swap          system -wi-ao----   2.00g

As can be seen for giant a check is needed (although 'C' is not thin-pools).
Unmounting the filesystem does not work (hangs).