Date:   Wed, 07 Sep 2022 11:11:58 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Trond Myklebust <trondmy@...merspace.com>,
        "bfields@...ldses.org" <bfields@...ldses.org>
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Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Wed, 2022-09-07 at 15:04 +0000, Trond Myklebust wrote:
> On Wed, 2022-09-07 at 10:05 -0400, 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?
> 
> Serialising reads is not the problem. The problem is ensuring that
> knfsd is able to provide an atomic change_info4 structure when the
> client modifies the directory.
> i.e. the requirement is that if the directory changed, then that
> modification is atomically accompanied by an update of the change
> attribute that can be retrieved by knfsd and placed in the reply to the
> client.
> 

I think we already do that for directories today via the i_rwsem. We
hold that exclusively over directory-morphing operations, and the
i_version is updated while holding that lock.

> > 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?
> 
> The problem outlined by Bruce above isn't a big deal. Just check the
> I_VERSION_QUERIED flag after the 'update_page_cache' bit, and bump the
> i_version if that's the case. The real problem is what happens if you
> then crash during writeback...
> 

It's a uglier than it looks at first glance. As Jan pointed out, thIt's
possible for the initial file_modified call to succeed and then a second
one to fail. If the time got an initial update and then the data was
copied in, should we fail the write at that point?

We may be better served by trying to also do this with the i_rwsem. I'm
looking at that now, though it's a bit hairy given that
vfs_getattr_nosec can be called either with or without it held.
-- 
Jeff Layton <jlayton@...nel.org>

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