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[2003:d8:2f20:da00:b70a:d502:3b51:1f2d]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e766659ea0sm2434868f8f.43.2025.09.12.02.15.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Sep 2025 02:15:29 -0700 (PDT) Message-ID: <0177c2db-a62f-4ed2-954b-6de8d3974ef3@redhat.com> Date: Fri, 12 Sep 2025 11:15:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/2] userfaultfd: remove anon-vma lock for moving folios in MOVE ioctl To: Lokesh Gidra , akpm@linux-foundation.org Cc: linux-mm@kvack.org, kaleshsingh@google.com, ngeoffray@google.com, Lorenzo Stoakes , Peter Xu , Suren Baghdasaryan , Barry Song References: <20250908044950.311548-1-lokeshgidra@google.com> <20250908044950.311548-2-lokeshgidra@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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20250908044950.311548-2-lokeshgidra@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: JDZCvX94HHbNvlrBRcUWj4Z4HX8CCeda8-4-q_7O1lU_1757668530 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: oqcudx5nfnmugd7qci3grmmicrrykhib X-Rspamd-Queue-Id: 559961C0006 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1757668533-346636 X-HE-Meta: U2FsdGVkX18q2xhd8hdWRqhilMxChLWYi32xA4UZAsxg3QpWThFQKb+M39SNqp7u4GcDayUUr/iMst+u4Dl7gCLQOFLc1owvLfxTaD7LumW2jah+TGmyZ8lwACwCRZn73Y90NCUCo9pP53UHO8w4vEVSTcEogUIKj2ygeXkaNRqnxdQ49ki96X0KCT5KkeQSdfyRzfAbBQ/jDnhcgZe5aeDukEC2n7hKXfovaBulUmiKv/8MmiIC0hyyTNCNM5v9Wg8i3zd3byUfaxmTDPPBInH4QzZ0yP55f1/7LGNYT2bfzzXuW1UAUDvoWDOQ1VeRkCFf9mi0d7djQrJIBi3hnljBsRtD5G+nrFTLmea0OEbyKUixCmOqwx/z68poKq4/i+D5I4LAxXJubQ39mg2BCLEgXmUom0+2gbWgvB1mwLKNLUIvefu/oKIjOn8rA/g+NJZmrAFyhQggjAtDM92JkhD3AbLd7+oIaidT8t5IANJjWCrXquXjxI2N5+tfIgPNfry5S7kG5PXZHE9e8Lxjc1yGTYYu/QCtwLE8M+wiQ8YyejnE46kG1EWOF8v8IehoNN1N+80UTjFiW4IMe/Biu1VYV+uTOvrN9bELzbtF939R2OBDNAUklrsRuiuS7zcUoSsd9D2azQTsiAxdzr/TYo6DWtG8iRb3oWY/5f6GBhh4HLaLVG3fJTE7qiiR9McjiEzRnXFGx/6Njna2VGtF0k/XXUtdmYw44mjP4nYDoaUSCYvjeHuykT+ksbnN3q0L/GqDCiK8gP6YdoNPT3YocyH2lGeDYg2p+VbUPqT5e8jAjvqqXpQNe38p1gjvE3I4XIrU7yIBsUTJB6r5idOULd79VCJ7qFdelHPDfRNaKw30EeTHn4DPwsBhoF0D42ekHLyPi5oB74d5oqc4pbQPfVBbzmDgQlsB7QXHLAPvVbMasbLig0dwSUORxsIIiWsuBIabeWWwB4M+ZbOeYIq 6rAjiU7X Qc5kqbWs8WhFhcAeTDTCO6f9n4TYG7V/tpg/gQDhsZN4NHhLPZYPs5/Nvt9mKTgals2h082MO0yw0HIRa27udXdO/HXpsbYDy4jqyZ007FqFkArjvxne4XQVJ9V5vWKKVf7NS8fYSpaKknAbZpvL9gHbtzOzYJjkS0IFGt8jdc1KqklemEhGXB2NzVgZXvi6JrN7SFtnc6+aoTg7Y+yC2FO9hdcOxC+InogZtpgEXo19R7Oo+UxTP/9pkhD1yxGg2tAPNqS5hhUG1V+0rMGRA1TRQfk7uJPxLTYF9rjxfqYywb3VtIKv6UNWY4gHDdhap+mL/XGVSuW0tLp2MOSZ/A1qEIb2PyM84SrN57iKC8wJbz22gSlHCWII5f8X9qJITy4zvFirFnIUXXpOv+1xeTpwdZpGwTna4KACPohR+P8sRDUE74E1no+i+jXlf0QNJw94gxQCEsOLWDHpXW2ceF515mscSyd1fcMXPmA0ttdOp7+8= 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 08.09.25 06:49, Lokesh Gidra wrote: > Since rmap_walk() is now always called on locked anon folios, we don't > have to serialize on anon_vma lock when updating folio->mapping. I would write that as "Now that rmap_walk() is guaranteed to be called with the folio lock held, we can stop serializing on the src VMA anon_vma lock when moving an exclusive folio from one a src VMA to a dst VMA. When moving a folio, we modify folio->mapping through folio_move_anon_rmap() and adjust folio->index accordingly. Doing that while we could have concurrent RMAP walks would be dangerous. folio_move_anon_rmap() already enforces that the folio is locked. So when we have the folio locked we can no longer race with concurrent rmap_walk() as used by folio_referenced() and the anon_vma lock is no longer required. Note that this handling is now the same as for other folio_move_anon_rmap() users that also do not hold the anon_vma lock -- namely COW reuse handling. These users never required the anon_vma lock as they are only moving the anon VMA closer to the anon_vma leaf of the VMA, for example, from an anon_vma root to a leaf of that root. rmap walks were always able to tolerate that scenario." Something like that. > > This helps avoid contention on src anon_vma when multiple threads are > simultaneously moving distinct pages from the same src vma. > > CC: David Hildenbrand > CC: Lorenzo Stoakes > CC: Peter Xu > CC: Suren Baghdasaryan > CC: Barry Song > Signed-off-by: Lokesh Gidra > --- -- Cheers David / dhildenb