netdev - Re: [PATCH net 1/1] netfilter: conntrack: Check offload bit on table dump

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Date:   Wed, 3 Feb 2021 13:50:35 +0100
From:   Florian Westphal <fw@...len.de>
To:     Roi Dayan <roid@...dia.com>
Cc:     Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netdev@...r.kernel.org, Paul Blakey <paulb@...dia.com>,
        Oz Shlomo <ozsh@...dia.com>
Subject: Re: [PATCH net 1/1] netfilter: conntrack: Check offload bit on table
 dump

Roi Dayan <roid@...dia.com> wrote:
> > Do you think rhashtable_insert_fast() in flow_offload_add() blocks for
> > dozens of seconds?
> 
> I'm not sure. but its not only that but also the time to be in
> established state as only then we offload.

That makes it even more weird.  Timeout for established is even larger.
In case of TCP, its days... so I don't understand at all :/

> > Thats about the only thing I can see between 'offload bit gets set'
> > and 'timeout is extended' in flow_offload_add() that could at least
> > spend *some* time.
> > 
> > > We hit this issue before more easily and pushed this fix
> > > 
> > > 4203b19c2796 netfilter: flowtable: Set offload timeout when adding flow
> > 
> > This fix makes sense to me.
> 
> I just noted we didn't test correctly Pablo's suggestion instead of
> to check the bit and extend the timeout in ctnetlink_dump_table() and
> ct_seq_show() like GC does.

Ok.  Extending it there makes sense, but I still don't understand
why newly offloaded flows hit this problem.

Flow offload should never see a 'about to expire' ct entry.
The 'extend timeout from gc' is more to make sure GC doesn't reap
long-lived entries that have been offloaded aeons ago, not 'prevent
new flows from getting zapped...'