From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma Date: Thu, 31 Jan 2019 09:13:55 +0100 Message-ID: <20190131081355.GC26495@lst.de> References: <20190129234752.GR3176@redhat.com> <655a335c-ab91-d1fc-1ed3-b5f0d37c6226@deltatee.com> <20190130041841.GB30598@mellanox.com> <20190130080006.GB29665@lst.de> <20190130190651.GC17080@mellanox.com> <840256f8-0714-5d7d-e5f5-c96aec5c2c05@deltatee.com> <20190130195900.GG17080@mellanox.com> <35bad6d5-c06b-f2a3-08e6-2ed0197c8691@deltatee.com> <20190130215019.GL17080@mellanox.com> <07baf401-4d63-b830-57e1-5836a5149a0c@deltatee.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <07baf401-4d63-b830-57e1-5836a5149a0c@deltatee.com> Sender: linux-kernel-owner@vger.kernel.org To: Logan Gunthorpe Cc: Jason Gunthorpe , Christoph Hellwig , Jerome Glisse , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Greg Kroah-Hartman , "Rafael J . Wysocki" , Bjorn Helgaas , Christian Koenig , Felix Kuehling , "linux-pci@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , Marek Szyprowski , Robin Murphy , Joerg Roedel , "iommu@lists.linux-foundation.org" List-Id: iommu@lists.linux-foundation.org On Wed, Jan 30, 2019 at 03:52:13PM -0700, Logan Gunthorpe wrote: > > *shrug* so what if the special GUP called a VMA op instead of > > traversing the VMA PTEs today? Why does it really matter? It could > > easily change to a struct page flow tomorrow.. > > Well it's so that it's composable. We want the SGL->DMA side to work for > APIs from kernel space and not have to run a completely different flow > for kernel drivers than from userspace memory. Yes, I think that is the important point. All the other struct page discussion is not about anyone of us wanting struct page - heck it is a pain to deal with, but then again it is there for a reason. In the typical GUP flows we have three uses of a struct page: (1) to carry a physical address. This is mostly through struct scatterlist and struct bio_vec. We could just store a magic PFN-like value that encodes the physical address and allow looking up a page if it exists, and we had at least two attempts at it. In some way I think that would actually make the interfaces cleaner, but Linus has NACKed it in the past, so we'll have to convince him first that this is the way forward (2) to keep a reference to the memory so that it doesn't go away under us due to swapping, process exit, unmapping, etc. No idea how we want to solve this, but I guess you have some smart ideas? (3) to make the PTEs dirty after writing to them. Again no sure what our preferred interface here would be If we solve all of the above problems I'd be more than happy to go with a non-struct page based interface for BAR P2P. But we'll have to solve these issues in a generic way first.