netdev - Re: [PATCH net-next 07/16] net: bridge: Maintain number of MDB entries in net_bridge_mcast

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Date:   Mon, 30 Jan 2023 16:02:07 +0100
From:   Petr Machata <petrm@...dia.com>
To:     Nikolay Aleksandrov <razor@...ckwall.org>
CC:     Petr Machata <petrm@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>, Roopa Prabhu <roopa@...dia.com>,
        <netdev@...r.kernel.org>, <bridge@...ts.linux-foundation.org>,
        Ido Schimmel <idosch@...dia.com>
Subject: Re: [PATCH net-next 07/16] net: bridge: Maintain number of MDB
 entries in net_bridge_mcast_port


Nikolay Aleksandrov <razor@...ckwall.org> writes:

> On 26/01/2023 19:01, Petr Machata wrote:
>> Note that the per-port-VLAN mcast_max_groups value gets reset when VLAN
>> snooping is enabled. The reason for this is that while VLAN snooping is
>> disabled, permanent entries can be added above the limit imposed by the
>> configured maximum. Under those circumstances, whatever caused the VLAN
>> context enablement, would need to be rolled back, adding a fair amount of
>> code that would be rarely hit and tricky to maintain. At the same time,
>> the feature that this would enable is IMHO not interesting: I posit that
>> the usefulness of keeping mcast_max_groups intact across
>> mcast_vlan_snooping toggles is marginal at best.
>> 
>
> Hmm, I keep thinking about this one and I don't completely agree. It
> would be more user-friendly if the max count doesn't get reset when
> mcast snooping is toggled. Imposing order of operations (first enable
> snooping, then config max entries) isn't necessary and it makes sense
> for someone to first set the limit and then enable vlan snooping.

If you are talking about mcast_snooping, that can be disabled while
mcast_vlan_snooping is enabled. So you can configure everything, then
turn snooping on.

If you are talking about configuring max while mcast_vlan_snooping is
off, then I assumed one shouldn't touch the VLAN context if
br_multicast_port_ctx_vlan_disabled(). So we would need to track the n
and max in some other entity than in the multicast context. But maybe
I'm wrong.

> Also it would be consistent with port max entries, I'd prefer if we
> have the same behaviour for port and vlan pmctxs. If we allow to set
> any maximum at any time we don't need to rollback anything, also we
> already always lookup vlans in br_multicast_port_vid_to_port_ctx() to
> check if snooping is enabled so we can keep the count correct
> regardless, the same as it's done for the ports. Keeping both limits
> with consistent semantics seems better to me.

The idea of requiring max >= current felt so natural to me that I didn't
even check what mcast_hash_max was doing. Sure -- let's be consistent.
This will incidentally make all the rollbacks go away, and happily makes
sense WRT locking, too: since the relation between max and n is somewhat
loose, we don't need to worry too much about sequencing inc-/dec-n vs.
set-max.