GPU buffer invalidation does not tear-down CPU mappings obtained via gdr_map()

[drossetti]

As of today, the nvidia_p2p_get_pages callback does not tear down the CPU mappings created via gdr_map().
This is a potential security threat, as those BAR1 pages could be reused later to expose some other GPU device memory, possibly belonging to a different OS process colocated on the same GPU.

[pakmarkthub]

Merged the PR.

Closing this issue.