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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2FD06C433EF for ; Sat, 25 Jun 2022 03:17:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232149AbiFYDRc (ORCPT ); Fri, 24 Jun 2022 23:17:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57246 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232029AbiFYDRa (ORCPT ); Fri, 24 Jun 2022 23:17:30 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 618BC49B52 for ; Fri, 24 Jun 2022 20:17:29 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4LVJz21KbLzkWVj; Sat, 25 Jun 2022 11:15:42 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Sat, 25 Jun 2022 11:17:26 +0800 Subject: Re: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE in remove_migration_pmd To: Zach O'Keefe CC: "Kirill A. Shutemov" , , , , , References: <20220622170627.19786-1-linmiaohe@huawei.com> <20220622170627.19786-3-linmiaohe@huawei.com> <20220623031401.wdyt5ylin4aijzhh@box.shutemov.name> <7ee4a597-d7cb-f387-5613-f51b17262745@huawei.com> From: Miaohe Lin Message-ID: Date: Sat, 25 Jun 2022 11:17:26 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/6/25 2:40, Zach O'Keefe wrote: > On 23 Jun 20:03, Miaohe Lin wrote: >> On 2022/6/23 11:14, Kirill A. Shutemov wrote: >>> On Thu, Jun 23, 2022 at 01:06:13AM +0800, Miaohe Lin wrote: >>>> vma->vm_page_prot is read lockless from the rmap_walk, it may be updated >>>> concurrently. Using READ_ONCE to prevent the risk of reading intermediate >>>> values. >>> >>> Have you checked all other vm_page_prot reads that they hold mmap_lock? >> >> I took a glance when I made this patch. >> >>> >>> I think the right fix would be to provide a helper to read vm_page_prot >>> which does READ_ONCE() and use it everywhere. This seems more sustainable. >>> >> >> This patch is inspired from the below commit >> 6d2329f8872f ("mm: vm_page_prot: update with WRITE_ONCE/READ_ONCE") >> >> It changed all the places that need to use READ_ONCE. But remove_migration_pmd >> is missed due to it's introduced later. It looks fine to add a helper to read >> vm_page_prot which does READ_ONCE() but READ_ONCE is unneeded while under the >> mmap_lock, so might it be a little overkill to add a helper because the helper >> is used iff mmap_lock is not held? >> >> Thanks. > > IMO adding the READ_ONCE() as proposed in fine. Adding a helper to be called > dependent on locking context still requires the caller / dev to know what the > locking context is - so I don't think it provides much benefit. I tend to agree with Zach. Thanks! > . >