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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2023-10-31 at 13:22 +0100, Jan Kara wrote:
> On Tue 31-10-23 07:04:53, Jeff Layton wrote:
> > On Tue, 2023-10-31 at 09:37 +1100, Dave Chinner wrote:
> > > I have suggested mechanisms for using masked off bits of timestamps
> > > to encode sub-timestamp granularity change counts and keep them
> > > invisible to userspace and then not using i_version at all for XFS.
> > > This avoids all the problems that the multi-grain timestamp
> > > infrastructure exposed due to variable granularity of user visible
> > > timestamps and ordering across inodes with different granularity.
> > > This is potentially a general solution, too.
> > 
> > I don't really understand this at all, but trying to do anything with
> > fine-grained timestamps will just run into a lot of the same problems we
> > hit with the multigrain work. If you still see this as a path forward,
> > maybe you can describe it more detail?
> 
> Dave explained a bit more details here [1] like:
> 
> Another options is for XFS to play it's own internal tricks with
> [cm]time granularity and turn off i_version. e.g. limit external
> timestamp visibility to 1us and use the remaining dozen bits of the
> ns field to hold a change counter for updates within a single coarse
> timer tick. This guarantees the timestamp changes within a coarse
> tick for the purposes of change detection, but we don't expose those
> bits to applications so applications that compare timestamps across
> inodes won't get things back to front like was happening with the
> multi-grain timestamps....
> -
> 
> So as far as I understand Dave wants to effectively persist counter in low
> bits of ctime and expose ctime+counter as its change cookie. I guess that
> could work and what makes the complexity manageable compared to full
> multigrain timestamps is the fact that we have one filesystem, one on-disk
> format etc. The only slight trouble could be that if we previously handed
> out something in low bits of ctime for XFS, we need to keep handing the
> same thing out until the inode changes (i.e., no rounding until the moment
> inode changes) as the old timestamp could be stored somewhere externally
> and compared.
> 
> 								Honza
> 
> [1] https://lore.kernel.org/all/ZTjMRRqmlJ+fTys2@dread.disaster.area/
> 
> 

Got it. That makes sense and could probably be made to work.
Doing that all in XFS won't be simple though. You'll need to reimplement
stuff like file_modified() and file_update_time(). Those get called from
deep within the VFS and from page fault handlers.

FWIW, that's the main reason the multigrain work was so invasive, even
though it was a filesystem-specific feature.
-- 
Jeff Layton <jlayton@kernel.org>