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From:   Rick Macklem <rick.macklem@gmail.com>
Date:   Tue, 14 Feb 2023 14:57:24 -0800
Message-ID: <CAM5tNy71pXhzf4wA+KuhF6kCOYspzpfNa8HR=d1XM=G_gng9dQ@mail.gmail.com>
Subject: Re: [PATCH 3/3] nfsd: simplify write verifier handling
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Trond Myklebust <trondmy@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>, willy@infradead.org,
        linux-nfs@vger.kernel.org
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On Tue, Feb 14, 2023 at 5:53 AM Jeff Layton <jlayton@kernel.org> wrote:
>
> On Mon, 2023-02-13 at 22:28 -0500, Trond Myklebust wrote:
> > On Mon, 2023-02-13 at 16:49 -0800, Rick Macklem wrote:
> > > On Mon, Feb 13, 2023 at 1:14 PM Jeff Layton <jlayton@kernel.org>
> > > wrote:
> > > >
> > > > CAUTION: This email originated from outside of the University of
> > > > Guelph. Do not click links or open attachments unless you recognize
> > > > the sender and know the content is safe. If in doubt, forward
> > > > suspicious emails to IThelp@uoguelph.ca
> > > >
> > > >
> > > > The write verifier exists to tell the client when the server may
> > > > have
> > > > forgotten some unstable writes. The typical way that this happens
> > > > is if
> > > > the server crashes, but we've also extended nfsd to change it when
> > > > there
> > > > are writeback errors as well.
> > > >
> > > > The way it works today though, we call something like vfs_fsync
> > > > (e.g.
> > > > for a COMMIT call) and if we get back an error, we'll reset the
> > > > write
> > > > verifier.
> > > >
> > > > This is non-optimal for a couple of reasons:
> > > >
> > > > 1/ There could be significant delay between an error being
> > > > recorded and the reset. It would be ideal if the write verifier
> > > > were to
> > > > change as soon as the error was recorded.
> > > >
> > > > 2/ It's a bit of a waste, in that if we get a writeback error on a
> > > > single inode, we'll end up resetting the write verifier for
> > > > everything,
> > > > even on inodes that may be fine (e.g. on a completely separate fs).
> > > >
> > > Here's the snippet from RFC8881:
> > >    The final portion of the result is the field writeverf.  This
> > > field
> > >    is the write verifier and is a cookie that the client can use to
> > >    determine whether a server has changed instance state (e.g.,
> > > server
> > >    restart) between a call to WRITE and a subsequent call to either
> > >    WRITE or COMMIT.  This cookie MUST be unchanged during a single
> > >    instance of the NFSv4.1 server and MUST be unique between
> > > instances
> > >    of the NFSv4.1 server.  If the cookie changes, then the client
> > > MUST
> > >    assume that any data written with an UNSTABLE4 value for committed
> > >    and an old writeverf in the reply has been lost and will need to
> > > be
> > >    recovered.
> > >
> > > I've always interpreted the writeverf as "per-server" and not  "per-
> > > file".
> > > Although I'll admit the above does not make that crystal clear, it
> > > does make
> > > it clear that the writeverf applies to a "server instance" and not a
> > > file or
> > > file system on the server.
> > >
> > > The FreeBSD client assumes it is "per-server" and re-writes all
> > > uncommitted
> > > writes for the server, not just ones for the file (or file system)
> > > the
> > > writeverf is
> > > replied with.  (I vaguely recall Solaris does the same?)
> > >
> > > At the very least, I think you should run this past the IETF working
> > > group
> > > (nfsv4@ietf.org) to see what they say w.r.t. the writeverf being
> > > "per-file" vs
> > > "per-server".
> > >
> >
> > As I recall, we've already had this discussion on the IETF NFSv4
> > working group mailing list:
> > https://mailarchive.ietf.org/arch/msg/nfsv4/99Ow2muMylXKWd9lzi9_BX2LJDY/
> >
> >
> > That's why I kept it a global in the first place.
> >
> > Now note that RFC8881 does also clarify in Section 18.3.3 that:
> >
> >
> >    The server must vary the value of the write
> >    verifier at each server event or instantiation that may lead to a
> >    loss of uncommitted data.  Most commonly this occurs when the server
> >    is restarted; however, other events at the server may result in
> >    uncommitted data loss as well.
> >
> > So I feel it is quite OK to use the verifier the way we do now in order
> > to signify that a fatal write error has occurred and that clients must
> > resend any data that was uncommitted.
> >
>
> Thanks, I missed that discussion. I think you guys have convinced me
> that we have to keep this per-server. I won't bother starting a new
> thread on it.
>
> It's a pity. It would have been a lot more elegant as a per-inode thing!
>
If you think it is worth the effort, you could propose an extension to
4.2. Something like Write_plus, Commit_plus operations.

rick

> Chuck, I think that means we'll just want to keep patch #1 in this
> series?
>
> Thanks,
> --
> Jeff Layton <jlayton@kernel.org>