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Mon, 23 Feb 2026 11:26:16 +0000 (UTC) X-FDA: 84475492752.25.4E58D00 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf10.hostedemail.com (Postfix) with ESMTP id DA785C0011 for ; Mon, 23 Feb 2026 11:26:14 +0000 (UTC) Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=UFgvymcp; spf=pass (imf10.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1771845975; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=sGqf2qwi7NORHRV6israjwuD6PzQni0n4UJJYcY2V5Y=; b=kyKZ5SZ+ZSWrYEOmGq0xYas0xlniT24CAHAUus/LpuaGgHRZiC61RmFf9p5XAfZCiuoEbr w5asoNA5istix0KrteXwd2qXTtkUOCxuHXqLGDQJ/s1NFl8QzLjyl8XI1mN5eeALphTS4O C2FWIuPywq6oxdaE3No/UTholM2YbfA= ARC-Seal: i=1; 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b=UFgvymcp0RVrTIuY7ko5yNeNvUrMjuwgZ0vqGTrFO2CKK1OCuYxFHel9s5ZS29euX Y+9XRwHjmGaZNlZ13uEEgJsvtB4qcMIBnNBYmOjjY1oj+0vSuqsIj6Kt7t5Ey8+yC+ /XHeKhfg3np7MaRo8JaW31RYzK0kl27mVTJvHeSPlin93nQJm97GoUpQCwBE7OcnJp IIOLEUjzZ4T+mt4IO59RzjZq9LbdJBez/cLAf9jib5lVWU+2CfcAETAqqfwPyW14Nd UGdhnVNUNWWaAgINjnNCMYLEGZbzocnr11HP0dedcjL9v+DyhacLEi34t7wtu1MChy v6Y1+twzPT2pA== Message-ID: <71247244-73fe-44ce-ab05-69c852f3aa1a@kernel.org> Date: Mon, 23 Feb 2026 12:26:10 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: "David Hildenbrand (Arm)" To: lsf-pc@lists.linux-foundation.org References: <9d5af75e-e41d-4052-bade-8e51c9622523@kernel.org> Content-Language: en-US Cc: "linux-mm@kvack.org" 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 Subject: LSF/MM/BPF TOPIC] Deferred folio splitting: extend or remove/replace? In-Reply-To: <9d5af75e-e41d-4052-bade-8e51c9622523@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: DA785C0011 X-Stat-Signature: 5a5w8u76qxouh74jbcxe4m4jan6z5te8 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1771845974-640154 X-HE-Meta: U2FsdGVkX1+ynqxMOoeeOE+DxdvwKxo+9FuW5eMp6hxEmoGTjnYtLKxu55TX6zmB7qdRAQXJqMV1pGoDFSDwQIwTxR2b9/4WmdFgW9NCQD1Qdjf7gCn/iFTT6yg4kd1+zKKgS+VCMM+VuxPylGUb+CHMAwDuMb8/PX+l0FMjQJVoBN80LtnKYzHe43WSllUlUKMFqN4HmmxttCeQTbIT83HVia/L/hKgymLRVMzfQGMp4Ulowgr9zea75HDRWBkdQnXSJoC/tYwlXXH9jZ0oiWeDVqre56+005hhKhqU44F4zuuB0OxyrV2e04t6lef4YTyBjMayNAe/IzxAV3+IaA5rcjSPm6OQmrHkGTlFXAZaZfQs55J7MVvmxChjySYT2L+BDjKUrWE4Qjz1TAxTqoMHiBS2jenwcYR1fS4a7l0nJNr4FgFT3qksWUo/A2/HbltUpKKr3iSAIDEchKYo1IVQpZBG2AVNwSlp1fgw0+57PnFgWHVBA5bwEnLfSM5lwo1pcQhexdbB/0bVqncI59QgqNaUTJhtImiheNlCcffNbJqujDf5pXPDNDBvJ2pfpG9HMTyyEPam57AiPduQNN31AH9oA4f6KWlbTQ34IW3Uy0tgGM3EpRTZ10/7k4mYASmR5bFv0iMhv6zGeGzbS0IViAN1FFutys2X0Nf0oGZGS6YaTAQEbJKQNcMDY6FDw8JPa7dsybkiR36XsXRoDqzV/ePfNXxOWYu+/BpHTbzkFz4A/7inrEtD/CaFI8OKqHX58G1yTY8rsV8NxFxtNj/fngyFq8FMt68LMKXV9H7QXTrM+kB1P7yJ1Lp7foz+CDxVUrZsUIaMdRjUhhP0Uk4gMwSPKeKm4LvzecCdw9z1lDDfsndKWYpOqueXiLZW4wGfjtC74vSzPcSGUbnAc/BTYu4gplj0++V9zay4+C3b9YGYvbD+P6Db/FsSpTWB88LW28/0JnBrBzcqMoO xfrZa5NF Nu8YKyR/dGk2mKEME32wrYtdgnyCK0dOkTj2ZevwRt7zs849w0zVEG/jwdZoZLoJUHxpDrLCjxvsDWkI3zjFRnG+WVxNLeYOXx4ryq5j4EF5WaEKMnnoWPweG9LS4vXTVymqjfAUJU80kFT1Wwm4EPenH6STmp6Xw0l/FWC7SDg64eXquNST+oQyv1NDs/um4KTv32bRJyWIIfi5nn9YEY1jjck2KIEPnCTPH4z9iyPXlYT9euTMBNV0+3u87Obja1hjmgrSx0/FS02S4ssk4Tq+gt6pILV20IO82z8kjWp3jZHXW7jkxu98Krw== 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, the "deferred" folio splitting mechanism was originally introduced to reclaim memory from partially-mapped anonymous THPs under memory pressure. Partially-mapped anonymous folios (often) guarantee that at least some memory can get reclaimed easily. Ever since, we extended it to also handle smaller large folios (mTHPs), but also to scan for mostly-zero-filled mTHPs that can be reclaimed by replacing them with the shared zeropage. Nowadays, each and every (m)THP we allocate gets added to the deferred split list as default, as "split_underused_thp = true", which is rather suboptimal. Historically, handling deferred splits has been rather complicated and error-prone. So I've been wondering whether that complexity still warranted. Could we instead just let LRU scanning deal with that and get rid of the shrinker? Or could a shrinker instead use existing LRU lists? We could flag folios for being * Partially mapped (which we already do) * Possibly over-allocated (unscanned for zero since added) To prioritize processing them. One of my motivations for looking into this topic is that with CONFIG_NO_PAGE_MAPCOUNT, we can sometimes not reliably detect "partially mapped" folios and only know that a folio "might be partially mapped". To figure out whether it is actually partially-mapped, we have to walk the RMAP. While I could extend the deferred splitting mechanism to handle such partially-mapped folios as well, it would imply taking the deferred split locks more frequently when unmapping pages ... not having to mess with another list might simplify things. -- Cheers, David