reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=7f18b4ec38272a00b16cb2995ebb06281e0f26d0
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch --no-tags akpm-mm mm-nonmm-unstable
        git checkout 7f18b4ec38272a00b16cb2995ebb06281e0f26d0
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs