reproduce (this is a W=1 build):
        git remote add hch-block git://git.infradead.org/users/hch/block.git
        git fetch --no-tags hch-block block-limit-flags
        git checkout ecd2eb65b1f6eec3edf6afe2044167e723b91107
        # 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 drivers/block/