reproduce (this is a W=1 build):
        # https://github.com/ceph/ceph-client/commit/f223d3366fa1de01d0119237ac6a4e7bbb85bec7
        git remote add ceph-client https://github.com/ceph/ceph-client.git
        git fetch --no-tags ceph-client wip-krb5-only
        git checkout f223d3366fa1de01d0119237ac6a4e7bbb85bec7

        # 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 arch/x86/kernel/fpu/ net/ceph/