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[2003:d8:2f28:c100:2225:10aa:f247:7b85]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3cf276c788esm596872f8f.17.2025.08.28.13.38.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 28 Aug 2025 13:38:51 -0700 (PDT) Message-ID: <56819052-d3f5-4209-824d-5cfbf49ff6e9@redhat.com> Date: Thu, 28 Aug 2025 22:38:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration To: Hugh Dickins Cc: Will Deacon , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Keir Fraser , Jason Gunthorpe , John Hubbard , Frederick Mayle , Andrew Morton , Peter Xu , Rik van Riel , Vlastimil Babka , Ge Yang References: <20250815101858.24352-1-will@kernel.org> <9e7d31b9-1eaf-4599-ce42-b80c0c4bb25d@google.com> <8376d8a3-cc36-ae70-0fa8-427e9ca17b9b@google.com> <3194a67b-194c-151d-a961-08c0d0f24d9b@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: <3194a67b-194c-151d-a961-08c0d0f24d9b@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: lHFXoi8B3VelGMMdGV9IkuOlDMFmXG3PHTUhcr0XAkk_1756413533 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 4B1B420004 X-Rspam-User: X-Stat-Signature: s81hfg3z7o86jjrgtnr3f1yjabhuwdme X-Rspamd-Server: rspam09 X-HE-Tag: 1756413941-663945 X-HE-Meta: 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 1S/RrDDF Ko7b75tXpypNwO7Y030FVyQD4+LD4dKWN4cf0mjYbCJu8bZ540Nipi5arNpdTVWoPqv48v9VaWcAJYC8aBy54alEEopRw+g+/MBa9NmUUdTVjGare9rFKGUm5HkDn8zciKWF5Gv428XN/jlBYJyztBdpvs1OPY2IrWXOj73+a2avc28DECO7AM5KVgSqyCiadk/x/lz63aGmsz4/JHBqyYojrDorlb5RDwMZV0Ecl6dGCOVV8ifed9BU/YY+1DgfglODSir7fjIjN/e8hljJHhiLdqypJGUXBKqL0bpSuQ06fpCMIj5EAiVwbMhwWtearEDgE8zKuRJbzj8jp5yLsVf27nwKiArA6HMTKPuRGrbc5SMfUrr5hBkOzv+B7ZzFh4c/M6E68NnG24ek= 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 28.08.25 18:12, Hugh Dickins wrote: > On Thu, 28 Aug 2025, David Hildenbrand wrote: >> On 28.08.25 10:47, Hugh Dickins wrote: > ... >>> It took several days in search of the least bad compromise, but >>> in the end I concluded the opposite of what we'd intended above. >>> >>> There is a fundamental incompatibility between my 5.18 2fbb0c10d1e8 >>> ("mm/munlock: mlock_page() munlock_page() batch by pagevec") >>> and Ge Yang's 6.11 33dfe9204f29 >>> ("mm/gup: clear the LRU flag of a page before adding to LRU batch"). >>> >>> It turns out that the mm/swap.c folio batches (apart from lru_add) >>> are all for best-effort, doesn't matter if it's missed, operations; >>> whereas mlock and munlock are more serious. Probably mlock could >>> be (not very satisfactorily) converted, but then munlock? Because >>> of failed folio_test_clear_lru()s, it would be far too likely to >>> err on either side, munlocking too soon or too late. >>> >>> I've concluded that one or the other has to go. If we're having >>> a beauty contest, there's no doubt that 33dfe9204f29 is much nicer >>> than 2fbb0c10d1e8 (which is itself far from perfect). But functionally, >>> I'm afraid that removing the mlock/munlock batching will show up as a >>> perceptible regression in realistic workloadsg; and on consideration, >>> I've found no real justification for the LRU flag clearing change. >> >> Just to understand what you are saying: are you saying that we will go back to >> having a folio being part of multiple LRU caches? > > Yes. Well, if you count the mlock/munlock batches in as "LRU caches", > then that has been so all along. Yes ... > >> :/ If so, I really rally >> hope that we can find another way and not go back to that old handling. > > For what reason? It sounded like a nice "invariant" to keep in mind, > but it's a limitation, and what purpose was it actually serving? I liked the semantics that if !lru, there could be at most one reference from the LRU caches. That is, if there are two references, you don't even have to bother with flushing anything. > > If it's the "spare room in struct page to keep the address of that one > batch entry ... efficiently extract ..." that I was dreaming of: that > has to be a future thing, when perhaps memdescs will allow an extensible > structure to be attached, and extending it for an mlocked folio (to hold > the mlock_count instead of squeezing it into lru.prev) would not need > mlock/munlock batching at all (I guess: far from uppermost in my mind!), > and including a field for "efficiently extract" from LRU batch would be > nice. > > But, memdescs or not, there will always be pressure to keep the common > struct as small as possible, so I don't know if we would actually go > that way. > > But I suspect that was not your reason at all: please illuminate. You are very right :) Regarding the issue at hand: There were discussions at LSF/MM about also putting (some) large folios onto the LRU caches. In that context, GUP could take multiple references on the same folio, and a simple folio_expected_ref_count() + 1 would no longer do the trick. I thought about this today, and likely it could be handled by scanning the page array for same folios etc. A bit nasty when wanting to cover all corner cases (folio pages must not be consecutive in the passed array ) ... Apart from that issue, I liked the idea of a "single entry in the cache" for other reasons: it gives clear semantics. We cannot end up in a scenario where someone performs OPX and later someone OPY on a folio, but the way the lru caches are flushed we might end up processing OPX after OPY -- this should be able to happen in case of local or remote flushes IIRC. Anyhow, I quickly scanned your code. The folio_expected_ref_count() should solve the issue for now. It's quite unfortunate that any raised reference will make us now flush all remote LRU caches. Maybe we just want to limit it to !folio_test_large(), because flushing there really doesn't give us any chance in succeeding right now? Not sure if it makes any difference in practice, though, we'll likely be flushing remote LRU caches now more frequently either way. -- Cheers David / dhildenb