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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id n6-20020a056000170600b0033e45930f35sm8541285wrc.6.2024.03.07.03.59.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 03:59:28 -0800 (PST) Message-ID: <0644814b-869b-4694-bdb1-bab4e6186136@redhat.com> Date: Thu, 7 Mar 2024 12:59:26 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable To: Richard Weinberger Cc: linux-mm , linux-fsdevel , linux-kernel , Linux Doc Mailing List , upstream+pagemap , adobriyan , wangkefeng wang , ryan roberts , hughd , peterx , avagin , lstoakes , vbabka , Andrew Morton , usama anjum , Jonathan Corbet References: <20240306232339.29659-1-richard@nod.at> <1058679077.23275.1709809843605.JavaMail.zimbra@nod.at> <7d9321db-a3c1-4593-91fa-c7f97bd9eecd@redhat.com> <1525238492.23321.1709812267495.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: <1525238492.23321.1709812267495.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-Rspamd-Queue-Id: 3AC0040002 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: wjcb5tqedmqzep4u5f3653xbsrxefrbu X-HE-Tag: 1709812772-879424 X-HE-Meta: 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 P0Qw6x6g 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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 07.03.24 12:51, Richard Weinberger wrote: > ----- Ursprüngliche Mail ----- >> Von: "David Hildenbrand" >>> I'm currently investigating why a real-time application faces unexpected >>> page faults. Page faults are usually fatal for real-time work loads because >>> the latency constraints are no longer met. >> >> Are you concerned about any type of page fault, or are things like a >> simple remapping of the same page from "read-only to writable" >> acceptable? ("very minor fault") > > Any page fault has to be avoided. > To give you more background, the real time application runs on Xenomai, > a real time extension for Linux. > Xenomai applies already many tweaks to the kernel to trigger pre-faulting of > memory areas. But sometimes the application does not use the Xenomai API > correctly or there is an bug in Xenomai it self. > Currently I'm suspecting the latter. > Thanks for the details! >>> >>> So, I wrote a small tool to inspect the memory mappings of a process to find >>> areas which are not correctly pre-faulted. While doing so I noticed that >>> there is currently no way to detect CoW mappings. >>> Exposing the writable property of a PTE seemed like a good start to me. >> >> Is it just about "detection" for debugging purposes or about "fixup" in >> running applications? > > It's only about debugging. If an application fails a test I want to have > a tool which tells me what memory mappings are wonky or could cause a fault > at runtime. One destructive way to find out in a writable mapping if the page would actually get remapped: a) Read the PFN of a virtual address using pagemap b) Write to the virtual address using /proc/pid/mem c) Read the PFN of a virtual address using pagemap to see if it changed If the application can be paused, you could read+write a single byte, turning it non-destructive. But that would still "hide" the remap-writable-type faults. > > I fully understand that my use case is a corner case and anything but mainline. > While developing my debug tool I thought that improving the pagemap interface > might help others too. I'm fine with this (can be a helpful debugging tool for some other cases as well, and IIRC we don't have another interface to introspect this), as long as we properly document the corner case that there could still be writefaults on some architectures when the page would not be accessed/dirty yet. -- Cheers, David / dhildenb