Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field

[Trond Myklebust <trondmy@hammerspace.com>]

On Thu, 2022-09-08 at 10:31 +1000, NeilBrown wrote:
> On Wed, 07 Sep 2022, Trond Myklebust wrote:
> > On Wed, 2022-09-07 at 09:12 -0400, Jeff Layton wrote:
> > > On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote:
> > > > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote:
> > > > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote:
> > > > > > On Wed, 07 Sep 2022, Jeff Layton wrote:
> > > > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic
> > > > > > > with
> > > > > > > respect to the
> > > > > > > +other changes in the inode. On a write, for instance,
> > > > > > > the
> > > > > > > i_version it usually
> > > > > > > +incremented before the data is copied into the
> > > > > > > pagecache.
> > > > > > > Therefore it is
> > > > > > > +possible to see a new i_version value while a read still
> > > > > > > shows the old data.
> > > > > > 
> > > > > > Doesn't that make the value useless?
> > > > > > 
> > > > > 
> > > > > No, I don't think so. It's only really useful for comparing
> > > > > to an
> > > > > older
> > > > > sample anyway. If you do "statx; read; statx" and the value
> > > > > hasn't
> > > > > changed, then you know that things are stable. 
> > > > 
> > > > I don't see how that helps.  It's still possible to get:
> > > > 
> > > >                 reader          writer
> > > >                 ------          ------
> > > >                                 i_version++
> > > >                 statx
> > > >                 read
> > > >                 statx
> > > >                                 update page cache
> > > > 
> > > > right?
> > > > 
> > > 
> > > Yeah, I suppose so -- the statx wouldn't necessitate any locking.
> > > In
> > > that case, maybe this is useless then other than for testing
> > > purposes
> > > and userland NFS servers.
> > > 
> > > Would it be better to not consume a statx field with this if so?
> > > What
> > > could we use as an alternate interface? ioctl? Some sort of
> > > global
> > > virtual xattr? It does need to be something per-inode.
> > 
> > I don't see how a non-atomic change attribute is remotely useful
> > even
> > for NFS.
> > 
> > The main problem is not so much the above (although NFS clients are
> > vulnerable to that too) but the behaviour w.r.t. directory changes.
> > 
> > If the server can't guarantee that file/directory/... creation and
> > unlink are atomically recorded with change attribute updates, then
> > the
> > client has to always assume that the server is lying, and that it
> > has
> > to revalidate all its caches anyway. Cue endless
> > readdir/lookup/getattr
> > requests after each and every directory modification in order to
> > check
> > that some other client didn't also sneak in a change of their own.
> 
> NFS re-export doesn't support atomic change attributes on
> directories.
> Do we see the endless revalidate requests after directory
> modification
> in that situation?  Just curious.

Why wouldn't NFS re-export be capable of supporting atomic change
attributes in those cases, provided that the server does? It seems to
me that is just a question of providing the correct information w.r.t.
atomicity to knfsd.

...but yes, a quick glance at nfs4_update_changeattr_locked(), and what
happens when !cinfo->atomic should tell you all you need to know.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com