reproduce (this is a W=1 build):
        # https://github.com/koverstreet/bcachefs/commit/1ac51de3b5327ce2d597e30a39ff7e6809349a93
        git remote add koverstreet-bcachefs https://github.com/koverstreet/bcachefs
        git fetch --no-tags koverstreet-bcachefs bcachefs-testing
        git checkout 1ac51de3b5327ce2d597e30a39ff7e6809349a93

        # 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/bcachefs/