Re: [RFC PATCH v2] statx, inode: document the new STATX_INO_VERSION field

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

On Tue, Sep 06, 2022 at 01:04:05PM -0400, Jeff Layton wrote:
> On Tue, 2022-09-06 at 12:41 -0400, Jeff Layton wrote:
> > On Tue, 2022-09-06 at 14:17 +0200, Florian Weimer wrote:
> > > * Jeff Layton:
> > > 
> > > > All of the existing implementations use all 64 bits. If you were to
> > > > increment a 64 bit value every nanosecond, it will take >500 years for
> > > > it to wrap. I'm hoping that's good enough. ;)
> > > > 
> > > > The implementation that all of the local Linux filesystems use track
> > > > whether the value has been queried using one bit, so there you only get
> > > > 63 bits of counter.
> > > > 
> > > > My original thinking here was that we should leave the spec "loose" to
> > > > allow for implementations that may not be based on a counter. E.g. could
> > > > some filesystem do this instead by hashing certain metadata?
> > > 
> > > Hashing might have collisions that could be triggered deliberately, so
> > > probably not a good idea.  It's also hard to argue that random
> > > collisions are unlikely.
> > > 
> > 
> > In principle, if a filesystem could guarantee enough timestamp
> > resolution, it's possible collisions could be hard to achieve. It's also
> > possible you could factor in other metadata that wasn't necessarily
> > visible to userland to try and ensure uniqueness in the counter.
> > 
> > Still...

I've got one other nagging worry, about the ordering of change attribute
updates with respect to their corresponding changes.  I think with
current implementations it's possible that the only change attribute
update(s) may happen while the old file data is still visible, which
means a concurrent reader could cache the old data with the new change
attribute, and be left with a stale cache indefinitely.

For the purposes of close-to-open semantics I think that's not a
problem, though.

There may be some previous discussion of this in mailing list archives.

--b.