On Thu, Feb 07, 2019 at 04:55:37PM +0000, Christopher Lameter wrote: > One approach that may be a clean way to solve this: > 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. So, ummm, how do we do block allocation then, which is done on demand during writes? IOWs, this requires the application to set up the file in the correct state for the filesystem to lock it down so somebody else can write to it. That means the file can't be sparse, it can't be preallocated (i.e. can't contain unwritten extents), it must have zeroes written to it's full size before being shared because otherwise it exposes stale data to the remote client (secure sites are going to love that!), they can't be extended, etc. IOWs, once the file is prepped and leased out for RDMA, it becomes an immutable for the purposes of local access. Which, essentially we can already do. Prep the file, map it read/write, mark it immutable, then pin it via the longterm gup interface which can do the necessary checks. Simple to implement, the reasons for errors trying to modify the file are already documented and queriable, and it's hard for applications to get wrong. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
