[PATCH 1/4] vfs: report change attribute in statx for IS_I_VERSION inodes

[Darrick J. Wong]

On Tue, Aug 16, 2022 at 11:32:24AM -0400, Jeff Layton wrote:
> On Tue, 2022-08-16 at 16:15 +0100, David Howells wrote:
> > Jeff Layton <jlayton at kernel.org> wrote:
> > 
> > > I think we'll just have to ensure that before we expose this for any
> > > filesystem that it conforms to some minimum standards. i.e.: it must
> > > change if there are data or metadata changes to the inode, modulo atime
> > > changes due to reads on regular files or readdir on dirs.
> > > 
> > > The local filesystems, ceph and NFS should all be fine. I guess that
> > > just leaves AFS. If it can't guarantee that, then we might want to avoid
> > > exposing the counter for it.
> > 
> > AFS monotonically increments the counter on data changes; doesn't make any
> > change for metadata changes (other than the file size).
> > 
> > But you can't assume NFS works as per your suggestion as you don't know what's
> > backing it (it could be AFS, for example - there's a converter for that).
> > 
> 
> In that case, the NFS server must synthesize a proper change attr. The
> NFS spec mandates that it change on most metadata changes.
> 
> > Further, for ordinary disk filesystems, two data changes may get elided and
> > only increment the counter once.
> > 
> 
> Not a problem as long as nothing queried the counter in between the
> changes.
> 
> > And then there's mmap...
> > 
> 
> Not sure how that matters here.
> 
> > It might be better to reduce the scope of your definition and just say that it
> > must change if there's a data change and may also be changed if there's a
> > metadata change.
> > 
> 
> I'd prefer that we mandate that it change on metadata changes as well.

...in that case, why not leave the i_version bump in
xfs_trans_log_inode?  That will capture all changes to file data,
attribues, and metadata. ;)

--D

> That's what most of the in-kernel users want, and what most of the
> existing filesystems provide. If AFS can't give that guarantee then we
> can just omit exposing i_version on it.
> -- 
> Jeff Layton <jlayton at kernel.org>