On Fri, Oct 13, 2017 at 11:31:45AM -0600, Jason Gunthorpe wrote: > I don't think that really represents how lots of apps actually use > RDMA. > > RDMA is often buried down in the software stack (eg in a MPI), and by > the time a mapping gets used for RDMA transfer the link between the > FD, mmap and the MR is totally opaque. > > Having a MR specific notification means the low level RDMA libraries > have a chance to deal with everything for the app. > > Eg consider a HPC app using MPI that uses some DAX aware library to > get DAX backed mmap's. It then passes memory in those mmaps to the > MPI library to do transfers. The MPI creates the MR on demand. > I suspect one of the more interesting use cases might be a file server, for which that's not the case. But otherwise I agree with the above, and also thing that notifying the MR handle is the only way to go for another very important reason: fencing. What if the application/library does not react on the notification? With a per-MR notification we can unregister the MR in kernel space and have a rock solid fencing mechanism. And that is the most important bit here.
