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 1F096EE2082 for ; Fri, 6 Feb 2026 10:24:00 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 86D386B0089; Fri, 6 Feb 2026 05:23:59 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 823326B0092; Fri, 6 Feb 2026 05:23:59 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 719356B0093; Fri, 6 Feb 2026 05:23:59 -0500 (EST) 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 5B42C6B0089 for ; Fri, 6 Feb 2026 05:23:59 -0500 (EST) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 0E0818C541 for ; Fri, 6 Feb 2026 10:23:59 +0000 (UTC) X-FDA: 84413646198.28.3213EA1 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf24.hostedemail.com (Postfix) with ESMTP id 2E41918000D for ; Fri, 6 Feb 2026 10:23:56 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=lkgsOVjM; spf=pass (imf24.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=1770373437; 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=+KNZN6z/krldVP2Y4BBko3jZMXeqf1Yu6t3X9QVX4lQ=; b=jEaX/PnQHs5l0OrA+45bI1b1jtUtSOta/wyYk11xx5KkoMg+guxunlFhsAleRcBMJCRXPD PoQ02rQXqs8xBDuw1EMdTuaSwvGWjkr6C8qq2biwrizLAFId0x8/k54ZFPtpGCZpzwSje+ 6woXrdJh4MZuTkG9/2xeixis5rMbJNc= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=lkgsOVjM; spf=pass (imf24.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-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770373437; a=rsa-sha256; cv=none; b=na4nhxv3vraBmkXg9hTK3Lc9aUv6ZUwFKiqE5An0lNUlnjN57NFEij4YVGG4jLI/WXfJkn Q2DJwaS+M07jhatOZwKhkyl1C4EP1QrTk2lCqmLuNVSuLCtUx/4YkgLELCE/JZDdIHqDx+ 9i0omRgA310SU6doqblyQpR0+BRqjUk= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 01C7444214; Fri, 6 Feb 2026 10:23:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EA86DC19422; Fri, 6 Feb 2026 10:23:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770373435; bh=CVRfjenhPNZ53C5ptXLpHu6G+l5qThn6T1w+S3R43J4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=lkgsOVjM8345QQjYJiRfQfJ0oSMWqIL/dF3b44KB/OYvtFa5wtksT4epKW7dc7zQK n13iIxkGzKpBzMaIzGDTNhk7k+VRqM3ICVJzDUOySg0Dee4z13wjFn7Cl2EN1ekcLW XWt4/jPFLIpPhf3JY1BeyzKTbyedA1v9JDaYVUAjI9tYMJtEuJsfrVQR+lDly5qhC1 zCwWghcKG4Qbwjg/A1hG6Xsrw28MryrTIl0yqvYqtw7HzK9njc9BhbXFrF7jt0cFdj aloqve3suzVNAbni2tS3Lc0IjWEudBVqDr37kT5BZXF3tUmnGa4ntBhXN5Von7/OEX EKyVfiMLATw8A== Message-ID: Date: Fri, 6 Feb 2026 11:23:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 15/17] mm: Remove the branch from compound_head() To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-16-kas@kernel.org> 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: <20260202155634.650837-16-kas@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 2E41918000D X-Stat-Signature: iwcgtzmtdydqcuucpaox9kykwh8psjtk X-Rspam-User: X-HE-Tag: 1770373436-694331 X-HE-Meta: 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 8pdkPDJ5 RLx8Ub9eixNbJG3zRdVSBIt9N/G+NeMmt0S6tcNEoptU8LIn70PJ9lBSh4B3bJXlvXJ/kiBhss9SRuCEVBQGAi5y4AvLlOyXtD18xeucE3F3+ngeDRykTJ+5hjNQbJLEFJh0xAIjSV88gs+hmg/gg+13CMBxSi9j1n5WnDAxbHOeY4/5TtEKRd/DdKoRFkZ41lQUxcxWQGsIS5lwCc5PgpIpUO+FDO/7d8lIknu4HBd4HTIxGcQSBIEeV+1PxmMUN6fM9WElrDtBT1E8V9OoTQFi21fsbjd6WJv1ayGGe1ugrtKaXoqlD7AIlQKHuc5nbxI4PioMfTK5hSShYFIkepEJbxAtx0qYnlZIl 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: On 2/2/26 16:56, Kiryl Shutsemau wrote: > The compound_head() function is a hot path. For example, the zap path > calls it for every leaf page table entry. > > Rewrite the helper function in a branchless manner to eliminate the risk > of CPU branch misprediction. > > Signed-off-by: Kiryl Shutsemau > Reviewed-by: Muchun Song > Reviewed-by: Zi Yan > --- > include/linux/page-flags.h | 27 +++++++++++++++++---------- > 1 file changed, 17 insertions(+), 10 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 1aaa604f4b9b..16384cb6f962 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void) > static __always_inline unsigned long _compound_head(const struct page *page) > { > unsigned long info = READ_ONCE(page->compound_info); > + unsigned long mask; > + > + if (!compound_info_has_mask()) { > + /* Bit 0 encodes PageTail() */ > + if (info & 1) > + return info - 1; > > - /* Bit 0 encodes PageTail() */ > - if (!(info & 1)) > return (unsigned long)page; > - > - /* > - * If compound_info_has_mask() is false, the rest of compound_info is > - * the pointer to the head page. > - */ > - if (!compound_info_has_mask()) > - return info - 1; > + } > > /* > * If compoun_info_has_mask() is true the rest of the info encodes > * the mask that converts the address of the tail page to the head page. > * > * No need to clear bit 0 in the mask as 'page' always has it clear. > + * > + * Let's do it in a branchless manner. > */ > - return (unsigned long)page & info; > + > + /* Non-tail: -1UL, Tail: 0 */ > + mask = (info & 1) - 1; > + > + /* Non-tail: -1UL, Tail: info */ > + mask |= info; > + > + return (unsigned long)page & mask; > } > > #define compound_head(page) ((typeof(page))_compound_head(page)) Nice! Acked-by: David Hildenbrand (Arm) -- Cheers, David