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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4500e1dd6c8sm28645685e9.37.2025.05.28.10.32.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 10:33:00 -0700 (PDT) Message-ID: <8b307f83-6a16-46c1-b71b-64af9ca0f592@redhat.com> Date: Wed, 28 May 2025 19:32:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: fix COW mapping handing in generic_access_phys To: Jason Gunthorpe , Peter Xu Cc: Jinjiang Tu , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, wangkefeng.wang@huawei.com References: <20250528015617.302681-1-tujinjiang@huawei.com> <0d4f0180-52e6-47c9-b141-54e7e7c86880@redhat.com> <5b9f5952-9979-426f-857a-dffa9b7963af@redhat.com> <20250528162915.GR61950@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <20250528162915.GR61950@nvidia.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: bGONUZLLEUWn-1EEduzyrxmsFOez7lAUtJjskdKjLFs_1748453581 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 28CEB80003 X-Stat-Signature: 61914ugyenf89k896iaieam8yyducowy X-Rspam-User: X-HE-Tag: 1748453585-881510 X-HE-Meta: U2FsdGVkX18MmfD9Vh6wr4BXi0qwSeLpBPUCpvrk2RF8wU+XU5NO6btZlYJRyo4py0xz+pwNxAQt+gAbpp4V8PNQxwfj4hM/2P/O9NwFvYnJlj+nIr+uYK7KbV7Flo6khyDN9YqeNBde6c7MpRePX5U4BEyGLCkB3KnI3CrCZZB7bhfiqoqGZjjvsPYr/x8eACRykQ5j3F71WR3MUbJJzUNJX5YJV7W2OObbZSJiTXdBPQ4p1kJ0/hHwz8EQ7yimTkJZr16CgW9NyxsaOMaiBlxcqrqsk6yzGzao0qAPRHrskeRX9JxGIiX0IuE2AKdh0zmsKdlJ1PpCpwtSKgCwdOlHoTYvBULoaiYRj3yKkbhXmDULfOce//WGv9CHl0+FbD3pW8dvTLFkZKzYfOiQhbpTADPZzBpFd/M7K4uAhK1JpH4lIDi52FC+lByQhSeHi83z4kixIUqqBmmEgkGMiQ1ry50rcp2SPXs8CGSDRzRktMzadahqpadItT84PIMFr2ezh3CG9MvOOl8jHo2+3/mA09A8dc95JzC65MxcvmhjDcpnI5rA5x/1ZV2S3+Vm5eHmtUwod6/kSlLJ62Ii0p7zl/ZAvjVfBszrZSAcaiNRAoVtPEEDWWCEop7VHzzO/+62cNtc6e/7pnrNch37Z48Go65/1LYafI+5TNJpkyz3npz18F7QY3ajr03f/oDKwndG2ldImNVZ9TLNXxd+hHrDWF5o2gPEXYprhw3mdZzgdAIn93NActKo3zrS7FH8mlnJX9dI1JUOfBZCPRLxQBfexCHbo/bRDnldLH/mQAhkzWRHUwq1/BzYeoFqWKc8baNGiI/ThUaSEsYgb9ZoygV+v8PyyeE345TInT4IkDCymij45a9syS9H2V/p+b3cL48GOZYXZ3ZmimxXjsdGAZJTW/WynVBtp3dHV+SX/QfhV3bErBdiwRNWvoJO7iB4D71wfYQGqbEtduqL/wS 2RTNXqJj OPaGzidSttDhkZpsp/xS4pkcbtvWYIxc0yUSxC8nHSAPLFqzQkg+pKcwZfT1KcJ5+kfoIYhxiT0YP39CrEI0K2/w/xeAa5jXizC0DdyjVEKSYp6KKFccrznR2UVuYZWKkGmB+wYH5v59AVDRmCk+exeUOb8B0K3BbEY+mrecmLERU3bKlUzdU0dB8ZMmnKVG3+rsjI1JYjHTSRe/saPBcuwEDQr2XosgJEb+Gi4dvFErYrmolJwEYeCRD8657gMG8biIYlTAOd7g74L6VMJejZu8GtwWwrOPsRHAYgaqylTwYiaKCweVQgCBgq6KnCPe3qGP/C8R1xM6pARqOkinTo/srNxfW3HK8ABY7fWhP4S5oOJ6jfgPJAASFCRCKBA8QA8QcxuYDmJhiTCeve0MaRyxyddGNGOQc7DAYwkxx+i7+HLJIyABIlAa0rMfRdfbbzDqHLsEYd3/uHns= 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 28.05.25 18:29, Jason Gunthorpe wrote: > On Wed, May 28, 2025 at 12:06:07PM -0400, Peter Xu wrote: >> #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ >> >> I'm not confident to blame any driver yet to have those special cases for >> VM_PFNMAP, because it only says "managed without struct page", it didn't >> say "it must not contain struct page".. Hence it hints the core mm "please >> do not manage these mappings with struct page at all". Still sounds fair >> contract, even if not ideal. > > I think it is pretty clear, if a VMA has VM_PFNMAP then nothing must > ever try to obtain a struct page from any PTEs in it, for any reason, > even if things in it might have a struct page. In practice it means > nothing can call vm_normal_page() on a VM_PFNMAP. No, not until we remove any COW mappings of VM_PFNMAP. But maybe I misunderstood what you mean. > > It would be nice to update the comment to make it clearer. > > If the VMA owner wanted to permit access to the struct page then it > should have used VM_MIXEDMAP. > > The fundamental difference between PFNMAP and MIXEDMAP is that > vm_normal_page() is allowed on MIXEDMAP. That comes with some extra > rules and restrictions to support arches without the special pte bit. > > VM_IO | VM_PFNMAP further means that all the pfns in the VMA require > the use of io accessors (writel/readl) to access them. > > No idea what VM_IO | VM_MIXEDMAP is supposed to mean. Only the special > ptes need io accessors? > > In either case GUP doesn't really work on the VMA. PFNMAP is totally > blocked, and for MIXEDMAP userspace has no way to discover which > subset of the VMA is GUPable. I think that GUP is supported on > MIXEDMAP at all is a bit of a weirdo thing. IIRC (after recent discussions with Lorenzo) there are use cases for that. And there is no way to block GUP-fast either way without pte_special(). And pte_special() ... is not for refcounted pages. -- Cheers, David / dhildenb