reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git/commit/?id=169a645279f9f90fa48436e556fe2817db2896a4
        git remote add sandeen https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git
        git fetch --no-tags sandeen convert-9p
        git checkout 169a645279f9f90fa48436e556fe2817db2896a4
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash fs/ net/