On Mon, Sep 12, 2022 at 02:15:16PM +0000, Trond Myklebust wrote: > On Mon, 2022-09-12 at 09:51 -0400, J. Bruce Fields wrote: > > On Mon, Sep 12, 2022 at 08:55:04AM -0400, Jeff Layton wrote: > > > Because of the "seen" flag, we have a 63 bit counter to play with. > > > Could > > > we use a similar scheme to the one we use to handle when "jiffies" > > > wraps? Assume that we'd never compare two values that were more > > > than > > > 2^62 apart? We could add i_version_before/i_version_after macros to > > > make > > > it simple to handle this. > > > > As far as I recall the protocol just assumes it can never wrap. I > > guess > > you could add a new change_attr_type that works the way you describe. > > But without some new protocol clients aren't going to know what to do > > with a change attribute that wraps. > > > > I think this just needs to be designed so that wrapping is impossible > > in > > any realistic scenario. I feel like that's doable? > > > > If we feel we have to catch that case, the only 100% correct behavior > > would probably be to make the filesystem readonly. > > > > Which protocol? If you're talking about basic NFSv4, it doesn't assume > anything about the change attribute and wrapping. > > The NFSv4.2 protocol did introduce the optional attribute > 'change_attr_type' that tries to describe the change attribute > behaviour to the client. It tells you if the behaviour is monotonically > increasing, but doesn't say anything about the behaviour when the > attribute value overflows. > > That said, the Linux NFSv4.2 client, which uses that change_attr_type > attribute does deal with overflow by assuming standard uint64_t wrap > around rules. i.e. it assumes bit values > 63 are truncated, meaning > that the value obtained by incrementing (2^64-1) is 0. Yeah, it was the MONOTONIC_INCRE case I was thinking of. That's interesting, I didn't know the client did that. --b.
