the method above doesn't fully work, since even though grub doesn't pass the 
subvol= option to the kernel, it still tries to find kernel at full path, 
including the subvolume name.
e.g. in /boot/grub/grub.cfg there are lines like:
linux   /@/boot/vmlinuz-5.13.0-20-generic 

to make it work as easy as on OpenSuse i have made separate subvolume
for /boot

1. 
in /etc/grub.d/10_linux
i have removed (commented out) the lines:

# case x"$GRUB_FS" in
#     xbtrfs)
#       rootsubvol="`make_system_path_relative_to_its_root /`"
#       rootsubvol="${rootsubvol#/}"
#       if [ "x${rootsubvol}" != x ]; then
#           GRUB_CMDLINE_LINUX="rootflags=subvol=${rootsubvol} 
${GRUB_CMDLINE_LINUX}"
#       fi;;
#     xzfs)
#       # We have a more specialized ZFS handler, with multiple system in 
10_linux_zfs.
#       if [ -e "`dirname $(readlink -f $0)`/10_linux_zfs" ]; then
#         exit 0
#       fi
#       rpool=`${grub_probe} --device ${GRUB_DEVICE} --target=fs_label 
2>/dev/null || true`
#       bootfs="`make_system_path_relative_to_its_root / | sed -e "s,@$,,"`"
#       LINUX_ROOT_DEVICE="ZFS=${rpool}${bootfs%/}"
#       ;;
# esac

Though i could have commented out only the btrfs related path, but i
don't use zfs anyway

2.
Then mounted the btrfs root subvolume (subvolid=5 option) sudo mount -L sys -o 
subvolid=5 /mnt
and created @boot subvolume there, and copied (cp -a) the boot directory from 
my previous boot.

3.
created fstab record for the new boot:
LABEL=sys /boot           btrfs   defaults,subvol=@boot      0       2
and mounted it 
mount -a
mount /boot/efi

4.
update-grub


Now works fine even on top of luks.
I have also created separate subvolume for /var/log but this is optional.

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https://bugs.launchpad.net/bugs/1734496

Title:
  snapper rollback does not work on ubuntu for btrfs

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