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

[Trond Myklebust <trondmy@kernel.org>]

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:
> > 
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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.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com