On Tue, 2017-03-21 at 15:13 -0400, J. Bruce Fields wrote: > On Tue, Mar 21, 2017 at 02:46:53PM -0400, Jeff Layton wrote: > > On Tue, 2017-03-21 at 14:30 -0400, J. Bruce Fields wrote: > > > On Tue, Mar 21, 2017 at 01:23:24PM -0400, Jeff Layton wrote: > > > > On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote: > > > > > - It's durable; the above comparison still works if there were reboots > > > > > between the two i_version checks. > > > > > - I don't know how realistic this is--we may need to figure out > > > > > if there's a weaker guarantee that's still useful. Do > > > > > filesystems actually make ctime/mtime/i_version changes > > > > > atomically with the changes that caused them? What if a > > > > > change attribute is exposed to an NFS client but doesn't make > > > > > it to disk, and then that value is reused after reboot? > > > > > > > > > > > > > Yeah, there could be atomicity there. If we bump i_version, we'll mark > > > > the inode dirty and I think that will end up with the new i_version at > > > > least being journalled before __mark_inode_dirty returns. > > > > > > So you think the filesystem can provide the atomicity? In more detail: > > > > > > > Sorry, I hit send too quickly. That should have read: > > > > "Yeah, there could be atomicity issues there." > > > > I think providing that level of atomicity may be difficult, though > > maybe there's some way to make the querying of i_version block until > > the inode update has been journalled? > > No idea. Anyway, I'd like to figure out some reasonable requirement > that we can document. > > > > > > - if I write to a file, a simultaneous reader should see either > > > (old data, old i_version) or (new data, new i_version), not a > > > combination of the two. > > > - ditto for metadata modifications. > > > - the same should be true if there's a crash. > > > > > > (If that's not possible, then I think we could live with a brief window > > > of (new data, old i_version) as long as it doesn't persist beyond sync?) > > > > > > > That said, I suppose it is possible for us to bump the counter, hand > > > > that new counter value out to a NFS client and then the box crashes > > > > before it makes it to the journal. > > > > > > > > Not sure how big a problem that really is. > > > > > > The other case I was wondering about may have been unclear. Represent > > > the state of a file by a (data, i_version) pair. Say: > > > > > > - file is modified from (F, V) to (F', V+1). > > > - client reads and caches (F', V+1). > > > - server crashes before writeback, so disk still has (F, V). > > > - after restart, someone else modifies file to (F'', V+1). > > > - original client revalidates its cache, sees V+1, concludes > > > file data is still F'. > > > > > > This may not cause a real problem for clients depending only on > > > traditional NFS close-to-open semantics. > > > > > > > > > > No, I think that is a legitimate problem. > > > > That said, after F'', the mtime would almost certainly be different > > from the time after F', and that would likely be enough to prevent > > confusion in NFS clients. > > Oh, good point. So, may be worth saying that anyone wanting to make > sense of these across reboot should compare times as well (maybe that > should be in nfs rfc's too). I think that should be ctime not mtime, > though? > Yes, it might be worth a mention there. IIRC, it does mention that you shouldn't just look at a single attribute for cache validation purposes, but the wording is a bit vague. I can't find the section at the moment though. The more I think about it though, simply ensuring that we don't publish a new change attr until the inode update has hit the journal may be the best we can do. I'd have to think about how to implement that though. -- Jeff Layton <jlayton@xxxxxxxxxx>
