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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2022-09-08 at 11:56 -0400, J. Bruce Fields wrote:
> On Thu, Sep 08, 2022 at 11:44:33AM -0400, Jeff Layton wrote:
> > On Thu, 2022-09-08 at 11:21 -0400, Theodore Ts'o wrote:
> > > On Thu, Sep 08, 2022 at 10:33:26AM +0200, Jan Kara wrote:
> > > > It boils down to the fact that we don't want to call mark_inode_dirty()
> > > > from IOCB_NOWAIT path because for lots of filesystems that means journal
> > > > operation and there are high chances that may block.
> > > > 
> > > > Presumably we could treat inode dirtying after i_version change similarly
> > > > to how we handle timestamp updates with lazytime mount option (i.e., not
> > > > dirty the inode immediately but only with a delay) but then the time window
> > > > for i_version inconsistencies due to a crash would be much larger.
> > > 
> > > Perhaps this is a radical suggestion, but there seems to be a lot of
> > > the problems which are due to the concern "what if the file system
> > > crashes" (and so we need to worry about making sure that any
> > > increments to i_version MUST be persisted after it is incremented).
> > > 
> > > Well, if we assume that unclean shutdowns are rare, then perhaps we
> > > shouldn't be optimizing for that case.  So.... what if a file system
> > > had a counter which got incremented each time its journal is replayed
> > > representing an unclean shutdown.  That shouldn't happen often, but if
> > > it does, there might be any number of i_version updates that may have
> > > gotten lost.  So in that case, the NFS client should invalidate all of
> > > its caches.
> > > 
> > > If the i_version field was large enough, we could just prefix the
> > > "unclean shutdown counter" with the existing i_version number when it
> > > is sent over the NFS protocol to the client.  But if that field is too
> > > small, and if (as I understand things) NFS just needs to know when
> > > i_version is different, we could just simply hash the "unclean
> > > shtudown counter" with the inode's "i_version counter", and let that
> > > be the version which is sent from the NFS client to the server.
> > > 
> > > If we could do that, then it doesn't become critical that every single
> > > i_version bump has to be persisted to disk, and we could treat it like
> > > a lazytime update; it's guaranteed to updated when we do an clean
> > > unmount of the file system (and when the file system is frozen), but
> > > on a crash, there is no guaranteee that all i_version bumps will be
> > > persisted, but we do have this "unclean shutdown" counter to deal with
> > > that case.
> > > 
> > > Would this make life easier for folks?
> > > 
> > > 						- Ted
> > 
> > Thanks for chiming in, Ted. That's part of the problem, but we're
> > actually not too worried about that case:
> > 
> > nfsd mixes the ctime in with i_version, so you'd have to crash+clock
> > jump backward by juuuust enough to allow you to get the i_version and
> > ctime into a state it was before the crash, but with different data.
> > We're assuming that that is difficult to achieve in practice.
> 
> But a change in the clock could still cause our returned change
> attribute to go backwards (even without a crash).  Not sure how to
> evaluate the risk, but it was enough that Trond hasn't been comfortable
> with nfsd advertising NFS4_CHANGE_TYPE_IS_MONOTONIC.
> 
> Ted's idea would be sufficient to allow us to turn that flag on, which I
> think allows some client-side optimizations.
> 

Good point.

> > The issue with a reboot counter (or similar) is that on an unclean crash
> > the NFS client would end up invalidating every inode in the cache, as
> > all of the i_versions would change. That's probably excessive.
> 
> But if we use the crash counter on write instead of read, we don't
> invalidate caches unnecessarily.  And I think the monotonicity would
> still be close enough for our purposes?
> 
> > The bigger issue (at the moment) is atomicity: when we fetch an
> > i_version, the natural inclination is to associate that with the state
> > of the inode at some point in time, so we need this to be updated
> > atomically with certain other attributes of the inode. That's the part
> > I'm trying to sort through at the moment.
> 
> That may be, but I still suspect the crash counter would help.
> 

Yeah, ok. That does make some sense. So we would mix this into the
i_version instead of the ctime when it was available. Preferably, we'd
mix that in when we store the i_version rather than adding it afterward.

Ted, how would we access this? Maybe we could just add a new (generic)
super_block field for this that ext4 (and other filesystems) could
populate at mount time?
-- 
Jeff Layton <jlayton@kernel.org>