Currently Lustre uses the Linux kernel shrinkers for several subsystems to handle freeing data when the system is under pressure. In newer kernels shrinker specific interface  has been developed that allows one to flush the data related to the shrinker. This is a handy feature and can be applied to the super block shrinker as well which Lustre currently doesn't do any special handling.

Information on this new interface can be found here:

https://lwn.net/Articles/896274

https://docs.kernel.org/admin-guide/mm/shrinker_debugfs.html