reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?id=31d6568e294e51ef7c3338bb773b1aef0d050682
        git remote add device-mapper-dm https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git
        git fetch --no-tags device-mapper-dm dm-vdo-wip
        git checkout 31d6568e294e51ef7c3338bb773b1aef0d050682
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs