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 124A4C433E0 for ; Mon, 25 Jan 2021 22:20:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C94CB2072C for ; Mon, 25 Jan 2021 22:20:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726468AbhAYWUb (ORCPT ); Mon, 25 Jan 2021 17:20:31 -0500 Received: from mga18.intel.com ([134.134.136.126]:45039 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732358AbhAYWTZ (ORCPT ); Mon, 25 Jan 2021 17:19:25 -0500 IronPort-SDR: iBtGRh4x4jC5jQqTwbFKNe/Si9NZfLZhB3Z5LYMK934cFoz8jumtgrXvTdOBxuFv93xCaZI1J0 aEL7jfW35ITQ== X-IronPort-AV: E=McAfee;i="6000,8403,9875"; a="167485735" X-IronPort-AV: E=Sophos;i="5.79,374,1602572400"; d="scan'208";a="167485735" Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Jan 2021 14:18:39 -0800 IronPort-SDR: DugOZsV+ToMjSGqjsXLvGijFi0PtI0XJE4I4LV/dYgGFrabp02LiqxEDb9/wwuIe3v5gJVNFfF HGVTIeRsiaLA== X-IronPort-AV: E=Sophos;i="5.79,374,1602572400"; d="scan'208";a="393535238" Received: from yyu32-mobl1.amr.corp.intel.com (HELO [10.212.60.232]) ([10.212.60.232]) by orsmga007-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Jan 2021 14:18:38 -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: 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 , Peter Zijlstra , 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> <8084836b-4990-90e8-5c9a-97a920f0239e@intel.com> <20210125215558.GK23070@zn.tnic> From: "Yu, Yu-cheng" Message-ID: Date: Mon, 25 Jan 2021 14:18:37 -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: <20210125215558.GK23070@zn.tnic> 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 1/25/2021 1:55 PM, Borislav Petkov wrote: > On Mon, Jan 25, 2021 at 01:27:51PM -0800, Yu, Yu-cheng wrote: >>> 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... >> >> *ptep can change concurrently. > > Care to elaborate? > For example, when a thread reads a W=1, D=0 PTE and before changing it to W=0,D=0, another thread could have written to the page and the PTE is W=1, D=1 now. When try_cmpxchg() detects the difference, old_pte is read again.