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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id r12-20020adfe68c000000b0033df5710fabsm20100194wrm.44.2024.03.07.04.09.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 04:09:30 -0800 (PST) Message-ID: Date: Thu, 7 Mar 2024 13:09:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable From: David Hildenbrand 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> <0644814b-869b-4694-bdb1-bab4e6186136@redhat.com> 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: <0644814b-869b-4694-bdb1-bab4e6186136@redhat.com> 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: rspam12 X-Rspamd-Queue-Id: 6A83D80012 X-Stat-Signature: srf139r85wuu9feqsyrqno6nr3be4az3 X-HE-Tag: 1709813374-589680 X-HE-Meta: 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 nNMvBVz+ L39A01jCo/sUmmbmxsvHeHISMyhGrVFyPCiH5VYSrihQWWgzLghtDqGOlvaiXpjSBPlIRGAb15XloTl/44+BDu5f9c/uiJgEl9LnCZRC0wkN7++FUxZBpoZqXbtRsWgLMBdGbQkdYfxTPnf1jyhckhRd+NjGHM+pYZl/v9qsub1gW4Wjg/Ts7QbPAZrZHkclLvO++PAkY4l3WgUQHI8KNSNtyq6gJ9m3H41zcg7qWhsnPYhdFUOLablTjzA== 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 12:59, David Hildenbrand wrote: > 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. > [and I just recall, there are some other corner cases. For example, pages in a shadow stack can be pte_write(), but they can only be written by HW indirectly when modifying the stack, and ordinary write access would still fault] -- Cheers, David / dhildenb