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

[Trond Myklebust]

On Mon, 2022-09-12 at 10:50 -0400, J. Bruce Fields wrote:
> 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.
> 

If you look at where we compare version numbers, it is always some
variant of the following:

static int nfs_inode_attrs_cmp_monotonic(const struct nfs_fattr *fattr,
                                         const struct inode *inode)
{
        s64 diff = fattr->change_attr - inode_peek_iversion_raw(inode);
        if (diff > 0)
                return 1;
        return diff == 0 ? 0 : -1;
}

i.e. we do an unsigned 64-bit subtraction, and then cast it to the
signed 64-bit equivalent in order to figure out which is the more
recent value.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx