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[2003:cb:c700:2400:6b79:2aa:9602:7016]) by smtp.gmail.com with ESMTPSA id v3-20020adfedc3000000b003062c0ef959sm6522031wro.69.2023.05.02.09.12.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 May 2023 09:12:42 -0700 (PDT) Message-ID: <1f29fe90-1482-7435-96bd-687e991a4e5b@redhat.com> Date: Tue, 2 May 2023 18:12:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings Content-Language: en-US To: Jason Gunthorpe Cc: Peter Xu , Matthew Rosato , Christian Borntraeger , Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , Dave Chinner , Theodore Ts'o References: <1ffbbfb7-6bca-0ab0-1a96-9ca81d5fa373@redhat.com> <3c17e07a-a7f9-18fc-fa99-fa55a5920803@linux.ibm.com> <4fd5f74f-3739-f469-fd8a-ad0ea22ec966@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org 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