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[2003:d8:2f20:da00:b70a:d502:3b51:1f2d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45e037d741asm53707575e9.23.2025.09.12.02.03.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 12 Sep 2025 02:03:47 -0700 (PDT) Message-ID: <3f37af16-abf2-4ed4-9894-0028a9f02f76@redhat.com> Date: Fri, 12 Sep 2025 11:03:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/2] mm: always call rmap_walk() on locked folios To: Lorenzo Stoakes , Lokesh Gidra Cc: akpm@linux-foundation.org, linux-mm@kvack.org, kaleshsingh@google.com, ngeoffray@google.com, Harry Yoo , Peter Xu , Suren Baghdasaryan , Barry Song , SeongJae Park References: <20250908044950.311548-1-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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: wlSV6fa3I6dzzBtLkUkFg-531x-TrO0GhH1cKx_H2Pk_1757667829 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: tx63iw1wuda5dahb7wt7zjm3ojk6dun1 X-Rspam-User: X-Rspamd-Queue-Id: 9D0731C0006 X-Rspamd-Server: rspam04 X-HE-Tag: 1757667835-289089 X-HE-Meta: U2FsdGVkX1+JOPn7FdVhmFj5wz3rkcRnYJrsRo4DFDzXrxg2Us/8xfTlIrtlx0CnqpMzyJm2YaP+Sf/UbR0cnDswcMlcKHfRc7XYxwuEz2LRAp6CRzuBiv2/5JhRK+f7w5EoUVSEZVU1cUSz7ojnNz58ZJHSDTEWgKNijBEIRH3+YsgDJ+qD7xeGowOB2uq6hJjy8/fRCw7bh2lKqHP8ZLwQwqBPl0JB23tNfhYhCM2QaNFbb8EI5Tc0MpQ5Y8z3O5kgW3/Lhy42g4bxf4IPO3h+j52SA/iJbZyYISsE1lKGvt+Gh32bLLYEmhp+poEYzsL7TtWfuoma3V76fY3c/9s9a/MWxx4tcV6h1VQrQKKCg3AylAvK5cis5ho51P6tENFrEwDRLo+zNoQeFfiy/q+1zI0gqCNH6sPeX8yu3+e2jdZCzwWVGvvgNcGmcDtiVGI1THAYADn81IDvtQSDW8YGIe4Q8gjp8dGr4We9RNsgh7XiK0R1ibwHvZe3Sk2l7d73WFEWGAhyER+z4XP/DIAI2lq2CEgWJjnhJuyr6x7+tycuvx/tlabbRjBQ8XuRXOyf/KB3iX+00b7V2lDg7RvOusVGHRlL3OEXeg0MllQJ5O6UcOcsWSyNUI9IgFWdzivsb2i4Tx/Q9cSCVhb9xq0v6mbTrCAiEYwI2SnCKa5yFtaM8W6AJ7I+QDkIPdb96my3kS7FsPqHt0bbtEvHQcITIUecyuvgbolTuBuyDIai0YQZ8bc45GgYHvL77QK5n0gXpNLoDP7ExicCQWJ3yF0FT+/wx0qhXk+Xkh4VEPg8j9tu+eLTGWLJHjqt6NRzptSdums79j+Ogoz1sg5+LANprAyVMSBsaI6KnO9cnIG3qx0+VLbWhA7hM76VDvLD7HcKsWdjOEr4sevK8PM4nPRWKdd1Bhrf8ZzZ2gGpQAqTAAYvhwo/VPhu4rR44AGSEmPM59ay4uGMJVUjeS8 QDOR3d/i /DtC+d1oxc4d0328BsOURZBfpnJ/9n3kPjBEoWeEs+GG/zH+MKHF1cWnOrKLC1HvukKLO3CPQe5PTzpmLROcTyYSraqLI4g1gacx1bU4P5REF6LjuzNdrVpQtGNV0t8CvvGSkO7jya6p6MWacT55IRwz/DaKibzz5ruWT00RCvLOOdvq3s6WIYIMwkr4+B/qHx1mfVSqrqqTEfqnWCPqwxSx716JdKHWfkoKnVDoMd7Wudvy4b370UWA7rzMADJH0Qq3YnirFTRWJfj2hONQvb1p2vmMSXqqbhiBmgaH7em17FSdnX509rJc3MydqEzJ9AH7NK/7LDC79V4YQAnzAbC7EZSyxH2eSDZyI8O98w0jgxXbS8wxpGngDE+Ma34eIKwjJe99GI9Q6abS0EMYFALLe76ABwfRqEUwMZRCncFFJS/Oz6d08Y62dsWKGgCzNA1L/Ptn3VdxoPlD2xsPfOdLydb0yjG7OvMOL 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 11.09.25 21:39, Lorenzo Stoakes wrote: > Please please please use a cover letter if there's more than 1 patch :) > > I really dislike the 2/2 replying to the 1/2. +1000 > > On Sun, Sep 07, 2025 at 09:49:49PM -0700, Lokesh Gidra wrote: >> Prior discussion about this can be found at [1]. >> >> rmap_walk() requires all folios, except non-KSM anon, to be locked. This >> implies that when threads update folio->mapping to an anon_vma with >> different root (currently only done by UFFDIO MOVE), they have to > > I said this on the discussion thread, but can we please stop dancing around > and acting as if this isn't an entirely uffd-specific patch please :) > > Let's very explicitly say that's why we're doing this. > >> serialize against rmap_walk() with write-lock on the anon_vma, hurting >> scalability. Furthermore, this necessitates rechecking anon_vma when >> pinning/locking an anon_vma (like in folio_lock_anon_vma_read()). > > THis is really quite confusing, you're compressing far too much information > into a single sentence. > > Let's reword this to make it clearer like: > > Userfaultfd has a scaling issue with its UFFDIO_MOVE operation, an > operation that is heavily used in android [insert reason why]. > > The issue arises because UFFDIO_MOVE updates folio->mapping to an > anon_vma with a different root. It acquires the folio lock to do > so, but this is insufficient, because rmap_walk() has a mode in > which a folio lock need not be acquired, exclusive to non-KSM > anonymous folios. > > This means that UFFDIO_MOVE has to acquire the anon_vma write lock > of the root anon_vma belonging to the folio it wishes to move. > > This has resulted in scalability issues due to contention between > [insert contention information]. We have observed: > > [ insert some data to back this up ] > > This patch resolves the issue by removing this exception. This is > less problematic than it might seem, as the only caller which > utilises this mode is shrink_active_list(). > > Something like this is _a lot_ clearer I think. Yes, fully agreed. > >> >> This can be simplified quite a bit by ensuring that rmap_walk() is >> always called on locked folios. Among the few callers of rmap_walk() on >> unlocked anon folios, shrink_active_list()->folio_referenced() is the >> only performance critical one. > > Let's please not call this a simplification, I mean yes we simplify the > code per se, but we're fundamentally changing the locking logic. > > Let's explicitly say that. > > Also I find it odd that you say shrink_active_list()->folio_referenced() is > 'performance critical', I mean if so, surely this series is broken then? > > I'd delete that, the entire basis of this being ok is that it's _not_ > performance critical to make this change. I think we can mention that as a side-effect of this performance optimization for uffd, folio_get_anon_vma() gets simpler and we no langer handle locking of anon folios different to locking of other (pagecache, ksm) folios. -- Cheers David / dhildenb