netdev - Re: [PATCH net-next 2/2] bonding: use l4 hash if available

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Date:	Tue, 15 Sep 2015 22:11:10 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Mahesh Bandewar <maheshb@...gle.com>
Subject: Re: [PATCH net-next 2/2] bonding: use l4 hash if available

On Tue, Sep 15, 2015 at 5:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2015-09-15 at 16:45 -0700, Tom Herbert wrote:
>> > +       if (bond->params.xmit_policy == BOND_XMIT_POLICY_ENCAP34 &&
>> > +           skb->l4_hash)
>> > +                       return skb->hash;
>> > +
>> >         if (bond->params.xmit_policy == BOND_XMIT_POLICY_LAYER2 ||
>> >             !bond_flow_dissect(bond, skb, &flow))
>> >                 return bond_eth_hash(skb);
>> >
>> >
>> Ugh, bond_flow_dissect is yet another instance of customized flow
>> dissection! We should really clean this up. I suggest that in cases
>> were we want L4 hash a call to skb_get_hash should suffice. We can
>> create skb_get_l3hash when caller explicitly wants an L3 hash-- this
>> would return skb->hash if it's valid and skb->l4_hash is not set, else
>> call flow dissector with FLOW_DISSECTOR_F_STOP_AT_L3 and then do the
>> normal hash over flow keys (don't save result in skb->hash in this
>> case).
>
> This code predates all the change you did recently ;)
>
> BTW, the simple xor weakness is showing up after
> our change favoring even ports at connect() time, for a bonding device
> with 2 or 4 slaves.
>
Right, xor as a packet hash should be eliminated. It seems possible
that all these modes can be implemented using flow_dissector and the
jhash. If I'm reading the meaning of modes correctly:

BOND_XMIT_POLICY_ENCAP34 is equivalent to skb_get_hash

BOND_XMIT_POLICY_LAYER23 is flow dissection with
FLOW_DISSECTOR_F_STOP_AT_L3 and then normal hash

BOND_XMIT_POLICY_LAYER34 is flow dissection with FLOW_DISSECTOR_F_STOP_AT_ENCAP

BOND_XMIT_POLICY_LAYER2 would be flow dissection with
FLOW_DISSECTOR_F_STOP_AT_L2 (new flag) and then normal hash

BOND_XMIT_POLICY_ENCAP23 is a little more interesting. This could be
accomplished with custom flow dissector targets that exclude L4
information (ports, flow label, key ID).

Also noticed a little bug in flow_dissector, we should get out on
FLOW_DISSECTOR_F_STOP_AT_L3 before IPv6 flow label is processed
(that's considered L4). I'll fix that.

Tom

> (commit 07f4c90062f8fc7c8c26f8f95324cbe8fa3145a5
> "tcp/dccp: try to not exhaust ip_local_port_range in connect()")
>
>
>
>
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