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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 7E285C433FF for ; Wed, 14 Aug 2019 06:46:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5EE70206C2 for ; Wed, 14 Aug 2019 06:46:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727038AbfHNGqP (ORCPT ); Wed, 14 Aug 2019 02:46:15 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:3938 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726373AbfHNGqO (ORCPT ); Wed, 14 Aug 2019 02:46:14 -0400 Received: from DGGEMM401-HUB.china.huawei.com (unknown [172.30.72.54]) by Forcepoint Email with ESMTP id DAF2632E9AED86E941B1; Wed, 14 Aug 2019 14:46:12 +0800 (CST) Received: from dggeme762-chm.china.huawei.com (10.3.19.108) by DGGEMM401-HUB.china.huawei.com (10.3.20.209) with Microsoft SMTP Server (TLS) id 14.3.439.0; Wed, 14 Aug 2019 14:46:12 +0800 Received: from 138 (10.175.124.28) by dggeme762-chm.china.huawei.com (10.3.19.108) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1591.10; Wed, 14 Aug 2019 14:46:12 +0800 Date: Wed, 14 Aug 2019 15:03:21 +0800 From: Gao Xiang To: Matthew Wilcox CC: "Darrick J. Wong" , Dave Chinner , xfs , linux-fsdevel Subject: Re: [PATCH v3] vfs: fix page locking deadlocks when deduping files Message-ID: <20190814070321.GB28602@138> References: <20190813151434.GQ7138@magnolia> <20190813154010.GD5307@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20190813154010.GD5307@bombadil.infradead.org> User-Agent: Mutt/1.11.3 (2019-02-01) X-Originating-IP: [10.175.124.28] X-ClientProxiedBy: dggeme703-chm.china.huawei.com (10.1.199.99) To dggeme762-chm.china.huawei.com (10.3.19.108) X-CFilter-Loop: Reflected Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Aug 13, 2019 at 08:40:10AM -0700, Matthew Wilcox wrote: > On Tue, Aug 13, 2019 at 08:14:34AM -0700, Darrick J. Wong wrote: > > + /* > > + * Now that we've locked both pages, make sure they still > > + * represent the data we're interested in. If not, someone > > + * is invalidating pages on us and we lose. > > + */ > > + if (src_page->mapping != src->i_mapping || > > + src_page->index != srcoff >> PAGE_SHIFT || > > + dest_page->mapping != dest->i_mapping || > > + dest_page->index != destoff >> PAGE_SHIFT) { > > + same = false; > > + goto unlock; > > + } > > It is my understanding that you don't need to check the ->index here. > If I'm wrong about that, I'd really appreciate being corrected, because > the page cache locking is subtle. > > You call read_mapping_page() which returns the page with an elevated > refcount. That means the page can't go back to the page allocator and > be allocated again. It can, because it's unlocked, still be truncated, > so the check for ->mapping after locking it is needed. But the check > for ->index being correct was done by find_get_entry(). > > See pagecache_get_page() -- if we specify FGP_LOCK, then it will lock > the page, check the ->mapping but not check ->index. OK, it does check > ->index, but in a VM_BUG_ON(), so it's not something that ought to be > able to be wrong. That is my understanding as well. In details... The page data get ready after read_mapping_page() is successfully returned. However, if someone needs to get a stable untruncated page, lock_page() and recheck page->mapping are needed as well. I have no idea how page->index can be changed safely without reallocating the page, even some paths could keep using some truncated page temporarily with some refcounts held but I think those paths cannot add these pages directly to some page cache again without freeing since it seems really unsafe..... Thanks, Gao Xiang >