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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-Stat-Signature: udo44ufefipms7xx1zxix5hn5g6hq8mz X-Rspam-User: X-Rspamd-Queue-Id: 75C8D40007 X-Rspamd-Server: rspam02 X-HE-Tag: 1723800638-849649 X-HE-Meta: U2FsdGVkX1+4KwNkrOJ68db31U+zM7NP+MlMcOlgGMaGwjTt+y+vOfzKQtIXhm/dRmLs3zdrkw8dH0YcWRQXD5lDmmJncPuSNSg1M+TrtX3a+Um5E3vV7ElErAV8nfMCz7ztWarPQfS9RiQQ/hxKBkRvOTF6QQB2p9zuhYHAjHE9F4GtrStMxBNoTKNnHgwVOm4uMstB+2GlWphEul2/ymZ6yuO/O7fvf2jjzpiJ3D5Oj/NwihOJV/k8PCuACLiG3Ss3Ju9NdLRkr5bdyMBb5V2G6XvhMLSWv5H2z5ASSAt24dJDe3qMTRlWmrGdrpUeXB2b3Ng+g7GfTRdfQNvwM2YaDpqtQB0Vdgf1kLmgIIY2C3r3f/EfqJPIhUq7/6yuq3fYxQqy/1h1C5/JTyX+d6Fr8me3qL25TdEATVD/XfwIOfrwyNwG5T4FXY3A/Mz1WaWNYvvYoLe3jRUEGHUpPgS9Q3YmKFOTRiSg17ois1JumFX42rlIsODAofgqWT0is0OL/v95J8VuevXDybYolqokgJR4jPvgpBFyNAtXInw/dR6nzSxjIa0qgN3COY6dVLbDFqN7yw9tnFdzCHDQ/icVtmMQ/d/cl4wUgz89bl4VjN/KGv2UIDzIMaOH2GEetKo6uChV/W2F/DPnh1VxOQI/9cL5lxmUev2KBewi+JvnQbGvbeclL7L3w/k9H8ZRWFZEKvzwnH4DoYfSCvKc8EBdjcsvLwTJ1ARlR36McVRhmjTy9GHT1QCQmVFuIJwBkCAzAvGFX4a5euMOZZKHiAAjkbuPI1qq+SyUZmgRHrNV09jcJvH6biasuZhGYrOgNyrddFZuYBxNWDTgcd+neoO6nyQDqRNwj/V7nScChb0t+OEBhZTdIfnOBSyB2n/Y42jkcN1yZ79MZldEpKlQclq4Ab3Z7z3mNeScYlwMatQfc9RE56a1vdbSEId+lZBnDlkSWsoEnniOph1O14B /PbOXBRN 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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 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