From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 70D65C98318 for ; Thu, 24 Sep 2026 18:52:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 83C7C6B00A9; Thu, 24 Sep 2026 14:52:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8147E6B00AA; Thu, 24 Sep 2026 14:52:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 72A746B00AB; Thu, 24 Sep 2026 14:52:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 3F16B6B00A9 for ; Thu, 24 Sep 2026 14:52:07 -0400 (EDT) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay05.hostedemail.com (Postfix) with ESMTP id BEE0E4036D for ; Thu, 24 Sep 2026 18:52:06 +0000 (UTC) X-FDA: 85249550652.17.F95F4D9 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf13.hostedemail.com (Postfix) with ESMTP id 13D3E2000D for ; Thu, 24 Sep 2026 18:52:04 +0000 (UTC) Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=hmKwra2T; spf=pass (imf13.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 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=1790275925; 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=8fQuNmR+wyKKa/NE7gKqnglhXs6oXfwgf7LqT0NLqJM=; b=zi+PhiqS7uC4VryftFziFzeFLvablBRSCklvV4PHgNs9wESjXwax3/Td099EWd1D6npr4l QdIsqR5QLVmgtJLG7U9U1ofSCAaPcq9fibkO1oc930gQNnWOkCMWRrUu9ywoR27Rg19ktb OAt1CWQtR9xkVZIMkhYzX5dOC4MEDbk= ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=hmKwra2T; spf=pass (imf13.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790275925; b=KEwQEKDgsDob0nbt8dXkQ1/+ObAhXKPuzamSFmuS9BAPxkkw6rX1U+cWhR8yUAhLgJD9hl X3P9p7U0enVk1RsSk3qv+N9J2x/ZMHp3M1MzQK4zxhWBnHlMfVT+rWDfWhaqmDFuNyd0Yn wD5vyJqDNVzmvTeu2THoOoPdEaKM7/c= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id AEDB060236; Thu, 24 Sep 2026 18:52:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50E851F00893; Thu, 24 Sep 2026 18:51:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790275924; bh=8fQuNmR+wyKKa/NE7gKqnglhXs6oXfwgf7LqT0NLqJM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hmKwra2TGt4NKXHAA/TkRcGPes5yT9c5nvFOd+aZbIBb7FPyc7k5fI9FVjH9eU4GG sODuhzOfxulRS+W1TQ3EiWAHBIdTotMlSZba1Zftbd3TRwCMVH/BoCBE0RnoxRtNik oXOPyebbT1FC9GA19P4NRoEdFGMIwEzKIX/PEN4ZgpKlbbq7Wn06FFrvEdj9QL+WWx MgKLgK3yz6RTLmkPgA8Xu6Vs+paDS02BGoGLXXkQew1Mxmq75NIxaodkh1HOvLTjr2 AOtX5ao831YTZLHT9N9YMT57zQXlry8BCfVNNahANt8tvX99YtRVHO8EYnTQok5MF+ 50pdsWexQGSSQ== Message-ID: <570cf649-a8bf-4073-be71-207fbb2bef40@kernel.org> Date: Thu, 24 Sep 2026 20:51:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 3/3] mm: implement page refcount locking via dedicated bit To: Ilya Gladyshev , Zi Yan Cc: akpm@linux-foundation.org, andrew+netdev@lunn.ch, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, edumazet@google.com, harry.yoo@oracle.com, hramamurthy@google.com, ivgorbunov@me.com, joshwash@google.com, kirill@shutemov.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, pfalcato@suse.de, rppt@kernel.org, surenb@google.com, torvalds@linuxfoundation.org, vbabka@suse.cz, willy@infradead.org, yuzhao@google.com References: <56e036665dbeb0a8f51d697220d5ed84f210994c@linux.dev> 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: <56e036665dbeb0a8f51d697220d5ed84f210994c@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 13D3E2000D X-Stat-Signature: z33iaqh4xrc9wi65pktza77u8byse577 X-Rspam-User: X-HE-Tag: 1790275924-632912 X-HE-Meta: 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 fOe88nox cNs/EWr9Mszp9xXHFvRaj5vT/M4s8iwREf6QVGCZO6eZa35zFnW3Jm4hTNCunWhXKhql1Eh5MpiE4Bic5s6zq15tCqiVXXvlrK3WALpzsVPurXOtbmpgvIC7U1hpLmaRomfXTZ1Y4SfoeUpXPKNAs1Ynm0StugJ/zjBdGh3Pea7id1kVehtDWF2jmg6NRsRGWsEaFRAdfTaN+uKW3mloYFw4Lg3wsji+jMZxsJ8N4xE9D4/C3U5BNJ2DhdioCb58JdG2LroUSgL6xUzJvE/cMbt1sCwrPxOa+m+Ql7vCmWnnPo5iBoYPHj5crmqPZwno4PKMFSA43bO7hkdHGHgm4dPAfVmTGx/VHJqnHzU+/9d0dGQ0KNrgQovnS494cdNO454MjTZyucZI9XFM= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/23/26 