why is short-circuiting nfs_lookup() for mkdir(2) et.al. dependent upon v3 or later?

[Al Viro <viro@zeniv.linux.org.uk>]

        In nfs_lookup() we have
        /*
         * If we're doing an exclusive create, optimize away the lookup
         * but don't hash the dentry.
         */
        if (nfs_is_exclusive_create(dir, flags) || flags & LOOKUP_RENAME_TARGET)
                return NULL;
OK, fair enough - we don't need to find out whether it's negative or not for
mkdir() et.al.; if it isn't, server will tell us to sod off and we can live
with not having it in cache - in the worst case, we'll have to do the same
lookup we'd skipped here at some later point.  Same for rename(2) destination -
if it wasn't in dcache, we are not going to bother with sillyrename anyway, and
that's the only thing where we might care about the destination.  If rename(2)
succeeds, we won't see whatever had been there anyway, and if it fails, we won't
lose anything from having lookup done later.

        What I don't get is why, unlike rename(2) target, mkdir(2) argument is
handled that way only for v3 and later.  It's been a long time since I looked
at NFSv2 servers, but shouldn't we get NFSERR_EXIST if the sucker turns out to
have already been there?

        What am I missing?