On Mon, Oct 16, 2017 at 10:43 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Mon, Oct 16, 2017 at 12:26 AM, Christoph Hellwig <hch@xxxxxx> 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. For this specific issue perhaps we promote FL_LAYOUT as a lease-type that can be set by fcntl(). -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
