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Message-ID: <a3576685-d389-55dc-1eac-fc190f8ea792@linux.ibm.com> Date: Tue, 2 May 2023 11:19:26 -0400 From: Matthew Rosato <mjrosato@...ux.ibm.com> To: David Hildenbrand <david@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, Lorenzo Stoakes <lstoakes@...il.com> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, 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>, Jason Gunthorpe <jgg@...dia.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>, Peter Xu <peterx@...hat.com> Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings On 5/2/23 10:57 AM, David Hildenbrand wrote: > On 02.05.23 15:35, Matthew Rosato wrote: >> On 5/2/23 9:04 AM, Christian Borntraeger wrote: >>> >>> >>> Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes: >>>> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote: >>>>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes: >>>>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken >>>>>> as we cannot rule out folios being cleaned and then a GUP user writing to >>>>>> them again and possibly marking them dirty unexpectedly. >>>>>> >>>>>> This is especially egregious for long-term mappings (as indicated by the >>>>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as >>>>>> we have already done in the slow path. >>>>> >>>>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery? >>>>> It would no longer work with file backed memory, correct? >>>>> >>>>> See >>>>> arch/s390/kvm/pci.c >>>>> >>>>> kvm_s390_pci_aif_enable >>>>> which does have >>>>> FOLL_WRITE | FOLL_LONGTERM >>>>> to >>>>> >>>> >>>> Does this memory map a dirty-tracked file? It's kind of hard to dig into where >>>> the address originates from without going through a ton of code. In worst case >>>> if the fast code doesn't find a whitelist it'll fall back to slow path which >>>> explicitly checks for dirty-tracked filesystem. >>> >>> It does pin from whatever QEMU uses as backing for the guest. >>>> >>>> We can reintroduce a flag to permit exceptions if this is really broken, are you >>>> able to test? I don't have an s390 sat around :) >>> >>> Matt (Rosato on cc) probably can. In the end, it would mean having >>> <memoryBacking> >>> <source type="file"/> >>> </memoryBacking> >>> >>> In libvirt I guess. >> >> I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working. >> > > That's ... unexpected. :) > > Either this series doesn't work as expected or you end up using a filesystem that is still compatible. But I guess most applicable filesystems (ext4, btrfs, xfs) all have a page_mkwrite callback and should, therefore, disallow long-term pinning with this series. > The memory backend file is on ext4 in my tests. A quick trace shows that pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable is still returning positive implying pages are being pinned.
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