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(p200300d82f0df000eec92b8d4913f32a.dip0.t-ipconnect.de. [2003:d8:2f0d:f000:eec9:2b8d:4913:f32a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4f00971e4sm19383632f8f.65.2025.06.03.11.37.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Jun 2025 11:37:30 -0700 (PDT) Message-ID: Date: Tue, 3 Jun 2025 20:37:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm/memory: ensure fork child sees coherent memory snapshot To: Matthew Wilcox , Jann Horn Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, Peter Xu , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250603-fork-tearing-v1-0-a7f64b7cfc96@google.com> <20250603-fork-tearing-v1-1-a7f64b7cfc96@google.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: HOaGF_OBQnz9EvygVxB39nySDBcvrHyP6JwqKEXAPTw_1748975851 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: rspam12 X-Rspamd-Queue-Id: 0DBAD10000B X-Stat-Signature: 77braoyq5p99sawyszncqfofh1yj988j X-Rspam-User: X-HE-Tag: 1748975853-738588 X-HE-Meta: U2FsdGVkX1/R7vSzlUbB+dM3SD2pwPWwOZ4vewy9lMWfNYpTi0UhL7pD+bJMRg1oN4A2sa5OmLqGDAOIpitmRy40RdB5E6BqbrZ+yzz1ILyq0mFe5PssBb8bqyEOK0dfhg9GQVFBH6AlUeYq64pTSEg2rLN4yY4/Jo33i3uZ+0NKJzAwzpNIdJbEsi8BVpcyYOFL/n7r6qG40NKb7KUMh0yLf8/OWCjAfpXft/9ibyvP7D697Bvv4wWLwgR23HvEcgfygeAQQ+kwoOHkHN5oMVX8xkxTw7TZuKDIQ6Ss/Q/9iH/cBzh5tyk7y1Qi3CkHxZ89g3XND9euwk1DtPJ500pjxmLYsU0Ei9krtND4InZTsUiNNeOoaNmpdRaOT/VBiP+HSBpbTwU05cv7NOZsJdvQzEGiCh7PLpk55OWkKOkV3SjsD9EDvWNV+CJ9dmUe0xD1D189Ev4Jo8cmdX1lOck+LVAx0oKhH6u6uyn5zEzHFpMCgYnI3irlSqJ+mhnT/132K6yIHlal9SILy/khT5rDFihtcFUTpKjPIC/9/OpRCF9NdMKq/LQv+tdujIFtjDH5NyNwBzvZnT8OyWjMf/h1gK2+d5phKHVIg/AUjoDvYpbk7hCHdVRrnts2nb6bLYfjDJzPW6AueVO8nFScxzAB6pC4CA+JavPUOrXjwcLil1gRPDFBTXkxvAroiy0P22jwyjEJYFFfRh4V7RqFmwS57kiBbHMts7w3TWpL0UrAhAdY4Y+xCZNBk1waFdYwO8wyaf2rjuGAvux0GF+On3d2C8S5aLwtQhC2+OeaTCQ163GHoeXC131Uobqg3acfJBfxlW5J660V87gmlL0ujfWBfnFPtInRNqb7d0nexs8a3/93ZQfRF2JnPKbCu+viUnybTdTQZ5qaN3WhKOVknLI9PKpM0GDRVPDxo3DQRNX5cFpDtw0+41++wdmJjGxuP19rGX6BEsbmgScPGez ZonGxOCd plkLHEcXY2Vhd8RMq2pQKbFvT9Et3s7jq73GL3J6pVh5ztN6lOfcOpMF7R/9RNGtgofNc1kquGgT3BpQ0exD/7v+lC2Y+ipF9hi923MYrUx9don3OciXBuw1HLETG5p3Cob4QLt/lkC4HMxfuYpvAf1UzOi2SMZ146xPzINliyCr4l0loh8221W2QWI7rNrCXeHvcZYNEJxWj/ycotoj9+v/Sc1biVQy0U7lhzfk03XCC3ThHEU2CsTYGYcQmzGKh58CWwKbq6H2gCVQVWYTt3funnNqGIGK1lsARv+0FFsjnJKpiodaqzhOvOv9sZL3BEVdaV1AOZhJFI2jN5xaVv5JGzNSnNIE6VT6LnOkTc8IBO7UwkdO+DmkEoUtpfVYXDjirsowcKU2JCkEetys+gYNhlQ== 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 03.06.25 20:29, Matthew Wilcox wrote: > On Tue, Jun 03, 2025 at 08:21:02PM +0200, Jann Horn wrote: >> When fork() encounters possibly-pinned pages, those pages are immediately >> copied instead of just marking PTEs to make CoW happen later. If the parent >> is multithreaded, this can cause the child to see memory contents that are >> inconsistent in multiple ways: >> >> 1. We are copying the contents of a page with a memcpy() while userspace >> may be writing to it. This can cause the resulting data in the child to >> be inconsistent. >> 2. After we've copied this page, future writes to other pages may >> continue to be visible to the child while future writes to this page are >> no longer visible to the child. >> >> This means the child could theoretically see incoherent states where >> allocator freelists point to objects that are actually in use or stuff like >> that. A mitigating factor is that, unless userspace already has a deadlock >> bug, userspace can pretty much only observe such issues when fancy lockless >> data structures are used (because if another thread was in the middle of >> mutating data during fork() and the post-fork child tried to take the mutex >> protecting that data, it might wait forever). > > Um, OK, but isn't that expected behaviour? POSIX says: > > : A process shall be created with a single thread. If a multi-threaded > : process calls fork(), the new process shall contain a replica of the > : calling thread and its entire address space, possibly including the > : states of mutexes and other resources. Consequently, the application > : shall ensure that the child process only executes async-signal-safe > : operations until such time as one of the exec functions is successful. > > It's always been my understanding that you really, really shouldn't call > fork() from a multithreaded process. I have the same recollection, but rather because of concurrent O_DIRECT and locking (pthread_atfork ...). Using the allocator above example: what makes sure that no other thread is halfway through modifying allocator state? You really have to sync somehow before calling fork() -- e.g., grabbing allocator locks in pthread_atfork(). For Linux we document in the man page "After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2)." -- Cheers, David / dhildenb