reproduce:
        # https://github.com/davidhildenbrand/linux/commit/21b8f7d4d9d70fec7ff6bbd973e485fa1226b156
        git remote add davidhildenbrand https://github.com/davidhildenbrand/linux
        git fetch --no-tags davidhildenbrand fork_rmap_batching_mm_unstable
        git checkout 21b8f7d4d9d70fec7ff6bbd973e485fa1226b156
        make W=1 O=/tmp/kselftest headers
        make W=1 O=/tmp/kselftest -C tools/testing/selftests
        make W=1 O=/tmp/kselftest -C tools/testing/selftests/bpf