reproduce (this is a W=1 build):
        git remote add kdave https://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux.git
        git fetch kdave for-next
        git checkout kdave/for-next
        b4 shazam https://lore.kernel.org/r/0102019bb2ff56b7-9302e783-c17d-452d-b6a7-11f773776ae7-000000@eu-west-1.amazonses.com

        # 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 fs/btrfs/