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

[Jeff Layton]

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.
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>