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

[NeilBrown]

On Thu, 08 Sep 2022, Jeff Layton wrote:
> On Wed, 2022-09-07 at 13:55 +0000, 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.
> > 
> 
> We generally hold the parent dir's inode->i_rwsem exclusively over most
> important directory changes, and the times/i_version are also updated
> while holding it. What we don't do is serialize reads of this value vs.
> the i_rwsem, so you could see new directory contents alongside an old
> i_version. Maybe we should be taking it for read when we query it on a
> directory?

We do hold i_rwsem today.  I'm working on changing that.  Preserving
atomic directory changeinfo will be a challenge.  The only mechanism I
can think if is to pass a "u64*" to all the directory modification ops,
and they fill in the version number at the point where it is incremented
(inode_maybe_inc_iversion_return()).  The (nfsd) caller assumes that
"before" was one less than "after".  If you don't want to internally
require single increments, then you would need to pass a 'u64 [2]' to
get two iversions back.

> 
> Achieving atomicity with file writes though is another matter entirely.
> I'm not sure that's even doable or how to approach it if so.
> Suggestions?

Call inode_maybe_inc_version(page->host) in __folio_mark_dirty() ??

NeilBrown