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 55708C636C8 for ; Fri, 16 Jul 2021 01:53:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 225C7613DA for ; Fri, 16 Jul 2021 01:53:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232953AbhGPB4u (ORCPT ); Thu, 15 Jul 2021 21:56:50 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:6940 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232276AbhGPB4s (ORCPT ); Thu, 15 Jul 2021 21:56:48 -0400 Received: from dggeme703-chm.china.huawei.com (unknown [172.30.72.56]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4GQvMG1HdLz7vBy; Fri, 16 Jul 2021 09:50:18 +0800 (CST) Received: from [10.174.178.125] (10.174.178.125) by dggeme703-chm.china.huawei.com (10.1.199.99) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2176.2; Fri, 16 Jul 2021 09:53:52 +0800 Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to anonymous LRU list To: John Hubbard CC: Michal Hocko , Yu Zhao , , , , , , , , , , , , , , Matthew Wilcox References: <20210710100329.49174-1-linmiaohe@huawei.com> <20210710100329.49174-2-linmiaohe@huawei.com> <9409189e-44f7-2608-68af-851629b6d453@huawei.com> <0634e9d6-9fcc-e65f-dc5e-bed13004b8fe@huawei.com> From: Miaohe Lin Message-ID: <1e1ce97a-2a0c-d0a4-85ad-cf70a85d9146@huawei.com> Date: Fri, 16 Jul 2021 09:53:51 +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: 8bit X-Originating-IP: [10.174.178.125] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To dggeme703-chm.china.huawei.com (10.1.199.99) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/7/16 8:01, John Hubbard wrote: > On 7/15/21 4:30 AM, Miaohe Lin wrote: > ... >>>>> So, it seems lockless GUP can redirty the MADV_FREE page. But is it ok to just release >>>>> a redirtied MADV_FREE pages? Because we hold the last reference here and the page will >>>>> be freed anyway... >>>> >>>> I don't see how lockless GUP can redirty the page.  It can grab the >>>> refcount, thus making the refcount here two.  Then the call to freeze >>>> here fails and the page stays on the list.  But the lockless GUP checks >>>> the page is still in the page table (and discovers it isn't, so releases >>>> the reference count).  Am I missing a path that lets lockless GUP dirty >>>> the page? >>>> >>> >>> If a device driver pins some pages using gup, and the device then uses dma >>> to write to those pages, then you could get there. That story is part of the >>> reasoning that led to creating pin_user_pages(), which btw does not yet >>> fully solve that case. >> >> Many thanks for your explanation. >> So the similar scenario that is clarified in the __remove_mapping() is possible: > > I probably should have added that the scenario I was describing is broken even > before any patches that you might apply here. I was just trying to ensure that > the complete list of scenarios was known. > Many thanks for doing this! :) > > > thanks,