reproduce (this is a W=1 build):
        # https://github.com/koverstreet/bcachefs/commit/559ae9ec988de991142b6a067c313aeb530c2b11
        git remote add koverstreet-bcachefs https://github.com/koverstreet/bcachefs
        git fetch --no-tags koverstreet-bcachefs bcachefs-dir-i-size
        git checkout 559ae9ec988de991142b6a067c313aeb530c2b11
        # 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/bcachefs/