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 752B1C636C9 for ; Fri, 16 Jul 2021 01:54:12 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id CE7D9613DA for ; Fri, 16 Jul 2021 01:54:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CE7D9613DA Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=huawei.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id ED78A8D00F4; Thu, 15 Jul 2021 21:54:11 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E60BD8D00EC; Thu, 15 Jul 2021 21:54:11 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CD9D58D00F4; Thu, 15 Jul 2021 21:54:11 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0042.hostedemail.com [216.40.44.42]) by kanga.kvack.org (Postfix) with ESMTP id A19568D00EC for ; Thu, 15 Jul 2021 21:54:11 -0400 (EDT) Received: from smtpin21.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 7467F181CBC27 for ; Fri, 16 Jul 2021 01:54:10 +0000 (UTC) X-FDA: 78366780660.21.16713F2 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by imf04.hostedemail.com (Postfix) with ESMTP id A5AA350000A3 for ; Fri, 16 Jul 2021 01:54:09 +0000 (UTC) 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 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 Authentication-Results: imf04.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=huawei.com; spf=pass (imf04.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.188 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: A5AA350000A3 X-Stat-Signature: 7n3g88nbbwnczbdu7x3tqs8zraoaberg X-HE-Tag: 1626400449-956876 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000009, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 her= e and the page will >>>>> be freed anyway... >>>> >>>> I don't see how lockless GUP can redirty the page.=C2=A0 It can grab= the >>>> refcount, thus making the refcount here two.=C2=A0 Then the call to = freeze >>>> here fails and the page stays on the list.=C2=A0 But the lockless GU= P checks >>>> the page is still in the page table (and discovers it isn't, so rele= ases >>>> the reference count).=C2=A0 Am I missing a path that lets lockless G= UP dirty >>>> the page? >>>> >>> >>> If a device driver pins some pages using gup, and the device then use= s 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 y= et >>> fully solve that case. >> >> Many thanks for your explanation. >> So the similar scenario that is clarified in the __remove_mapping() is= possible: >=20 > I probably should have added that the scenario I was describing is brok= en even > before any patches that you might apply here. I was just trying to ensu= re that > the complete list of scenarios was known. >=20 Many thanks for doing this! :) >=20 >=20 > thanks,