reproduce (this is a W=1 build):
        # https://github.com/btrfs/linux/commit/49c863d9d6690e1ba2ffa093994e03a0cefd50ad
        git remote add btrfs https://github.com/btrfs/linux.git
        git fetch --no-tags btrfs pr/1157
        git checkout 49c863d9d6690e1ba2ffa093994e03a0cefd50ad
        # 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