reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/commit/?id=1e8eb4117f27366855b192104098330a2f290f26
        git remote add vbabka-slab https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git
        git fetch --no-tags vbabka-slab slab/for-7.0/obj_metadata
        git checkout 1e8eb4117f27366855b192104098330a2f290f26

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash mm/