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

Jeff Layton
2022-09-07T13:12:48Z

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.

> >
> > > Surely the change number must
> > > change no sooner than the change itself is visible, otherwise stale data
> > > could be cached indefinitely.
> > >
> > > If currently implementations behave this way, surely they are broken.
> >
> > It's certainly not ideal but we've never been able to offer truly atomic
> > behavior here given that Linux is a general-purpose OS. The behavior is
> > a little inconsistent too:
> >
> > The c/mtime update and i_version bump on directories (mostly) occur
> > after the operation. c/mtime updates for files however are mostly driven
> > by calls to file_update_time, which happens before data is copied to the
> > pagecache.
> >
> > It's not clear to me why it's done this way. Maybe to ensure that the
> > metadata is up to date in the event that a statx comes in? Improving
> > this would be nice, but I don't see a way to do that without regressing
> > performance.
> > --
> > Jeff Layton <jlayton@kernel.org>

--
Jeff Layton <jlayton@kernel.org>