Re: [PATCH v9 0/6] MAP_DIRECT for DAX userspace flush

On Mon, Oct 16, 2017 at 12:26 AM, Christoph Hellwig <hch(a)lst.de&gt; wrote: > 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. While I agree with the need for a per-MR notification mechanism, one thing we lose by walking away from MAP_DIRECT is a way for a hypervisor to coordinate pass through of a DAX mapping to an RDMA device in a guest. That will remain a case where we will still need to use device-dax. I'm fine if that's the answer, but just want to be clear about all the places we need to protect a DAX mapping against RDMA from a non-ODP device.