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b=Lv2YMWDp7uE7yaxsztb3i0LlL2BjJ2uffnVurXVOYwqxR/7/HcGo2NUww6+rKZWkW DzFOU/ehCwBgorR6mzyrjt923aZ/asEIf8he+7sNbGqyCi9WgxKgNRkEf4zPCXO/Ry nz067tsW4OGWo8uWxR+1nMlIrb7W3N5rrcvnn//buxGjWkKov7eyDP1LVMTTj+9XtU PvzQyY0VyVfL32kS3eJlqk6sO7nbFKyHvlB1vzTqL5nvAoM0lxt9VjEpyMrTtcQF8t vL2bK4Zp/DmCpfIoiOj99ZDmmBFirNmNbTnsA3Jv5zqkW3czdKUVmClIB+y9g/2vrm Nui4+Ugnqt6hA== Message-ID: Date: Fri, 20 Feb 2026 11:35:20 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Towards removing CONFIG_PAGE_MAPCOUNT To: Zi Yan Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org References: From: "David Hildenbrand (Arm)" 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 In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 7znir1dtz7uu1448k5jyekxs6ppjk8qc X-Rspam-User: X-Rspamd-Queue-Id: 74BD4A000C X-Rspamd-Server: rspam01 X-HE-Tag: 1771583723-205579 X-HE-Meta: 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 ZnJafIcW pjyX8kk4PQQpaCjKrSZ9tqU+24fXdcHPlfRabIabVqs05nkBPIl1UAWRHE465KgKQ47STLHIS2WmDc8XPKyHCz3XGjqKGXLW5xbt6nVPFarv9YOcIzsku/0dLbRzTBjHb/LircHRhw8H1I6DwARpgbIbEeAJ04fCD/mUT0udfpHt0YFNHKcNaJmGHtw0WEsLQoo7T1TLvC2GzW9e9494p3toTfXL+wSlKJGNaud54MJvuX8PP2wgPq0odXdy4/d2gfvcqgnG1HI3THXC3QV5wA429oSIwx/YOZ0oQr8EUoIagj2HwKZpLXLxxMma2FsFP10v7PuuhGa/HD0M= 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 2/19/26 18:07, Zi Yan wrote: > On 17 Feb 2026, at 16:04, David Hildenbrand (Arm) wrote: > >> 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. > > Then, for PMD sized folio, _pmd_mapcount is its “_entire_mapcount”, for > PUD sized folio, _pud_mapcount is its “_entire_mapcount”. For mulit-PMD > or multi-PUD folio, _pmd_mapcount and _pud_mapcount are similar to > _nr_pages_mapped but with PMD_NR/PUD_NR multiplier. Maybe we would have > _pte_mapcount instead of _nr_pages_mapped? What we'd have is essentially (ignoring hugetlb) is * mapcount * pmd_mapcount * pud_mapcount pte_mapcount = mapcount - pmd_mapcount - pud_mapcount That is sufficient to calculate folio_average_page_mapcount(). There are some possible evolutions of this concept (but some other stuff would have to change), but above is what we would start with. > >> >> 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: > > Or let a workqueue to walk rmap to check if an unmapped subpage indeed has > no mapping left and put that folio in deferred_split list. Or let the > deferred_list_scan does the rmap walk and decide whether to split the folio. Exactly. That is one solution I had in mind and started prototyping at some point after LPC: flag folios as possibly-partially-mapped and let deferred splitting figure it out. -- Cheers, David