Re: [PATCH 3/3] nfsd: simplify write verifier handling

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2023-02-14 at 14:57 -0800, Rick Macklem wrote:
> 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
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> > > > > 
> > > > > 
> > > > > 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.
> 

I considered that, but I don't think it really helps. We'd have to bump
the verifier on a per-server basis anyway to keep up backward
compatibility. I think we're stuck unless we wanted to make a break with
the past.

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

-- 
Jeff Layton <jlayton@kernel.org>