reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=dd35ca2f2cac9d6439a2ba2f523b98b5a9a9f8f5
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch --no-tags akpm-mm mm-new
        git checkout dd35ca2f2cac9d6439a2ba2f523b98b5a9a9f8f5
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=x86_64 olddefconfig
        make W=1 ARCH=x86_64 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests