reproduce (this is a W=1 build):
        mkdir -p ~/bin
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git/commit/?id=2031cbae1532cadb0838ebc1fecbfd14698b5a0a
        git remote add jaegeuk-f2fs https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git
        git fetch --no-tags jaegeuk-f2fs dev-test
        git checkout 2031cbae1532cadb0838ebc1fecbfd14698b5a0a
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=x86_64 olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash fs/f2fs/