Puma is a Linux kernel module that enables a distributed page cache between multiple hosts. It relies on the existing cleancache API, but an updated Linux kernel is required to remove some limitations and improve its performance (see https://github.com/mlorrillere/linux-puma).
You need to pass the enable parameter to enable the module. Once loaded, you can add remotes by writing an ip:port string to the /sys/modules/remotecache/parameters/remotes file.