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=-1.2 required=3.0 tests=DATE_IN_PAST_03_06, 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 B3B01C33CAA for ; Tue, 21 Jan 2020 14:40:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8EE2B24125 for ; Tue, 21 Jan 2020 14:40:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729427AbgAUOkj (ORCPT ); Tue, 21 Jan 2020 09:40:39 -0500 Received: from mx2.suse.de ([195.135.220.15]:51572 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729096AbgAUOki (ORCPT ); Tue, 21 Jan 2020 09:40:38 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 03C37AE33; Tue, 21 Jan 2020 14:40:37 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 03AAE1E0A4A; Tue, 21 Jan 2020 12:36:27 +0100 (CET) Date: Tue, 21 Jan 2020 12:36:27 +0100 From: Jan Kara To: Matthew Wilcox Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Jeff Layton , Christoph Hellwig , Chris Mason Subject: Re: [RFC v2 0/9] Replacing the readpages a_op Message-ID: <20200121113627.GA1746@quack2.suse.cz> References: <20200115023843.31325-1-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200115023843.31325-1-willy@infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org Hello Matthew! On Tue 14-01-20 18:38:34, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" > > This is an attempt to add a ->readahead op to replace ->readpages. I've > converted two users, iomap/xfs and cifs. The cifs conversion is lacking > fscache support, and that's just because I didn't want to do that work; > I don't believe there's anything fundamental to it. But I wanted to do > iomap because it is The Infrastructure Of The Future and cifs because it > is the sole remaining user of add_to_page_cache_locked(), which enables > the last two patches in the series. By the way, that gives CIFS access > to the workingset shadow infrastructure, which it had to ignore before > because it couldn't put pages onto the lru list at the right time. > > v2: Chris asked me to show what this would look like if we just have > the implementation look up the pages in the page cache, and I managed > to figure out some things I'd done wrong last time. It's even simpler > than v1 (net 104 lines deleted). I have an unfinished patch series laying around that pulls the ->readpage / ->readpages API in somewhat different direction so I'd like to discuss whether it's possible to solve my problem using your API. The problem I have is that currently some operations such as hole punching can race with ->readpage / ->readpages like: CPU0 CPU1 fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) filemap_write_and_wait_range() down_write(inode->i_rwsem); truncate_pagecache_range(); readahead(fd, off, len) creates pages in page cache looks up block mapping removes blocks from inode and frees them issues bio - reads stale data - potential security issue Now how I wanted to address this is that I'd change the API convention for ->readpage() so that we call it with the page unlocked and the function would lock the page, check it's still OK, and do what it needs. And this will allow ->readpage() and also ->readpages() to grab lock (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching while we are adding pages to page cache and mapping underlying blocks. Now your API makes even ->readpages() (actually ->readahead) called with pages locked so that makes this approach problematic because of lock inversions. So I'd prefer if we could keep the situation that ->readpages / ->readahead gets called without any pages in page cache locked... Honza -- Jan Kara SUSE Labs, CR