reproduce (this is a W=1 build):
        # https://github.com/btrfs/linux/commit/ef0c33ef0f4020b31bca106645340968f1a497bb
        git remote add btrfs https://github.com/btrfs/linux.git
        git fetch --no-tags btrfs ci-kvm-misc-next
        git checkout ef0c33ef0f4020b31bca106645340968f1a497bb

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-20 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=x86_64 olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-20 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash fs/btrfs/ fs/proc/