reproduce (this is a W=1 build):
        # https://github.com/koverstreet/bcachefs/commit/d581cdb2a0705aed99b7534ad8c164ce30377a4e
        git remote add koverstreet-bcachefs https://github.com/koverstreet/bcachefs
        git fetch --no-tags koverstreet-bcachefs pr/739
        git checkout d581cdb2a0705aed99b7534ad8c164ce30377a4e
        # 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/