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[2003:cb:c721:b900:4f34:b2b7:739d:a650]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37189849a05sm3206277f8f.26.2024.08.16.02.30.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Aug 2024 02:30:34 -0700 (PDT) Message-ID: <81080764-7c94-463f-80d3-e3b2968ddf5f@redhat.com> Date: Fri, 16 Aug 2024 11:30:31 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start() To: Jason Gunthorpe Cc: Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sean Christopherson , Oscar Salvador , Axel Rasmussen , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Will Deacon , Gavin Shan , Paolo Bonzini , Zi Yan , Andrew Morton , Catalin Marinas , Ingo Molnar , Alistair Popple , Borislav Petkov , Thomas Gleixner , kvm@vger.kernel.org, Dave Hansen , Alex Williamson , Yan Zhao References: <20240809160909.1023470-1-peterx@redhat.com> <20240809160909.1023470-7-peterx@redhat.com> <20240814130525.GH2032816@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240816_023039_026556_D20B1DCD X-CRM114-Status: GOOD ( 26.53 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 14.08.24 15:05, Jason Gunthorpe wrote: > On Fri, Aug 09, 2024 at 07:25:36PM +0200, David Hildenbrand wrote: > >>>> That is in general not what we want, and we still have some places that >>>> wrongly hard-code that behavior. >>>> >>>> In a MAP_PRIVATE mapping you might have anon pages that we can happily walk. >>>> >>>> vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO, >>>> vm_normal_page_pud()] should be able to identify PFN maps and reject them, >>>> no? >>> >>> Yep, I think we can also rely on special bit. > > It is more than just relying on the special bit.. > > VM_PFNMAP/VM_MIXEDMAP should really only be used inside > vm_normal_page() because thay are, effectively, support for a limited > emulation of the special bit on arches that don't have them. There are > a bunch of weird rules that are used to try and make that work > properly that have to be followed. > > On arches with the sepcial bit they should possibly never be checked > since the special bit does everything you need. > > Arguably any place reading those flags out side of vm_normal_page/etc > is suspect. IIUC, your opinion matches mine: VM_PFNMAP/VM_MIXEDMAP and pte_special()/... usage should be limited to vm_normal_page/vm_normal_page_pmd/ ... of course, GUP-fast is special (one of the reason for "pte_special()" and friends after all). > >>> Here I chose to follow gup-slow, and I suppose you meant that's also wrong? >> >> I assume just nobody really noticed, just like nobody noticed that >> walk_page_test() skips VM_PFNMAP (but not VM_IO :) ). > > Like here.. > >>> And, just curious: is there any use case you're aware of that can benefit >>> from caring PRIVATE pfnmaps yet so far, especially in this path? >> >> In general MAP_PRIVATE pfnmaps is not really useful on things like MMIO. >> >> There was a discussion (in VM_PAT) some time ago whether we could remove >> MAP_PRIVATE PFNMAPs completely [1]. At least some users still use COW >> mappings on /dev/mem, although not many (and they might not actually write >> to these areas). > > I've squashed many bugs where kernel drivers don't demand userspace > use MAP_SHARED when asking for a PFNMAP, and of course userspace has > gained the wrong flags too. I don't know if anyone needs this, but it > has crept wrongly into the API. > > Maybe an interesting place to start is a warning printk about using an > obsolete feature and see where things go from there?? Maybe we should start with some way to pr_warn_ONCE() whenever we get a COW/unshare-fault in such a MAP_PRIVATE mapping, and essentially populate the fresh anon folio. Then we don't only know who mmaps() something like that, but who actually relies on getting anon folios in there. -- Cheers, David / dhildenb