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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id t70-20020a0dea49000000b0056d443372f0sm3297041ywe.119.2023.07.24.21.30.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 Jul 2023 21:30:36 -0700 (PDT) Date: Mon, 24 Jul 2023 21:30:27 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: "Kasireddy, Vivek" cc: Jason Gunthorpe , Alistair Popple , Gerd Hoffmann , "Kim, Dongwon" , David Hildenbrand , "Chang, Junxiao" , Hugh Dickins , Peter Xu , "linux-mm@kvack.org" , "dri-devel@lists.freedesktop.org" , Mike Kravetz Subject: RE: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages) In-Reply-To: Message-ID: <75e3a74a-68f5-df-9a49-a0553c04320@google.com> References: <20230718082858.1570809-1-vivek.kasireddy@intel.com> <20230718082858.1570809-2-vivek.kasireddy@intel.com> <87jzuwlkae.fsf@nvdebian.thelocal> <87pm4nj6s5.fsf@nvdebian.thelocal> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: CF605180008 X-Stat-Signature: gqe4kgi7383kfpx3wdnkb7jw36z9377i X-Rspam-User: X-HE-Tag: 1690259437-628873 X-HE-Meta: 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 H0aWrWFb gwZXrYTeXxKuSwjmZ0IB2vvv7SlF/CfyT6I57HXyx1pcHN21S/rz+rN9rsrZ8XGgCBzVyTTD4RM/OZZi1/YglC640W9Ab6GCJwWPCCK9f+N5ZjhYW9ltFpSNyZotDp8+6BW7so49G8s969TmxQhIWM+Xy+KHfsfdfnzNDZn0OEhW+Kze264yfubm0MCCT3qYI8652+tHwEt8kjs2o5EasskJNpT9TFuxT8mu4igAZTVr54CU13AR91L9fUTz2zs2vG0AYH5ZvMw3trdXPJuxhZs3JRy31g+yDojDq1/rQv2VD8dngIFiTLb5Lh39KKGgQE/8nWgWgbkU24v31G5Klx4raF3m5rSERROZlhUyS3AKW/Ro+hW98vmxdsVJ6u0gKSPFiPOCJGh6pzFLI+JSbrGHcaPsUIC3roQHKNp/r4rBN04AI6cMx7YFcc3qNbre252Izcs8x0yZeMjA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000058, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, 24 Jul 2023, Kasireddy, Vivek wrote: > Hi Jason, > > On Mon, Jul 24, 2023 at 07:54:38AM +0000, Kasireddy, Vivek wrote: > > > > > > I'm not at all familiar with the udmabuf use case but that sounds > > > > brittle and effectively makes this notifier udmabuf specific right? > > > Oh, Qemu uses the udmabuf driver to provide Host Graphics components > > > (such as Spice, Gstreamer, UI, etc) zero-copy access to Guest created > > > buffers. In other words, from a core mm standpoint, udmabuf just > > > collects a bunch of pages (associated with buffers) scattered inside > > > the memfd (Guest ram backed by shmem or hugetlbfs) and wraps > > > them in a dmabuf fd. And, since we provide zero-copy access, we > > > use DMA fences to ensure that the components on the Host and > > > Guest do not access the buffer simultaneously. > > > > So why do you need to track updates proactively like this? > As David noted in the earlier series, if Qemu punches a hole in its memfd > that goes through pages that are registered against a udmabuf fd, then > udmabuf needs to update its list with new pages when the hole gets > filled after (guest) writes. Otherwise, we'd run into the coherency > problem (between udmabuf and memfd) as demonstrated in the selftest > (patch #3 in this series). Wouldn't this all be very much better if Qemu stopped punching holes there? Hugh