reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git/commit/?id=825af80daf827d439c7dd3e24ef5648cc338bc98
        git remote add sj https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git
        git fetch --no-tags sj mm-new
        git checkout 825af80daf827d439c7dd3e24ef5648cc338bc98
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash