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@xxxxxxxxxx>
