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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-09-06 at 15:29 -0400, J. Bruce Fields wrote:
> 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.
> 

Yeah, that's a potential issue. The i_version is updated in
inode_update_time, which does happen before the write to the pagecache.

We should probably add a note to the manpage that one should not expect
any sort of atomicity between the change to the inode and the change in
the value. I'm not sure we can offer much in the way of mitigation for
that problem, otherwise.

> 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.
> 

-- 
Jeff Layton <jlayton@kernel.org>