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Message-ID: <6681789f-f70e-820d-a185-a17e638dfa53@redhat.com> Date: Tue, 2 May 2023 18:32:23 +0200 From: David Hildenbrand <david@...hat.com> To: Jason Gunthorpe <jgg@...dia.com> Cc: Peter Xu <peterx@...hat.com>, Matthew Rosato <mjrosato@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>, John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>, "Kirill A . Shutemov" <kirill@...temov.name>, Pavel Begunkov <asml.silence@...il.com>, Mika Penttila <mpenttil@...hat.com>, Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu> Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings On 02.05.23 18:19, Jason Gunthorpe wrote: > On Tue, May 02, 2023 at 06:12:39PM +0200, David Hildenbrand wrote: > >>> It missses the general architectural point why we have all these >>> shootdown mechanims in other places - plares are not supposed to make >>> these kinds of assumptions. When the userspace unplugs the memory from >>> KVM or unmaps it from VFIO it is not still being accessed by the >>> kernel. >> >> Yes. Like having memory in a vfio iommu v1 and doing the same (mremap, >> munmap, MADV_DONTNEED, ...). Which is why we disable MADV_DONTNEED (e.g., >> virtio-balloon) in QEMU with vfio. > > That is different, VFIO has it's own contract how it consumes the > memory from the MM and VFIO breaks all this stuff. > > But when you tell VFIO to unmap the memory it doesn't keep accessing > it in the background like this does. To me, this is similar to when QEMU (user space) triggers KVM_S390_ZPCIOP_DEREG_AEN, to tell KVM to disable AIF and stop using the page (1) When triggered by the guest explicitly (2) when resetting the VM (3) when resetting the virtual PCI device / configuration. Interrupt gets unregistered from HW (which stops using the page), the pages get unpinned. Pages get no longer used. I guess I am still missing (a) how this is fundamentally different (b) how it could be done differently. I'd really be happy to learn how a better approach would look like that does not use longterm pinnings. I don't see an easy way to not use longterm pinnings. When using mmu notifiers and getting notified about unmapping of a page (for whatever reason ... migration, swapout, unmap), you'd have to disable aif. But when to reenable it (maybe there would be a way)? Also, I'm not sure if this could even be visible by the guest, if it's suddenly no longer enabled. Something for the s390x people to explore ... if HW would be providing a way to deal with that somehow. -- Thanks, David / dhildenb
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