reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git/commit/?id=8beafe484c79bce5de4807d390d93250449fecee
        git remote add leon-rdma https://git.kernel.org/pub/scm/linux/kernel/git/leon/linux-rdma.git
        git fetch --no-tags leon-rdma dma-split-wip
        git checkout 8beafe484c79bce5de4807d390d93250449fecee
        # 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/