reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?id=da4b036e3dc46bc8ab5add0a7d4944de54182afe
        git remote add ast-bpf https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git
        git fetch --no-tags ast-bpf trylock
        git checkout da4b036e3dc46bc8ab5add0a7d4944de54182afe
        # 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 mm/