linux-ext4 - Re: [PATCH] fs: ext4: inode->i

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Message-ID: <20170707164700.GA18740@fieldses.org>
Date:   Fri, 7 Jul 2017 12:47:00 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        William Koh <kkc6196@...com>,
        Andreas Dilger <adilger@...ger.ca>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0.

On Fri, Jul 07, 2017 at 12:13:36PM -0400, Jeff Layton wrote:
> On Fri, 2017-07-07 at 11:51 -0400, Theodore Ts'o wrote:
> > 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?

RHEL 7 is current, I think you mean the 17-year-old Red Hat Linux 7.
Anyone that far back is on their own as far as any enterprise distro is
concerned.

There are some exceptions to the "lifetime of a mount" rule, none real
issues, I think:

	- fscache may keep fh's around across client boots, but I
	  suspect you just lose the benefit of the cache until it
	  expires data keyed under old filehandles and repopulates the
	  cache with new ones.

	- Does the client actually depend on stable filehandles across
	  client reboots if it might cache write data under a persistent
	  delegation?  But seeing as we don't even implement persistent
	  delegations, this is a non-issue.

	- nontraditional NFS clients could do any random thing.  NFS is
	  just a protocol, we have no idea how some weird application
	  that talks NFS directly to the server might use filehandles.
	  But this is purely hypothetical, I don't know of one.

--b.