On Tue, Aug 30, 2022 at 03:30:13PM -0400, Jeff Layton wrote:

On Tue, 2022-08-30 at 14:32 -0400, J. Bruce Fields wrote:

quoted

On Tue, Aug 30, 2022 at 01:02:50PM -0400, Jeff Layton wrote:

quoted

The fact that NFS kept this more loosely-defined is what allowed us to
elide some of the i_version bumps and regain a fair bit of performance
for local filesystems [1]. If the change attribute had been more
strictly defined like you mention, then that particular optimization
would not have been possible.

This sort of thing is why I'm a fan of not defining this any more
strictly than we require. Later on, maybe we'll come up with a way for
filesystems to advertise that they can offer stronger guarantees.

Yeah, the afs change-attribute-as-counter thing seems ambitious--I
wouldn't even know how to define what exactly you're counting.

My one question is whether it'd be worth just defining the thing as
*increasing*.  That's a lower bar.

That's a very good question.

One could argue that NFSv4 sort of requires that for write delegations
anyway. All of the existing implementations that I know of do this, so
that wouldn't rule any of them out.

I'm not opposed to adding that constraint. Let me think on it a bit
more.

quoted

(Though admittedly we don't quite manage it now--see again 1631087ba872
"Revert "nfsd4: support change_attr_type attribute"".)

Factoring the ctime into the change attr seems wrong, since a clock jump
could make it go backward. Do you remember what drove that change (see
630458e730b8) ?

It seems like if the i_version were to go backward, then the ctime
probably would too, and you'd still see a duplicate change attr.

See the comment--I was worried about crashes: the change attribute isn't
on disk at the time the client requests it, so after a crash the client
may see it go backward.  (And then could see it repeat a value, possibly
with different file contents.)

Combining it with the ctime means we get something that behaves
correctly even in that case--unless the clock goes backwards.

--b.