reproduce (this is a W=1 build):
        git remote add tj-cgroup https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git
        git fetch tj-cgroup for-next
        git checkout tj-cgroup/for-next
        b4 shazam https://lore.kernel.org/r/172616070094.2055617.17676042522679701515.stgit@firesoul
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash mm/