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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4eac7e919sm2106642f8f.33.2025.05.28.10.37.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 10:37:52 -0700 (PDT) Message-ID: <41ebd229-6640-4375-a528-98d079936b01@redhat.com> Date: Wed, 28 May 2025 19:37:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: fix COW mapping handing in generic_access_phys To: Peter Xu , Jason Gunthorpe 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: vyXYxUoFkKLdwPRzSrAAlxsEescJKYO1Oeg115JKwpU_1748453873 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: rspam05 X-Rspamd-Queue-Id: 91A8B40004 X-Stat-Signature: t3j1tpzkxkae9r7ij7r17qqywwd939ni X-Rspam-User: X-HE-Tag: 1748453876-159785 X-HE-Meta: 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 GqWHfOr1 1BP33jmkob+7t1my608Eu6W12VZbqTqY1HatFIAPrF1UjjhoDAbRjj+GG/zKjZb0jx3bqESqoYe3kJ7hKQHtV9eK//nSIQaTvKvw8oP5VVCcqQ9bcHCSYtO41sGM7JMra/0audX2XE3bSYoykI5wSCA0ZSt0dMK35rZpt9All+UChmpI9IAdrj/gY4RhWsMatABmXLTvv9tSOs1ucaficpQ7QVu5Cm2R2hzfDZrMrlki8u0Jhe8pxcWg8ukQ+nR13h+eDb2fuWNo+nDMUxUS6u2dHjYIjANVr917ktbwLFgTlPGBq6UUqBoarwyuvHyfG2qWinyOLJ1bj/AiYyGgxYREkC7kXQuM3pd0jFZ0bAcVTjkpuZx+getMlwde2FxMW0gHa0qPUE1Mi4p36myRyYmG2gHcRnH5jXFlMudGyBOuRMHPVUdYxqsMOxofkSU4mMjpJE1cf3oSkMqQ= 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: >> 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. > > Does it imply that in the ideal case one should use follow_pfnmap_start() > for MIXEDMAP? > > I don't have a strong feeling yet on how GUP should treat MIXEDMAP, either > (1) fail MIXEDMAP like you said, falling back to follow_pfnmap_start(), or > (2) allow MIXEDMAP only on page-backed mappings, then fallback to > follow_pfnmap_start() on non-page-backed mappings only. I think GUP should GUP when there is a page to GUP. If there is no page to GUP, then we have a PFNMAP and should fallback to follow_pfnmap_start() indeed. And now I realize that MIXEDMAP should indeed be working with follow_pfnmap_start(). But I think it should only deal with actual non-struct page thingies. GUP not supporting COW'ed pages in a VM_PFNMAP is something we should likely just fix. It would likely also fix the problem here, because the anonymous page would simply be returned by GUP. I mean, GUP-fast should work on COW'ed pages in a VM_PFNMAP already ... -- Cheers, David / dhildenb