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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6c340bfa6d0sm7797286d6.1.2024.08.29.11.45.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Aug 2024 11:45:19 -0700 (PDT) Date: Thu, 29 Aug 2024 14:45:16 -0400 From: Peter Xu To: David Hildenbrand Cc: Jason Gunthorpe , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Gavin Shan , Catalin Marinas , x86@kernel.org, Ingo Molnar , Andrew Morton , Paolo Bonzini , Dave Hansen , Thomas Gleixner , Alistair Popple , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Sean Christopherson , Oscar Salvador , Borislav Petkov , Zi Yan , Axel Rasmussen , Yan Zhao , Will Deacon , Kefeng Wang , Alex Williamson Subject: Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start() Message-ID: References: <20240826204353.2228736-1-peterx@redhat.com> <20240826204353.2228736-7-peterx@redhat.com> <9f9d7e96-b135-4830-b528-37418ae7bbfd@redhat.com> <20240828234652.GD3773488@nvidia.com> <2123f339-2487-4b1c-abb1-313e9a012242@redhat.com> MIME-Version: 1.0 In-Reply-To: <2123f339-2487-4b1c-abb1-313e9a012242@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: bsxrh9x39znn7j6toimrd5gm1phesi8a X-Rspamd-Queue-Id: 0E61E14001D X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1724957122-353041 X-HE-Meta: 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 AhmUOYNR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 29, 2024 at 08:35:49AM +0200, David Hildenbrand wrote: > Fortunately, we did an excellent job at documenting vm_normal_page(): > > * There are 2 broad cases. Firstly, an architecture may define a pte_special() > * pte bit, in which case this function is trivial. Secondly, an architecture > * may not have a spare pte bit, which requires a more complicated scheme, > * described below. > * > * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a > * special mapping (even if there are underlying and valid "struct pages"). > * COWed pages of a VM_PFNMAP are always normal. > * > * The way we recognize COWed pages within VM_PFNMAP mappings is through the > * rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit > * set, and the vm_pgoff will point to the first PFN mapped: thus every special > * mapping will always honor the rule > * > * pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT) > * > * And for normal mappings this is false. > * > > remap_pfn_range_notrack() will currently handle that for us: > > if (is_cow_mapping(vma->vm_flags)) { > if (addr != vma->vm_start || end != vma->vm_end) > return -EINVAL; > } > > Even if [1] would succeed, the is_cow_mapping() check will return NULL and it will > all work as expected, even without pte_special(). IMHO referencing vm_pgoff is ambiguous, and could be wrong, if without a clear contract. For example, consider when the driver setup a MAP_PRIVATE + VM_PFNMAP vma, vm_pgoff to be not the "base PFN" but some random value, then for a COWed page it's possible the calculation accidentally satisfies "pfn == vma->vm_pgoff + off". Then it could wrongly return NULL rather than the COWed anonymous page here. This is extremely unlikely, but just to show why it's wrong to reference it at all. > > Because VM_PFNMAP is easy: in a !COW mapping, everything is special. Yes it's safe for vfio-pci, as vfio-pci doesn't have private mappings. But still, I don't think it's clear enough now on how VM_PFNMAP should be mapped. -- Peter Xu