Comment 12 for bug 1793901

We have a reproducer now:

$ uname -a
Linux ln-sv-infr01 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 3.7T 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
└─sda2 8:2 0 3.7T 0 part
  ├─ln--sv--infr01--vg-root 253:0 0 301G 0 lvm /
  ├─ln--sv--infr01--vg-swap_1
  │ 253:1 0 976M 0 lvm [SWAP]
  └─ln--sv--infr01--vg-var 253:2 0 3.4T 0 lvm
    └─bcache0 252:0 0 3.4T 0 disk /var
nvme0n1 259:0 0 1.5T 0 disk
└─bcache0 252:0 0 3.4T 0 disk /var

$ sudo fstrim /var
# at this point it will immediately fail to kernel panic ... oops