Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752927AbdGEUtb (ORCPT ); Wed, 5 Jul 2017 16:49:31 -0400 Received: from fieldses.org ([173.255.197.46]:48656 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752030AbdGEUt3 (ORCPT ); Wed, 5 Jul 2017 16:49:29 -0400 Date: Wed, 5 Jul 2017 16:49:28 -0400 From: "J. Bruce Fields" To: "Darrick J. Wong" Cc: William Koh , Andreas Dilger , "Theodore Ts'o" , linux-ext4 , lkml , Kernel Team , linux-fsdevel , Trond Myklebust , xfs , neilb@suse.com Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0. Message-ID: <20170705204928.GA8151@fieldses.org> References: <20A40B3C-E179-432B-B56F-BDAAF0CD2E1F@dilger.ca> <7CD38230-D961-428F-B2E9-2C0E28CAF442@fb.com> <20170629045940.GB5865@birch.djwong.org> <20170629143551.GB1651@fieldses.org> <20170629172528.GA5869@birch.djwong.org> <20170629183053.GA4178@fieldses.org> <20170629185022.GB4178@fieldses.org> <20170704040446.GB4704@birch.djwong.org> <20170705011534.GC1420@fieldses.org> <20170705191933.GA6297@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170705191933.GA6297@magnolia> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3752 Lines: 82 On Wed, Jul 05, 2017 at 12:19:33PM -0700, Darrick J. Wong wrote: > On Tue, Jul 04, 2017 at 09:15:34PM -0400, J. Bruce Fields wrote: > > On Mon, Jul 03, 2017 at 09:04:46PM -0700, Darrick J. Wong wrote: > > > On Thu, Jun 29, 2017 at 02:50:22PM -0400, J. Bruce Fields wrote: > > > > On Thu, Jun 29, 2017 at 02:30:53PM -0400, J. Bruce Fields wrote: > > > > > On Thu, Jun 29, 2017 at 10:25:28AM -0700, Darrick J. Wong wrote: > > > > > > Was there ever a version of NFS (or more generally callers of the > > > > > > exportfs code) that couldn't deal with i_generation in the file handle, > > > > > > and therefore we invented this generation hack to work around the loss > > > > > > of the generation information? > > > > > > > > > > > > There's a comment in xfs_fs_encode_fh about not supporting 64bit inodes > > > > > > with subtree_check (which seems to require one ino/gen pair for the file > > > > > > and a second pair for the file's parent) on NFSv2 because v2 doesn't > > > > > > provide enough space for all the file handle information, but that's the > > > > > > furthest I got with lazy-mining the git history. :) > > > > > > > > > > There's a comment in fs/ext4/super.c:ext4_nfs_get_inode > > > > > > > > > > * Currently we don't know the generation for parent directory, so > > > > > * a generation of 0 means "accept any" > > > > > > > > > > But I don't see that used. > > > > > > > > > > It was used once upon a time; I see it actually used in old 2.5 code in > > > > > nfsd_get_dentry. Hm. > > > > > > > > Oh, maybe it's here in fs/libfs.c:generic_fh_to_parent: > > > > > > > > switch (fh_type) { > > > > case FILEID_INO32_GEN_PARENT: > > > > inode = get_inode(sb, fid->i32.parent_ino, > > > > (fh_len > 3 ? fid->i32.parent_gen : 0)); > > > > break; > > > > } > > > > > > > > I'm not sure under what conditions that filehandle encoding is used. > > > > > > The best guess I can come up with is the old nfs_fhbase_old style handles, > > > which (afaict) do not carry parent i_generation? > > > > Yeah, I just couldn't tell in the time I looked whether they could still > > be handed out. > > > > If not, then the only way they'd still be used is if a client had a > > server continually mounted while the server was upgraded from a kernel > > that still handed out the old filehandle. > > > > So if they haven't been given out for long enough it's possible nobody > > would notice if we dropped support. > > > > But, I didn't get far enough to figure that out. > > Hmm, so looking back through prehistory, Linux prior to 2.3.51 (11 March > 2000) gave out the old dentry style fhandles. After that, the kernel > only gave out the new style handles that we still use today. In 2.4.6 > (4 July 2001) the behavior was modified again to chain handle types, > i.e. if the client passed in an old style handle then it would get > another old style handle back. The changelog for -pre9 says that this > was done for compatibility reasons. Yeah, you're supposed to be able to reboot your NFS server for a kernel upgrade without your client applications experiencing anything worse than a temporary hang while you wait for the server to come back up. So, changing the filehandle format and returning ESTALE to everyone would be unpopular. > So, what's the probability that there are clients out there that started > talking to a 2.2-based knfsd and will now want to talk to a modern 4.13 > kernel seventeen years later? I think it's unlikely enough that we could drop that code; cc'ing Neil in case we overlooked anything. > (Do nfs handles persist across client restarts/remounts?) No. (Well, with maybe a couple exceptions (fscache and persistent NFSv4 delegations) but neither seem relevant here.) --b.