Comment # 1 on bug 1081427 from Matwey Kornilov

(none):/lib/modules/4.14.15-2-default/kernel/fs/btrfs # modprobe btrfs
modprobe: ERROR: ctx=0x1d46150
path=/lib/modules/4.14.15-2-default/kernel/crypto/xor.ko error=No such file or
directory
modprobe: ERROR: ctx=0x1d46150
path=/lib/modules/4.14.15-2-default/kernel/crypto/xor.ko error=No such file or
directory
modprobe: ERROR: could not insert 'btrfs': Unknown symbol in module, or unknown
parameter (see dmesg)

btrfs module is present but has missed dependency.