From the kernel patch series[1]:
"This series provides an asynchronous means of reporting free guest
pages to a hypervisor so that the memory associated with those pages can
be dropped and reused by other processes and/or guests on the host.
Using this it is possible to avoid unnecessary I/O to disk and greatly
improve performance in the case of memory overcommit on the host."[1]

The runner hosts may be memory overcommitted if there is too many
running VMs and containers at the same time, which this should help to
avoid.

[1] https://lore.kernel.org/linux-mm/20200211224416.29318.44077.stgit@localhost.localdomain/