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 X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3150FC43333 for ; Wed, 27 Jan 2021 00:20:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 02FF9206A1 for ; Wed, 27 Jan 2021 00:20:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2403992AbhA0ARu (ORCPT ); Tue, 26 Jan 2021 19:17:50 -0500 Received: from mga18.intel.com ([134.134.136.126]:20783 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389898AbhAZRHf (ORCPT ); Tue, 26 Jan 2021 12:07:35 -0500 IronPort-SDR: 9wuXSXAJ/fTnUCYvNVky7DM26HdRJlCUxFyLAx1+wFQwT0Ner8vwQY5NSzxsCcgR6uhcBZabko BSkpd4Ytrd7g== X-IronPort-AV: E=McAfee;i="6000,8403,9876"; a="167603534" X-IronPort-AV: E=Sophos;i="5.79,375,1602572400"; d="scan'208";a="167603534" Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Jan 2021 08:43:54 -0800 IronPort-SDR: eGwpQ92J/sX81sqArYH6+QIHP+J0S4EMZTaaYIKsSJxcLe0QKMCKSSF3yj0bW41zDIEqCl3g42 x8Jj9aQHSdrg== X-IronPort-AV: E=Sophos;i="5.79,375,1602572400"; d="scan'208";a="504585840" Received: from yyu32-mobl1.amr.corp.intel.com (HELO [10.212.153.84]) ([10.212.153.84]) by orsmga004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Jan 2021 08:43:51 -0800 Subject: Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW To: Peter Zijlstra , Borislav Petkov Cc: x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , Weijiang Yang , Pengfei Xu References: <20201229213053.16395-1-yu-cheng.yu@intel.com> <20201229213053.16395-12-yu-cheng.yu@intel.com> <20210125182709.GC23290@zn.tnic> From: "Yu, Yu-cheng" Message-ID: <2f69ab09-57e8-6da0-07ab-5b885758fc27@intel.com> Date: Tue, 26 Jan 2021 08:43:51 -0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/26/2021 1:40 AM, Peter Zijlstra wrote: > On Tue, Jan 26, 2021 at 09:46:36AM +0100, Peter Zijlstra wrote: >> On Mon, Jan 25, 2021 at 07:27:09PM +0100, Borislav Petkov wrote: >> >>>> + pte_t old_pte, new_pte; >>>> + >>>> + do { >>>> + old_pte = READ_ONCE(*ptep); >>>> + new_pte = pte_wrprotect(old_pte); >>> >>> Maybe I'm missing something but those two can happen outside of the >>> loop, no? Or is *ptep somehow changing concurrently while the loop is >>> doing the CMPXCHG and you need to recreate it each time? >>> >>> IOW, you can generate upfront and do the empty loop... >>> >>>> + >>>> + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); >>>> + >>>> + return; >>>> + } >> >> Empty loop would be wrong, but that wants to be written like: >> >> old_pte = READ_ONCE(*ptep); >> do { >> new_pte = pte_wrprotect(old_pte); >> } while (try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte)); > > ! went missing, too early, moar wake-up juice. > >> Since try_cmpxchg() will update old_pte on failure. Thanks Peter! I will fix that. -- Yu-cheng