reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git/commit/?id=5ff971ba3599d1e5d6a1721a85d50969a3c270ca
        git remote add sj https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git
        git fetch --no-tags sj damon/next
        git checkout 5ff971ba3599d1e5d6a1721a85d50969a3c270ca
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs