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[2003:cb:c70f:7e00:75dc:9c40:33fa:b5f8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434d5281229sm56908145e9.26.2024.12.05.02.53.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Dec 2024 02:53:06 -0800 (PST) Message-ID: <12445dbf-cdcb-4ac8-8d6f-5b0683df6a75@redhat.com> Date: Thu, 5 Dec 2024 11:53:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [QUESTION] anon_vma lock in khugepaged To: Dev Jain , ryan.roberts@arm.com, kirill.shutemov@linux.intel.com, willy@infradead.org, ziy@nvidia.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20241128062641.59096-1-dev.jain@arm.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: L3vpqF0iHMr5Q04rN1TLYf2VzODEBr-7e8ZKopYVxpw_1733395988 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: zfptkmnghn47h3boppncd13wdgufx6e6 X-Rspamd-Queue-Id: 060D2160013 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1733395981-427308 X-HE-Meta: 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 l5F4jAjx +UkLWau+F4/7BUBunV48e6kGAUeHPK9WPxEQy1OPWI+eS7iAei0BsSl2hJtaoaeJirpmCiPB7gQaQNkiJY2x73Degr0HyM7h0ydBJjbdUpn3IukSHlZ4nVHw51T1eqVZqTBonbuH4IRM3Lm2V6IF1HmwBION0d2aSscUrPW3TlgnXmAMSAK4Z4Azc2YAGoNrmn6SiV9uN8o9n2//vEwyjTL/OvJyvzkAhYB8+LQyE4HSii0Lmk8AG8QVxpF5sU6vh2+Jj75iLphDD4qOX06UwPkpOafKUTRUMYO1WY4MYY9D/D74jE+nyFyx9P2HO522azcmfTA6R/WtMNH4bLGo1PNAFLaz4E+0KPfeGvTmoHAu+HxQ6I2WRpRXKp+S1XvO54yiVEsB9T7pRPio4hfIblCiRb3G+p/8FhYXbJ2IraP6I3PfieA2THHY1eewfMy6tAFBg X-Bogosity: Ham, tests=bogofilter, spamicity=0.000007, 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 05.12.24 11:10, Dev Jain wrote: > > On 28/11/24 11:56 am, Dev Jain wrote: >> Hi, I was looking at khugepaged code and I cannot figure out what will the problem be >> if we take the mmap lock in read mode. Shouldn't just taking the PMD lock, then PTL, >> then unlocking PTL, then unlocking PMD, solve any races with page table walkers? >> >> > > Similar questions: > > 1. Why do we need anon_vma_lock_write() in collapse_huge_page()? AFAIK we need to walk anon_vma's either > when we are forking or when we are unmapping a folio and need to find all VMAs mapping it; the former path takes the > mmap_write_lock() and so we have no problem, and for the latter, if we just had anon_vma_lock_read(), then it > may happen that kswapd isolates folio from LRU, and traverses rmap and swaps the folio out and khugepaged fails in > folio_isolate_lru(), but then that is not a fatal problem but just a performance degradation due to a race (wherein > the entire code is racy anyways). What am I missing? > > 2. In what all scenarios does rmap come into play? Fork, swapping out, any other I am missing? > > 3. Please confirm the correctness: In stark contrast to page migration, we do not need to do rmap walk and nuke all > PTEs referencing the folio, because for anon non-shmem folios, the only way the folio can be shared is forking, > and, if that is the case, folio_put() will not release the folio in __collapse_huge_page_copy_succeeded() -> free_page_and_swap_cache(), > so the old folio is still there and child processes can read from it. Page migration requires that we are able > to deallocate the old folios. > Without going too much into detail: I think one reason is that try_to_unmap() and friends should not fail (not finding all PTEs/PMDs) simply because there is concurrent collapse going on. Observe how try_to_unmap() and friends take the folio lock first, and how collapse tries taking the folio locks only after it already invalidated the PMDP. Likely there are other reasons, but this is the "obvious" one I can think of. We really try preventing *any* concurrent page table walkers, and that requires the mmap lock+vma lock in write + rmap lock in write. -- Cheers, David / dhildenb