reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=6fa1bf6b010deb50b35d2b2a7c97e7e229f2747e
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch --no-tags akpm-mm mm-unstable
        git checkout 6fa1bf6b010deb50b35d2b2a7c97e7e229f2747e
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=i386 olddefconfig
        make W=1 ARCH=i386 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests