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Message-ID: <1f29fe90-1482-7435-96bd-687e991a4e5b@redhat.com> Date: Tue, 2 May 2023 18:12:39 +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 18:06, Jason Gunthorpe wrote: > On Tue, May 02, 2023 at 05:45:40PM +0200, David Hildenbrand wrote: >> On 02.05.23 17:36, Jason Gunthorpe wrote: >>> On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote: >>>>> How does s390 avoid mmu notifiers without having lots of problems?? It >>>>> is not really optional to hook the invalidations if you need to build >>>>> a shadow page table.. >>>> >>>> Totally no idea on s390 details, but.. per my read above, if the firmware >>>> needs to make sure the page is always available (so no way to fault it in >>>> on demand), which means a longterm pinning seems appropriate here. >>>> >>>> Then if pinned a must, there's no need for mmu notifiers (as the page will >>>> simply not be invalidated anyway)? >>> >>> And what if someone deliberately changes the mapping? memory hotplug >>> in the VM, or whatever? >> >> Besides s390 not supporting memory hotplug in VMs (yet): if the guest wants >> a different guest physical address, I guess that's the problem of the guest, >> and it can update it: >> >> KVM_S390_ZPCIOP_REG_AEN is triggered from QEMU via >> s390_pci_kvm_aif_enable(), triggered by the guest via a special instruction. >> >> If the hypervisor changes the mapping, it's just the same thing as mixing >> e.g. MADV_DONTNEED with longterm pinning in vfio: don't do it. And if you do >> it, you get to keep the mess you created for your VM. >> >> Linux will make sure to not change the mapping: for example, page migration >> of a pinned page will fail. >> >> But maybe I am missing something important here. > > 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. > > Functional bug or not, it is inconsistent with how this is designed to > work. Sorry to say, I *really* don't see how that is supposed to work with a page that *cannot* be faulted back in on demand. -- Thanks, David / dhildenb
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