Right, you need at least some dm-linear device mapper device in between to ensure a device is RO I think. Also, ext4 has separate "noload" option that actually

Right, you need at least some dm-linear device mapper device in between to ensure a device is RO I think. Also, ext4 has separate "noload" option that actually instructs it not to touch the disk (ofc relying solely on "ro,noload" to not mess with your broken FS is a bad idea)