Re: BUG: Linux 6.12 nfsd does not support FATTR4_WORD2_CHANGE_ATTR_TYPE in NFSv4.2 mode!!

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2025-01-13 at 05:06 +0100, Takeshi Nishimura wrote:
> On Sat, Jan 11, 2025 at 10:17 PM Rick Macklem <rick.macklem@gmail.com> wrote:
> > 
> > On Sat, Jan 11, 2025 at 12:08 PM Takeshi Nishimura
> > <takeshi.nishimura.linux@gmail.com> wrote:
> > > 
> > > Dear list,
> > > 
> > > We tried to use FATTR4_WORD2_CHANGE_ATTR_TYPE with Linux 6.12 nfsd,
> > > but the server does not set that attribute, while it is mandatory for
> > > NFSv4.2.
> > My understand is that nothing is mandatory in NFSv4.2. Everything is considered
> > optional extensions. I doubt any extant 4.2 server supports all of the optional
> > extensions in NFSv4.2.
> 
> That can't be true, or would be a bug in the NFSv4.2 spec then.
> "Everything optional" means feature support gets fragmented, and
> interoperability will cease to exist.

Not true. NFSv4.2 is composed entirely of optional features. It's
totally legitimate for an endpoint to just report that it supports
minorversion 2, and do nothing else, and it's now compliant.

For this attr, it's even written in the spec!

    12.1.  New RECOMMENDED Attributes - List and Definition References

That said, I'd be happy to help review patches that add this, but I'd
take a look 1631087ba8727db03c0ab2815dc06dc25d962b80 where we reverted
this before.

If the server experiences a clock jump backward, then you could end up
with a duplicate change attribute for a different file state. There is
some question about whether that means that we can't claim that the
attr is MONOTONIC.
-- 
Jeff Layton <jlayton@kernel.org>