reproduce (this is a W=1 build):
        # https://github.com/ceph/ceph-client/commit/cb4ac0e6565fe667dba2760b6a012306e6f796e0
        git remote add ceph-client https://github.com/ceph/ceph-client.git
        git fetch --no-tags ceph-client testing
        git checkout cb4ac0e6565fe667dba2760b6a012306e6f796e0

        # 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/