Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

[Christian Brauner <brauner@kernel.org>]

On Thu, Oct 19, 2023 at 07:28:48AM -0400, Jeff Layton wrote:
> On Thu, 2023-10-19 at 11:29 +0200, Christian Brauner wrote:
> > > Back to your earlier point though:
> > > 
> > > Is a global offset really a non-starter? I can see about doing something
> > > per-superblock, but ktime_get_mg_coarse_ts64 should be roughly as cheap
> > > as ktime_get_coarse_ts64. I don't see the downside there for the non-
> > > multigrain filesystems to call that.
> > 
> > I have to say that this doesn't excite me. This whole thing feels a bit
> > hackish. I think that a change version is the way more sane way to go.
> > 
> 
> What is it about this set that feels so much more hackish to you? Most
> of this set is pretty similar to what we had to revert. Is it just the
> timekeeper changes? Why do you feel those are a problem?

So I think that the multi-grain timestamp work was well intended but it
was ultimately a mistake. Because we added code that complicated
timestamp timestamp handling in the vfs to a point where the costs
clearly outweighed the benefits.

And I don't think that this direction is worth going into. This whole
thread ultimately boils down to complicating generic infrastructure
quite extensively for nfs to handle exposing xfs without forcing an
on-disk format change. That's even fine.

That's not a problem but in the same way I don't think the solution is
just stuffing this complexity into the vfs. IOW, if we make this a vfs
problem then at the lowest possible cost and not by changing how
timestamps work for everyone even if it's just internal.