Due to commit 11222ee , it is not possible update item limit if not specified a default value in the config in the first place.

# runc update --kernel-memory 50331648 root
failed to set memory.kmem.limit_in_bytes, because either tasks have already joined this cgroup or it has children

However, it is fine to update other memory limits even if did not specify default values in the config.json.

# runc update --memory-reservation 50331648 root
# runc update --memory 50331648 root
# runc update --memory-swap 50331648 root

@crosbymichael PTAL