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

[bfields@fieldses.org (J. Bruce Fields)]

On Tue, Aug 30, 2022 at 03:57:09PM -0400, Jeff Layton wrote:
> On Tue, 2022-08-30 at 15:46 -0400, J. Bruce Fields wrote:
> > 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.
> > 
> 
> Yeah ok, I vaguely remember discussing this.
> 
> That seems like it has its own problem though. If you mix in the ctime
> and the clock jumps backward, then you could end up with the same issue
> (a stale changeid, different contents). No crash required.

Yes, exactly.

My feeling was that I've got no control over power failures and such,
but the clock setting is something I can get right.  So I'd rather have
something that depended on the latter than the former.

I could be wrong.

--b.