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b=N39gBlZp8G7QwzHgp2lBy7tB/3hXBuKoUC9oUniAtB/hq9DOo37ureWaHJgBwe/on jzOYx24Wr6hHs8j7ex5wJeb4XgZwBy7hLH79GJcCcPi6My8Tjvl1UeOlbFZ+fa0kG2 d6xjxGG1iAaHxURXpH3qaKKzTlA51C+mfFbM3DXReaxOTTZK7M60X7QJPGRppLaSmh BS9Dz+oMRjWoGSk4Lv5tEAAt2vOf6MUBTOwMRK727nZhAGVC5gNfLESJBb5EieYSsG kMWXONb1Gnz6IIP75Kq+FLJ+xNvTCa7mOU94RAwMMTc8v5kdxWY7svtIz1JV2WukIS UI6GwYMMRv4VQ== Message-ID: Date: Tue, 17 Feb 2026 22:04:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: "David Hildenbrand (Arm)" To: lsf-pc@lists.linux-foundation.org References: Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+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 Cc: linux-mm@kvack.org Subject: [LSF/MM/BPF TOPIC] Towards removing CONFIG_PAGE_MAPCOUNT In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: A16AE40002 X-Rspamd-Server: rspam02 X-Stat-Signature: wfehrq8e9gt9rsc8uxe6nycjihicfbni X-HE-Tag: 1771362282-102186 X-HE-Meta: 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 V0tOkEVD iTrGeqmNUeDs1DAd9578LbVdsrVcXosyR556/g9LZjwU0IUOqwEHu6Lb0CM3ItiEqs1YwftCe1jF8QDCpTCp8jtfptxq5nMhtXNL/PfQQfHCN21VEWkhOcVoUVqrhyvDk0mMeojZnbHvePWm7A8IbI64igC45QNzlGMhM1Tij/ZIwLUJlPqX/MbF7fHNF8xiWMHvNDLYyN9wSfZ4qEAquBe9hTzwruyC4Vp0FZlprZLlaqaPkslZab9NpueRVmbdb4So8b/ytQRqx6Vvh9VhMGnSLNegRwi6iomXjWL/oIKrOhMiYCvRlKRTl9RP0Oyza7bIVUq7AuIprB679SsVoOjSxtRoc2jOhPPVgCFAk1YylNPs= 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: Hi, although I like mapcounts very much, I'd rather prefer to not have mapcount work on my todo list. We now have CONFIG_NO_PAGE_MAPCOUNT in the kernel that doesn't touch any mapcount values of tail pages, which is great. But we still have CONFIG_PAGE_MAPCOUNT around, being used as default. To make my dream come true, some things I have in mind are still pending. In particular, I want to: (a) Support mapping of folios > PMD through PMDs. (b) Get rid of CONFIG_PAGE_MAPCOUNT to stop messing with page->_mapcount on tail pages and to cleanup the rmap code. (c) Better detect partially-mapped anon folios with CONFIG_NO_PAGE_MAPCOUNT. + some other small things. I discussed some of these challenges at LSF/MM 2024 [1], before we had CONFIG_NO_PAGE_MAPCOUNT. No we have it and we can discuss the next steps. Sorting out (a) is fairly easy once we removed CONFIG_PAGE_MAPCOUNT: we'll primarily have to split folio->_entire_mapcount into folio->_pmd_mapcount and folio->_pud_mapcount. Sorting out (b) requires switching to CONFIG_NO_PAGE_MAPCOUNT first, which will imply some imprecision with large folios to: (1) Process memory stats: Pss + Uss accounting like "Pss" and "Shared_" vs "Private_" in /proc/$PID/smaps and /proc/$PID/smaps_rollup (2) PM_MMAP_EXCLUSIVE flag in /proc/$PID/pagemap (3) System memory stats: "mapped" memory like "AnonPages", "Mapped" and "Shmem" in /proc/meminfo And some other smaller things. While I think that all changes here should be fine, I want to be a bit careful and have a discussion on how to tackle it without realizing in a couple of releases that some use cases still require CONFIG_PAGE_MAPCOUNT. Sorting out (c) is a harder nut to crack, and I wonder to which degree we care and whether I am being too careful. I have some ideas that I want to discuss. One idea is to just remove the deferred split lists and let memory reclaim deal with that: but that one might be discussed in another session I'll propose around the deferred split lists. [1] https://lwn.net/Articles/1013649/ -- Cheers, David