reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git/commit/?id=e06d7dd780ab4a50f85115f8ee988e588e4aad57
        git remote add jlayton https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git
        git fetch --no-tags jlayton netns-debugfs
        git checkout e06d7dd780ab4a50f85115f8ee988e588e4aad57
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash fs/debugfs/