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 C631EF01832 for ; Fri, 6 Mar 2026 13:10:18 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id F40BF6B008C; Fri, 6 Mar 2026 08:10:17 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EEEBF6B0092; Fri, 6 Mar 2026 08:10:17 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DC3DC6B0093; Fri, 6 Mar 2026 08:10:17 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id C7E2C6B008C for ; Fri, 6 Mar 2026 08:10:17 -0500 (EST) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 6573D592C2 for ; Fri, 6 Mar 2026 13:10:17 +0000 (UTC) X-FDA: 84515671674.02.8B56643 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf17.hostedemail.com (Postfix) with ESMTP id A82D34000B for ; Fri, 6 Mar 2026 13:10:15 +0000 (UTC) Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=KPktBZVB; spf=pass (imf17.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=1772802615; 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=gp4QZ2wxY+gYbrvECxbnbJG/BDfCcjvw3OegdBalhRU=; b=08UlFQ2HtWj+zu/VhNBswWMIcyRRqZ5ngbQVzVPIAFVDiMRqY339i6SJCNhetRLZYuWJpU lDpGpzvg6+pqjOYOGF5U64Bid8fz5c17AZOI5RWZfvdmEc+XAUvWr6/KXKiU/UQiOE5GvK t51E0apoS+mO5QYxCOWJVQPjD96/e9Q= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1772802615; a=rsa-sha256; cv=none; b=FIXrnDqfT8mFHmPL+RID+8Jgye7duWgZyu3wIH3vx0S7znj85wQMXlZMnFu7JORW7ZfrUD ALpvp2FiBW/ExOpB4yyBPo28Peu3a4Uf9AWGNpkN2zh74xSpCI46Nsv62MufPeKIKC+CYs MHHH8gBqDXxloDHWdf+cM553t4DzTXM= ARC-Authentication-Results: i=1; imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=KPktBZVB; spf=pass (imf17.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 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 0B6F860127; Fri, 6 Mar 2026 13:10:15 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B553AC4CEF7; Fri, 6 Mar 2026 13:10:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772802614; bh=61lGynqWixTzQYwyPA6ZFruK9XjY6xGcpOGYFOEIcyc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=KPktBZVBidCYe4HR1PJvnvJ4qUzMyGkpu6Be2Y9L7neKHTFr5deLppeg5Usp5CcFn G1xhlgut9NR5gDZEHJmBit5FVSjzf3X76Nb7LnQyN35Ft0gouVBdVEsyB36llo5Zzi 9qk0B38nIVyK4fiAgXGOgD0NVao8jFSZubPSKnW1XSGviHuLx6FeBwUdiB6DRLffvy 8xyKvkDNPjO2rI8g22Xd/Z3T2zjrluHSr/0LBJmdtmGggj66MJ/X4kS5+cxJOhYwwC c2E/+ZIWIkisnuUYp/eA0pvLh2R4I5c7K4Y2GfGXzB8rDTizwS4QV+IijlK79bhVfp 40P+OQ6e7J5PQ== Message-ID: <54cc6e6d-bda7-4c5b-a998-df854ead1985@kernel.org> Date: Fri, 6 Mar 2026 14:10:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm: implement page refcount locking via dedicated bit To: Gladyshev Ilya Cc: Andrew Morton , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Harry Yoo , Matthew Wilcox , Yu Zhao , Baolin Wang , Alistair Popple , Gorbunov Ivan , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau , Linus Torvalds References: <6bf6eba6e2e6a74e2045a3bd08d58fd91bece7be.1772120327.git.gladyshev.ilya1@h-partners.com> <902d821b-e903-4bf5-89db-070851c95a1a@h-partners.com> 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: <902d821b-e903-4bf5-89db-070851c95a1a@h-partners.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: irbbjzmnqow9dngqkqkp157jg4ixq7cn X-Rspamd-Queue-Id: A82D34000B X-Rspamd-Server: rspam03 X-HE-Tag: 1772802615-265131 X-HE-Meta: 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 p7cRVNpX AmIpPorHhDVYzJCyUdOJY7vR/QfdzSzdsCmJ4VRjh5BP2A7HgCRwOjZLAEu3oMlJoo/b9+uI+gGmCxvoGTjI/gWDaw8wIv/mGwalxI2GU56TEHg2SC2k/puVDFO9cxEBHKDhCbfWcaGP26Pc9U9f8ZndfUGdb28MWpGMSqImSL+phuyMS1KcIuMwcEb84Nt6F576kICDduf0mt1yfYut/P2TdcbJVnSvDDAdJmuQqRrrEFbjsn/+Rl8mAr/SKGkWYmS/1xEzIZrd1MFZieRsAHBVEY3orXJYUQXT0 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> >>>       rcu_read_lock(); >>>       /* avoid writing to the vmemmap area being remapped */ >>> -    if (page_count_writable(page)) >>> -        ret = atomic_add_unless(&page->_refcount, nr, 0); >>> +    if (page_count_writable(page)) { >>> +        val = atomic_add_return(nr, &page->_refcount); >>> +        ret = !