reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/commit/?id=1e4fd024c798fbeff0b5a3c5f55cdcf710367c59
        git remote add harry https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git
        git fetch --no-tags harry kvfree-rcu-improvements-rfc-v2r1-wip
        git checkout 1e4fd024c798fbeff0b5a3c5f55cdcf710367c59

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash mm/