From: "NeilBrown" <neilb@suse.de>
To: "Trond Myklebust" <trondmy@hammerspace.com>
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Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field
Date: Thu, 08 Sep 2022 10:53:35 +1000 [thread overview]
Message-ID: <166259841505.30452.6627904866093896202@noble.neil.brown.name> (raw)
In-Reply-To: <9f8b9ee28dcc479ab6fb1105fc12ff190a9b5c48.camel@hammerspace.com>
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@hammerspace.com
>
>
>
next prev parent reply other threads:[~2022-09-08 0:54 UTC|newest]
Thread overview: 126+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-07 11:16 [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field Jeff Layton
2022-09-07 11:37 ` NeilBrown
2022-09-07 12:20 ` J. Bruce Fields
2022-09-07 12:58 ` Jeff Layton
2022-09-07 12:47 ` Jeff Layton
2022-09-07 12:52 ` J. Bruce Fields
2022-09-07 13:12 ` Jeff Layton
2022-09-07 13:51 ` Jan Kara
2022-09-07 14:43 ` Jeff Layton
2022-09-08 0:44 ` NeilBrown
2022-09-08 8:33 ` Jan Kara
2022-09-08 15:21 ` Theodore Ts'o
2022-09-08 15:44 ` J. Bruce Fields
2022-09-08 15:44 ` Jeff Layton
2022-09-08 15:56 ` J. Bruce Fields
2022-09-08 16:15 ` Chuck Lever III
2022-09-08 17:40 ` Jeff Layton
2022-09-08 18:22 ` J. Bruce Fields
2022-09-08 19:07 ` Jeff Layton
2022-09-08 23:01 ` NeilBrown
2022-09-08 23:23 ` Jeff Layton
2022-09-08 23:45 ` NeilBrown
2022-09-09 15:45 ` J. Bruce Fields
2022-09-09 16:36 ` Jeff Layton
2022-09-10 14:56 ` J. Bruce Fields
2022-09-12 11:42 ` Jeff Layton
2022-09-12 12:13 ` Florian Weimer
2022-09-12 12:55 ` Jeff Layton
2022-09-12 13:20 ` Florian Weimer
2022-09-12 13:49 ` Jeff Layton
2022-09-12 13:51 ` J. Bruce Fields
2022-09-12 14:02 ` Jeff Layton
2022-09-12 14:47 ` J. Bruce Fields
2022-09-12 14:15 ` Trond Myklebust
2022-09-12 14:50 ` J. Bruce Fields
2022-09-12 14:56 ` Trond Myklebust
2022-09-12 15:32 ` Trond Myklebust
2022-09-12 15:49 ` Jeff Layton
2022-09-12 12:54 ` J. Bruce Fields
2022-09-12 12:59 ` Jeff Layton
2022-09-13 0:29 ` John Stoffel
2022-09-13 0:41 ` Dave Chinner
2022-09-13 1:49 ` NeilBrown
2022-09-13 2:41 ` Dave Chinner
2022-09-13 3:30 ` NeilBrown
2022-09-13 9:38 ` Theodore Ts'o
2022-09-13 19:02 ` J. Bruce Fields
2022-09-13 23:19 ` NeilBrown
2022-09-14 0:08 ` J. Bruce Fields
2022-09-09 20:34 ` John Stoffel
2022-09-10 22:13 ` NeilBrown
2022-09-12 10:43 ` Jeff Layton
2022-09-12 13:42 ` J. Bruce Fields
2022-09-12 23:14 ` NeilBrown
2022-09-15 14:06 ` J. Bruce Fields
2022-09-15 15:08 ` Trond Myklebust
2022-09-15 16:45 ` Jeff Layton
2022-09-15 17:49 ` Trond Myklebust
2022-09-15 18:11 ` Jeff Layton
2022-09-15 19:03 ` Trond Myklebust
2022-09-15 19:25 ` Jeff Layton
2022-09-15 22:23 ` NeilBrown
2022-09-16 6:54 ` Theodore Ts'o
2022-09-16 11:36 ` Jeff Layton
2022-09-16 15:11 ` Jeff Layton
2022-09-18 23:53 ` Dave Chinner
2022-09-19 13:13 ` Jeff Layton
2022-09-20 0:16 ` Dave Chinner
2022-09-20 10:26 ` Jeff Layton
2022-09-21 0:00 ` Dave Chinner
2022-09-21 10:33 ` Jeff Layton
2022-09-21 21:41 ` Dave Chinner
2022-09-22 10:18 ` Jeff Layton
2022-09-22 20:18 ` Jeff Layton
2022-09-23 9:56 ` Jan Kara
2022-09-23 10:19 ` Jeff Layton
2022-09-23 13:44 ` Trond Myklebust
2022-09-23 13:50 ` Jeff Layton
2022-09-23 14:58 ` Frank Filz
2022-09-26 22:43 ` NeilBrown
2022-09-27 11:14 ` Jeff Layton
2022-09-27 13:18 ` Jeff Layton
2022-09-15 15:41 ` Jeff Layton
2022-09-15 22:42 ` NeilBrown
2022-09-16 11:32 ` Jeff Layton
2022-09-09 12:11 ` Theodore Ts'o
2022-09-09 12:47 ` Jeff Layton
2022-09-09 13:48 ` Theodore Ts'o
2022-09-09 14:43 ` Jeff Layton
2022-09-09 14:58 ` Theodore Ts'o
2022-09-08 22:55 ` NeilBrown
2022-09-08 23:59 ` Trond Myklebust
2022-09-09 0:51 ` NeilBrown
2022-09-09 1:05 ` Trond Myklebust
2022-09-09 1:07 ` NeilBrown
2022-09-09 1:10 ` Trond Myklebust
2022-09-09 2:14 ` Trond Myklebust
2022-09-09 6:41 ` NeilBrown
2022-09-10 12:39 ` Jeff Layton
2022-09-10 22:53 ` NeilBrown
2022-09-12 10:25 ` Jeff Layton
2022-09-12 23:29 ` NeilBrown
2022-09-13 1:15 ` Dave Chinner
2022-09-13 1:41 ` NeilBrown
2022-09-13 19:01 ` Jeff Layton
2022-09-13 23:24 ` NeilBrown
2022-09-14 11:51 ` Jeff Layton
2022-09-14 22:45 ` NeilBrown
2022-09-14 23:02 ` NeilBrown
2022-09-08 22:40 ` NeilBrown
2022-09-07 13:55 ` Trond Myklebust
2022-09-07 14:05 ` Jeff Layton
2022-09-07 15:04 ` Trond Myklebust
2022-09-07 15:11 ` Jeff Layton
2022-09-08 0:40 ` NeilBrown
2022-09-08 11:34 ` Jeff Layton
2022-09-08 22:29 ` NeilBrown
2022-09-09 11:53 ` Jeff Layton
2022-09-10 22:58 ` NeilBrown
2022-09-10 19:46 ` Al Viro
2022-09-10 23:00 ` NeilBrown
2022-09-08 0:31 ` NeilBrown
2022-09-08 0:41 ` Trond Myklebust
2022-09-08 0:53 ` NeilBrown [this message]
2022-09-08 11:37 ` Jeff Layton
2022-09-08 12:40 ` Trond Myklebust
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