Re: [PATCH v3 1/7] iversion: update comments with info about atime updates

J. Bruce Fields
2022-08-30T19:47:01Z

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