19:34, Ilya Gladyshev wrote: > September 15, 2026 at 5:42 AM, "Zi Yan" wrote: > > >> >> On Sat Sep 12, 2026 at 3:50 PM EDT, Ilya Gladyshev wrote: >> >>> >>> The current page refcount implementation uses a single counter value >>> (zero) as dead. So, to prevent incrementing a dead refcount in >>> folio_try_get(), it fundamentally requires a CAS loop. >>> >>> This CAS loop can act as a serialization point and can become a >>> significant bottleneck during high-frequency file read operations >>> [1][2]. >>> >>> This patch reallocates the refcount value range: >>> >>> (1) refcount < 0 means dead refcount (uninit / frozen) >>> (2) refcount = 0 allowed only as a temporary state (see below) >>> (3) refcount > 0 is a regular reference count >>> >>> In other words, refcount is now split into "dead bit" and a 31-bit >>> counter. >>> >>> Refcount decrement now works as follows: >>> 1. Counter decrement >>> 2. If it is now zero, try to put it deep inside the dead zone >>> (CAS to INT_MIN). Or you can view it as "set up frozen bit and reset >>> counter". >>> 3. This CAS can fail only if someone grabbed a reference in-between -- >>> that's okay, this page is their problem now. >>> >> I am not sure this works. David raised a concern on V4[1], where if a >> folio lands to page_frag_free()'s free_frozen_pages(), it will not be >> properly handled. "this page is their problem" only works if every >> folio/page free functions call a generic folio/page free function that >> checks the page type and does the proper handling. > >>> Refcount decrement now works as follows: >>> 1. Counter decrement >>> 2. If it is now zero, try to put it deep inside the dead zone >>> (CAS to INT_MIN). Or you can view it as "set up frozen bit and reset >>> counter". >>> 3. This CAS can fail only if someone grabbed a reference in-between -- >>> that's okay, this page is their problem now. >> >> I am not sure this works. David raised a concern on V4[1], where if a >> folio lands to page_frag_free()'s free_frozen_pages(), it will not be >> properly handled. "this page is their problem" only works if every >> folio/page free functions call a generic folio/page free function that >> checks the page type and does the proper handling. > > After some thinking, this scenario is indeed possible and problematic. The > complexity of the required setup give me hope that there are some implicit > barriers, however I wasn't able to find them. But I am not an mm guru, > so maybe I missed something :shrug: > > -- Below is a minimal buggy scenario -- > > Thread A: network code that calls page_frag_free() > Thread B: calls folio_try_get(). Maybe it is the DAMON paddr scanner, > maybe this folio was previously used in page cache > (however this noticeably complicates the scenario) > Thread C: user of folio's reincarnation. It also requires destruction via > folio_put() and not via free_frozen_pages() -- for example hugetlb > or memcg charged folio. > > Refcount operations as follows (FR = FROZEN): > > A: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: INC 0 -> 1 [ folio_try_get() ] => success > B: DEC 1 -> 0 [ page_ref_dec_and_test() ] > B: CAS 0 -> FR [ page_ref_dec_and_test() ] > B: * deallocates via __folio_put() * > > C: * re-allocates page * > C: DEC 1 -> 0 [ page_ref_dec_and_test() ] > A: CAS 0 -> FR [ page_ref_dec_and_test() ] => success > A: * deallocates directly via free_frozen_pages() * => BUG > > This can be fixed with a `s/page_frag_free/folio_put/`-style patch, however > it seems unreasonable... There needs to be a common page freeing function that dipatches stuff. folio_put() is not the best future-proof thing, because not all pages will be folios sooninsh. -- Cheers, David