From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 65A8036195A for ; Fri, 16 Oct 2026 15:14:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792163679; cv=none; b=nXWe//HlH2lbsUJvV6ZYwIgt8OcWcEnMw38uEpNNH0LlImSbTW4ZKlB35+ziB/x9NDRhdDKa8hQXuoXe2hzb23dVizWV7AfYu6iw8Dpgyv7D76+xTqKrZSL1jfiqXaRUaO3CEcGfceenrvCg5ZuDQsX58+5Cj6HzK225yIC5CgI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792163679; c=relaxed/simple; bh=92uiVq59fzFjGpCucqYXpQiP1PRGnn4Y2IaAxSLoQFY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=td4u5CVbFjJuL2MiYS+J3ALWppEZ0+6wxda2DNZ4yj0VNm3i7VumMbjoYJwwheh+LggFPAOHiN4fC8IIU/TBv2927YepbN7zS41Lq8HJJ57v8vXMD8AVDtBuq02tiBCFRa6bDRT86vqJy645MUT1BHVHOUW15pLOpyO388KElds= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LE03XN4r; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LE03XN4r" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 895321F000C4; Fri, 16 Oct 2026 15:14:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792163678; bh=5v3J6WKP+bHQTvLWIIelf/Yrhn+GxWTc6CC2vGMgLRc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LE03XN4rEVcp7wnxG0PrnjlUqFydFo3JZdPbDyihq74fSLSK3hhifkR7kEAr/RDRy +dYgIf3llk1dFjn3QPsishW8AlpPi1OmuYsFxR4G2oP5tRPEwbUWCMDtJEFiNXPklb r+eSQf4ZGIFgRo3CJ5L84/TkoOvxH6/9da83AFC6+KPulonX9kkPgH2s+lR/o6HpQH AznufVTctKJgt0XqtEut1qTg3/Y+v6VkeLJg5anI/8bnmgqAHUWHUdV4iZzWvQgqgq XYEgJq1PQ+2OZxVeJDQJ7u6T/7Td2y3tNKMhml/K7NQUYyzHBw1Z152IzHe3fKOOhi 2OUZps1SUKd2w== Message-ID: Date: Fri, 16 Oct 2026 17:14:32 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Improve page refcount scalability To: Kiryl Shutsemau , Linus Torvalds Cc: Ilya Gladyshev , Matthew Wilcox , Lorenzo Stoakes , Luis Chamberlain , Linux Kernel Mailing List , Linux-MM , "Paul E. McKenney" 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 Content-Transfer-Encoding: 7bit On 10/16/26 16:40, Kiryl Shutsemau wrote: > On Tue, Oct 13, 2026 at 11:43:14AM -0700, Linus Torvalds wrote: >> Comments? > > There's a regression: the lockless read loses on large reads from large > folios. > > In a 64-vCPU guest on a Threadripper 7975WX, 1 MiB reads of a cached > ext4 file are 9% slower from one thread with rcu-free-v3 than without > it, 64K reads 4% slower. > > The reference path looks a large folio up once and copies it in one go. > The lockless one walks i_pages from the root and calls > copy_page_to_iter() for every 4K page. > > I've added a cutoff to the lockless path: if the folio is large and the > read covers more than four of its pages, leave it to the slow path, > where one reference covers all of them. > > if (folio != prev) { > if (!filemap_read_rcu_can_copy(folio, index, *last_index)) > break; > /* Many pages of a large folio: one reference is cheaper */ > if (folio_test_large(folio) && > min_t(unsigned long, folio_next_index(folio) - index, > DIV_ROUND_UP(offset + iov_iter_count(iter), > PAGE_SIZE)) > 4) > break; > prev = folio; > } > > With it, ext4 1 MiB reads match base and 64K reads are within 2%. Small > reads do not change. The cut is on the folio, not on the read size, so > files in 4K folios keep the gain on large reads: 1 MiB tmpfs reads from > one thread are ~30% faster than base. > > Sounds reasonable? > > I've pushed my tree here: > > https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git rcu-free/rfc-v1 > > I'm thinking of posting it as an RFC to make it easier to review. > Objections? Yes please. I spent 6 hours today fixing all sorts of bugs after wanting to get something rather trivial fixed. I need the patch in my inbox to not get distracted by all the other stuff :) -- Cheers, David