One approach that may be a clean way to solve this: 1. Long term GUP usage requires the virtual mapping to the pages be fixed for the duration of the GUP Map. There never has been a way to break the pinnning and thus this needs to be preserved. 2. Page Cache Long term pins are not allowed since regular filesystems depend on COW and other tricks which are incompatible with a long term pin. 3. Filesystems that allow bypass of the page cache (like XFS / DAX) will provide the virtual mapping when the PIN is done and DO NO OPERATIONS on the longterm pinned range until the long term pin is removed. Hardware may do its job (like for persistent memory) but no data consistency on the NVDIMM medium is guaranteed until the long term pin is removed and the filesystems regains control over the area. 4. Long term pin means that the mapped sections are an actively used part of the file (like a filesystem write) and it cannot be truncated for the duration of the pin. It can be thought of as if the truncate is immediate followed by a write extending the file again. The mapping by RDMA implies after all that remote writes can occur at anytime within the area pinned long term.
