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@xxxxxxxxxx/ I wonder how accurate that impression is. Thanks, NeilBrown > > -- > Trond Myklebust > Linux NFS client maintainer, Hammerspace > trond.myklebust@xxxxxxxxxxxxxxx > > >
