reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?id=b2fd6d9020b74d49f3d0f7585cce82396b4a608e
        git remote add dhowells-fs https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git
        git fetch --no-tags dhowells-fs netfs-writeback
        git checkout b2fd6d9020b74d49f3d0f7585cce82396b4a608e
        # 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 fs/afs/