Re: [PATCH] fs: ext4: inode->i_generation not assigned 0.

["J. Bruce Fields" <bfields@fieldses.org>]

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.