Message-id: <166259841505.30452.6627904866093896202@noble.neil.brown.name>
Date:   Thu, 08 Sep 2022 10:53:35 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Trond Myklebust" <trondmy@...merspace.com>
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Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Thu, 08 Sep 2022, Trond Myklebust wrote:
> 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.

I don't know if it "could" but as far as I can see the Linux nfsd server
doesn't.
NFS sets EXPORT_OP_NOATOMIC_ATTR which causes ->fs_no_atomic_attr to be
set so cinfo->atomic reported back to the client is always false.

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

Yep, I can see that all the directory cache is invalidated.  I was more
wondering if anyone had noticed this causing performance problems.  I
suspect there are some workloads where is isn't noticeable, and others
where it would be quite unpleasant.

Chuck said recently:

> My impression is that pre/post attributes in NFSv3 have not
> turned out to be as useful as their inventors predicted.

https://lore.kernel.org/linux-nfs/8F16D957-F43A-4E5B-AA28-AAFCF43222E2@oracle.com/

I wonder how accurate that impression is.

Thanks,
NeilBrown



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

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