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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id v12-20020a05600c470c00b00412b4dca795sm2376146wmo.7.2024.03.07.03.15.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 03:15:39 -0800 (PST) Message-ID: <9ebc9faf-0bb0-4409-b93a-5beddfc4929b@redhat.com> Date: Thu, 7 Mar 2024 12:15:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit To: Richard Weinberger Cc: linux-mm , linux-fsdevel , linux-kernel , Linux Doc Mailing List , upstream+pagemap@sigma-star.at, adobriyan@gmail.com, wangkefeng wang , ryan roberts , hughd@google.com, peterx@redhat.com, avagin@google.com, lstoakes@gmail.com, vbabka , Andrew Morton , usama anjum , Jonathan Corbet References: <20240306232339.29659-1-richard@nod.at> <20240306232339.29659-2-richard@nod.at> <861682210.23281.1709809857201.JavaMail.zimbra@nod.at> 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: <861682210.23281.1709809857201.JavaMail.zimbra@nod.at> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 7C688A0011 X-Stat-Signature: yf7cp93gft4kjjzg7dh9pnpnpz64p3fk X-HE-Tag: 1709810144-568023 X-HE-Meta: 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 Z0vj3f3W 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.004771, 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 07.03.24 12:10, Richard Weinberger wrote: > ----- Ursprüngliche Mail ----- >> Von: "David Hildenbrand" >> An: "richard" , "linux-mm" >>> + Bit 58 is useful to detect CoW mappings; however, it does not indicate >>> + whether the page mapping is writable or not. If an anonymous mapping is >>> + writable but the write bit is not set, it means that the next write access >>> + will cause a page fault, and copy-on-write will happen. >> >> That is not true. > > Can you please help me correct my obvious misunderstanding? We'll perform a page copy of an anonymous page only if the page is not detected as exclusive to the process. So a better description could be: "In an private mapping, having the writable bit clear can indicate that next write access will result in copy-on-write during a page fault. Note that exclusive anonymous pages can be mapped read-only, and they might simply get remapped writable during the next write fault, avoiding a page copy." -- Cheers, David / dhildenb