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b=kBLSxfAU1kS/0AtX8wqwCfxoY5F+x5fc9wQ0BALrSlew8+78rhbFUu3HiNmlNMPWN acHIbB4M3SailIR2wHEYTyBHVTrPogo6QsSLd0tnMvvtUygZXR/nT2oE6BKt58ECnL 6g8eauuXi3CpmNAPK/y8tLhiSsPLMwzmrGIk+z7mYS++qhtItSGdTHIlr1gDXeXMC9 h79xDOLt6Wv1q95Rs0O3+GBBUbaFP/EvpOPSBy/bLubDnvLLK1+0TiDDHsG6EPkmqB CYJhwA0RSI7x49iH7zQmqd/3BB1l8elosRqkH/C2AAiZ6M4WiTnd47s6dCdoi1w3u+ 0GFbeTd8vFJ4g== Message-ID: Date: Fri, 8 May 2026 14:06:12 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Towards removing CONFIG_PAGE_MAPCOUNT From: "David Hildenbrand (Arm)" To: lsf-pc@lists.linux-foundation.org Cc: linux-mm@kvack.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 In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 46E09100004 X-Stat-Signature: z53cr5ry357c84sjc4t87fdsk9mwj4qc X-HE-Tag: 1778241977-533769 X-HE-Meta: U2FsdGVkX1+r73Ok65wAdYP5gP1dpCUt+f1TFpTdLemlz67mc4akdUpfs4Kkk3CHgBCAc4hhggqyD/B/VIzo2aDTObsJT+SXORHFPbMr2L3tmswMxQddAu9LWDD25gfkZveUw/GOIxTphGXCfurgWCe9RgoifmoduTQB/Z1WvOd54gRBOsX7LdqGyUygqG3A7R2wx2YZwHoGnDMWeI65uLBHl7iP2Js1IqSrqYwoAQXP8BVUMlFyhvJFWg/YPJKFRCjfLpBUGwb52+KjgoP1UEK1l7IFQ0VSigAE0A0LvWNPT6NKpxMwUj6xUtm57euzeinE/kSgs1h7HDPhoJPQwJtYABZ/rzQCoGiw4imva8dyGe/8fnwYpDcwCnXvVGcYEk3bl8PH8EReikCrK32jOqYKsXMCJq5rVktodLHOYT2AkBWMFijNFzq6fAh1QyGmVQy0vfzUh56ivcVRFODUQyyV9sZzkO1z5Ikf7rrCZsg7M1l7yEtMeeMsfxyAWZYMmgc+SthVRsbCRwkBhpfh4MybyOHdLHLiUGFaJhdfVOMwpQGsC3e8WPN8ZKp0p2S8WADnIC6oivPxwpiD+R0xYU/Ais0RhC7QUXXDc74Z4yemXVpt4WBkS587TNsucz6lvbCNNdATIzY120lttnW3yTWR2vbTRn/46MiJVmGdktqPlBwL6Jxa8lu6HKJOOYytQhgtZOeS2rPfG5Dy3fq+w2ZygQd+xnYaXlIyffOhscXajHuIdrMvu2r6DyPJt/EuQ7F0mG60OVR88MzP8btk97oJJTBfg5qEgEfCfoVLMYx1txHCkG7QWDbK0L+ypel9shDqFBTP6RbtyUHxAhMkxYN66YvOavR3Ie4Rwl2pzYRuuI54cxCrVrsQiilR1Zu4iLILOZL4K4ZsuyoopW6XknYxCq+9tGI7EzQ9lgo2x4DimXW4w3tu+enRB5QOw66Wkiz6Tqvnbb/u+t72oKX e0DhtkfY YKkopKh+lOnMLDgK7wzI98hKjah0YjadgRcECo8/oKe6SsZ4/DlHlD2DqmKHdq/cc6UZVai86YXgW+XWHnq/q2eHFOEcomcUre54CPojrEnZAGsfo9cnB1zexwhQHBiv7GQDIxe3Tb31Hu030/85RkhRYr8iYap2cGSKnSo/pXNIuFNMkM92aQSJ5jnoXMatk0IoNGyaAxOhngcTcYKYQ/YGyz/Y3hoit3SnKzj9Y2WhnDAJv1nWXU5N7aUOaSqSd3dCbYDjAyZkH4vEuJjNB1+9rX7fDutLSfWpNgO/XDg4RpyOaqAVP5F5gcY4f6RrSlVAj4iZATwq6oAvzt4u1tmy15m5Mo9TIKguoRJIZWZT6mhyNCUfviwOhfeNWe+3+WWqHkkKJO9YaV5xMOyBXBSOq6VXkM1/eEkxrmQP/ClhybWeZr5yJtD6oced8u6EcJ0He Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/17/26 22:04, 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. > > 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/ > Slides at: https://share.daveh.de/s/bQsDpD7xigJ6wD8/download?path=&files=LSF_MM%2726_Towards_Removing_CONFIG_PAGE_MAPCOUNT.pdf -- Cheers, David