(val & PAGEREF_LOCKED_BIT); >>> + >>> +        /* Undo atomic_add() if counter is locked and scary big */ >>> +        while (unlikely((unsigned int)val >= _PAGEREF_LOCKED_LIMIT)) >>> +            val = atomic_cmpxchg_relaxed(&page->_refcount, val, > PAGEREF_LOCKED_BIT); >> >> I assume we can't do an atomic_dec(), because we might have concurrent >> unfreezing (or similar things) happening that overwrote whatever was in >> there. > > Not only that, but also you probably don't want to handle "atomic_dec() > returned 0" situations. Right, and then we'd have to free the page here instead, which is rather awkward. >> >> I was wondering is whether page_ref_freeze() could actually leave the >> references set, and only set the PAGEREF_LOCKED_BIT bit, whereby >> page_ref_unfreeze() would only clear the PAGEREF_LOCKED_BIT bit. >> >> Similarly, the set_page_refcounted() could add a reference and clear the >> PAGEREF_LOCKED_BIT. That'd be more expensive on the allocation path ... >> and not sure if that would really help to turn this >> atomic_cmpxchg_relaxed() into an simpler atomic_dec() my brain could >> more easily understand 🙂 > > I believe resetting whole counter via single CAS (CPU-wise) is cheaper > than atomic_dec() for each individual attempt. > Right, that's what I meant with "more expensive". [...] >> >> For example, all pages we add to the page allocator through >> __free_pages_core(). > > You are right that refcount = 0 is tricky. However, for a bad outcome > you will need: > 1. Some external reference to this page, through which you try to > increment the refcount; > 2. set_page_count(0) somewhere between freeing and "it is safe to alloc" > state. That is way, way, way too dodgy. What you should likely do is (a) Make set_page_count(0) set it to PAGEREF_LOCKED_BIT (b) Make any places that might skip set_page_count(0) to use it (c) Document this all extremely thoroughly. Alternatively, disallow set_page_count(0) ccompletely and add something like "initialize_page_as_frozen()" or sth. like that. A refcount of 0 must really only be temporarily allowed in the system when transitioning from 0 to PAGEREF_LOCKED_BIT. Otherwise this just screams for trouble. > > So adding new pages with zeroed refcount to allocator is safe, because > there are no external references. Zeroing tail page's refcount is safe, > unless someone actually tries to increment its refcount (and this is bug). > > Generally, the only unsafe set_page_count() (or any other zeroing) will > be in allocator itself between freeing and allocating. Or maybe I missed > something, and this approach is indeed incorrect > > Probably we can think of some debug checks to prevent bugs in "safe" > scenarious Just take a look at do_migrate_range(), where we do a page = pfn_to_page(pfn); folio = page_folio(page); if (!folio_try_get(folio)) continue; If that is a page that was added to the buddy (set_page_count(0)) but either (a) not allocated yet or (b) allocated as frozen that's a problem. We really rely on (1) Frozen pages (2) Buddy pages (3) Tail pages (compound or non-compoud) To have a refcount of 0 that *reliably* makes folio_try_get() etc. fail. Anything else is just playing with fire. -- Cheers, David