netdev - Re: Netlink NLM_F_DUMP

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Date:   Thu, 23 Jun 2022 09:36:09 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Ismael Luceno <iluceno@...e.de>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Netlink NLM_F_DUMP_INTR flag lost

On Thu, 23 Jun 2022 10:17:17 -0600 David Ahern wrote:
> > Yup, the question for me is what's the risk / benefit of sending 
> > the empty message vs putting the _DUMP_INTR on the next family.
> > I'm leaning towards putting it on the next family and treating 
> > the entire dump as interrupted, do you reckon that's suboptimal?  
> 
> I think it is going to be misleading; the INTR flag needs to be set on
> the dump that is affected.

Right, it's a bit of a philosophical discussion but dump is delineated
but NLMSG_DONE. PF_UNSPEC dump is a single dump, not a group of multiple
independent per-family dumps. If we think of a nlmsg as a representation
of an object having an empty one is awkward. What if someone does a dump
to just count objects? Too speculative?

I guess one can argue either way, no empty messages is a weaker promise
and hopefully lower risk, hence my preference. Do you feel strongly for
the message? Do we flip a coin? :)

> All of the dumps should be checking the consistency at the end of the
> dump - regardless of any remaining entries on a particular round (e.g.,
> I mentioned this what the nexthop dump does). Worst case then is DONE
> and INTR are set on the same message with no data, but it tells
> explicitly the set of data affected.

Okay, perhaps we should put a WARN_ON_ONCE(seq && seq != prev_seq)
in rtnl_dump_all() then, to catch those who get it wrong.