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

From: "Theodore Ts'o" <tytso@mit.edu>
To: Jeff Layton <jlayton@redhat.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	William Koh <kkc6196@fb.com>, Andreas Dilger <adilger@dilger.ca>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Trond Myklebust <trond.myklebust@primarydata.com>,
	xfs <linux-xfs@vger.kernel.org>
Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0.
Date: Fri, 7 Jul 2017 11:51:17 -0400	[thread overview]
Message-ID: <20170707155117.4zp7qkpreeptwlgc@thunk.org> (raw)
In-Reply-To: <1499424697.5826.3.camel@redhat.com>

On Fri, Jul 07, 2017 at 06:51:37AM -0400, Jeff Layton wrote:
> 
> Right. That's the case today if we don't remove support for old
> filehandles. If we were to remove them, the clients would get back
> -ESTALE there if they tried to use the old 2.2-style fh's that they saw
> before the upgrade.
> 
> The main takeaway here is that NFS filehandle lifetime is really only
> bounded by the boot time of the oldest clients.

Well, and how long an NFS server is still up.  So one could construct
a use case where a (hypothetical) system administrator had a RHEL 7.0
system with a 2.2.16-22 kernel, and they try to update it to a
(hypothetical) RHEL 10 kernel in one fell swoop with a 4.13+ kernel
that no longer supports the 2-2-style fh's.  A client that had the
server mounted when it was running the 2.2 kernel might only be up for
a few hours, before the upgrade to RHEL 10 happened, and then the
client would get ESTALE errors.

Of course, I've stopped carrying about enterprise kernel support a
long time ago, so I just think that scenario is funny.  I recognize
that folks who work at Red Hat have to worry about such things --- and
I'm sorry.  :-)

In reality a server installed with RHEL 7.0 has probably died of old
age by now --- unless someone crazy is running it in a VMware VM
because they had some enterprise software package or some bar-code
printing module for which they don't have source code[1], and so they are
stuck on RHEL 7.0, even in 2017.  Have I mentioned I'm so glad I don't
have to worry these sorts of things any more?

					- Ted

[1] That wasn't a made up example; I once visited a customer on site,
back in the day, that had that exact problem, and so they were stuck
on some antique version of RHEL, and they expected me to help them.