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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 37765C43600 for ; Wed, 19 May 2021 10:57:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27000613AD for ; Wed, 19 May 2021 10:57:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240661AbhESK6h (ORCPT ); Wed, 19 May 2021 06:58:37 -0400 Received: from mx2.suse.de ([195.135.220.15]:60810 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229720AbhESK6g (ORCPT ); Wed, 19 May 2021 06:58:36 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B6396AC4B; Wed, 19 May 2021 10:57:15 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 086AA1F2C9C; Wed, 19 May 2021 12:57:14 +0200 (CEST) Date: Wed, 19 May 2021 12:57:14 +0200 From: Jan Kara To: Dave Chinner Cc: "Darrick J. Wong" , Jan Kara , linux-fsdevel@vger.kernel.org, Christoph Hellwig , ceph-devel@vger.kernel.org, Chao Yu , Damien Le Moal , "Darrick J. Wong" , Jaegeuk Kim , Jeff Layton , Johannes Thumshirn , linux-cifs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-mm@kvack.org, linux-xfs@vger.kernel.org, Miklos Szeredi , Steve French , Ted Tso , Matthew Wilcox Subject: Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock Message-ID: <20210519105713.GA26250@quack2.suse.cz> References: <20210512101639.22278-1-jack@suse.cz> <20210512134631.4053-3-jack@suse.cz> <20210512152345.GE8606@magnolia> <20210513174459.GH2734@quack2.suse.cz> <20210513185252.GB9675@magnolia> <20210513231945.GD2893@dread.disaster.area> <20210514161730.GL9675@magnolia> <20210518223637.GJ2893@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210518223637.GJ2893@dread.disaster.area> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-cifs@vger.kernel.org On Wed 19-05-21 08:36:37, Dave Chinner wrote: > On Fri, May 14, 2021 at 09:17:30AM -0700, Darrick J. Wong wrote: > > On Fri, May 14, 2021 at 09:19:45AM +1000, Dave Chinner wrote: > > > On Thu, May 13, 2021 at 11:52:52AM -0700, Darrick J. Wong wrote: > > > > On Thu, May 13, 2021 at 07:44:59PM +0200, Jan Kara wrote: > > > > > On Wed 12-05-21 08:23:45, Darrick J. Wong wrote: > > > > > > On Wed, May 12, 2021 at 03:46:11PM +0200, Jan Kara wrote: > > > > > > > +->fallocate implementation must be really careful to maintain page cache > > > > > > > +consistency when punching holes or performing other operations that invalidate > > > > > > > +page cache contents. Usually the filesystem needs to call > > > > > > > +truncate_inode_pages_range() to invalidate relevant range of the page cache. > > > > > > > +However the filesystem usually also needs to update its internal (and on disk) > > > > > > > +view of file offset -> disk block mapping. Until this update is finished, the > > > > > > > +filesystem needs to block page faults and reads from reloading now-stale page > > > > > > > +cache contents from the disk. VFS provides mapping->invalidate_lock for this > > > > > > > +and acquires it in shared mode in paths loading pages from disk > > > > > > > +(filemap_fault(), filemap_read(), readahead paths). The filesystem is > > > > > > > +responsible for taking this lock in its fallocate implementation and generally > > > > > > > +whenever the page cache contents needs to be invalidated because a block is > > > > > > > +moving from under a page. > > > > > > > + > > > > > > > +->copy_file_range and ->remap_file_range implementations need to serialize > > > > > > > +against modifications of file data while the operation is running. For blocking > > > > > > > +changes through write(2) and similar operations inode->i_rwsem can be used. For > > > > > > > +blocking changes through memory mapping, the filesystem can use > > > > > > > +mapping->invalidate_lock provided it also acquires it in its ->page_mkwrite > > > > > > > +implementation. > > > > > > > > > > > > Question: What is the locking order when acquiring the invalidate_lock > > > > > > of two different files? Is it the same as i_rwsem (increasing order of > > > > > > the struct inode pointer) or is it the same as the XFS MMAPLOCK that is > > > > > > being hoisted here (increasing order of i_ino)? > > > > > > > > > > > > The reason I ask is that remap_file_range has to do that, but I don't > > > > > > see any conversions for the xfs_lock_two_inodes(..., MMAPLOCK_EXCL) > > > > > > calls in xfs_ilock2_io_mmap in this series. > > > > > > > > > > Good question. Technically, I don't think there's real need to establish a > > > > > single ordering because locks among different filesystems are never going > > > > > to be acquired together (effectively each lock type is local per sb and we > > > > > are free to define an ordering for each lock type differently). But to > > > > > maintain some sanity I guess having the same locking order for doublelock > > > > > of i_rwsem and invalidate_lock makes sense. Is there a reason why XFS uses > > > > > by-ino ordering? So that we don't have to consider two different orders in > > > > > xfs_lock_two_inodes()... > > > > > > > > I imagine Dave will chime in on this, but I suspect the reason is > > > > hysterical raisins^Wreasons. > > > > > > It's the locking rules that XFS has used pretty much forever. > > > Locking by inode number always guarantees the same locking order of > > > two inodes in the same filesystem, regardless of the specific > > > in-memory instances of the two inodes. > > > > > > e.g. if we lock based on the inode structure address, in one > > > instancex, we could get A -> B, then B gets recycled and > > > reallocated, then we get B -> A as the locking order for the same > > > two inodes. > > > > > > That, IMNSHO, is utterly crazy because with non-deterministic inode > > > lock ordered like this you can't make consistent locking rules for > > > locking the physical inode cluster buffers underlying the inodes in > > > the situation where they also need to be locked. > > > > That's protected by the ILOCK, correct? > > > > > We've been down this path before more than a decade ago when the > > > powers that be decreed that inode locking order is to be "by > > > structure address" rather than inode number, because "inode number > > > is not unique across multiple superblocks". > > > > > > I'm not sure that there is anywhere that locks multiple inodes > > > across different superblocks, but here we are again.... > > > > Hm. Are there situations where one would want to lock multiple > > /mappings/ across different superblocks? The remapping code doesn't > > allow cross-super operations, so ... pipes and splice, maybe? I don't > > remember that code well enough to say for sure. > > Hmmmm. Doing read IO into a buffer that is mmap()d from another > file, and we take a page fault on it inside the read IO path? We're > copying from a page in one mapping and taking a fault in another > mapping and hence taking the invalidate_lock to populate the page > cache for the second mapping... > > I haven't looked closely enough at where the invalidate_lock is held > in the read path to determine if this is an issue, but if it is then > it is also a potential deadlock scenario... I was careful enough to avoid this problem - we first bring pages into pages cache (under invalidate_lock), then drop invalidate lock, just keep page refs, and copy page cache content into the buffer (which may grab invalidate_lock from another mapping as you say). Honza -- Jan Kara SUSE Labs, CR