Re: [PATCH net] ip: hash fragments consistently

[Eric Dumazet <eric.dumazet@gmail.com>]

On 07/23/2018 09:26 AM, Eric Dumazet wrote:
> 
> 
> On 07/23/2018 07:50 AM, Paolo Abeni wrote:
>> The skb hash for locally generated ip[v6] fragments belonging
>> to the same datagram can vary in several circumstances:
>> * for connected UDP[v6] sockets, the first fragment get its hash
>>   via set_owner_w()/skb_set_hash_from_sk()
>> * for unconnected IPv6 UDPv6 sockets, the first fragment can get
>>   its hash via ip6_make_flowlabel()/skb_get_hash_flowi6(), if
>>   auto_flowlabel is enabled
>>
>> For the following frags the hash is usually computed via
>> skb_get_hash().
>> The above can cause OoO for unconnected IPv6 UDPv6 socket: in that
>> scenario the egress tx queue can be selected on a per packet basis
>> via the skb hash.
>> It may also fool flow-oriented schedulers to place fragments belonging
>> to the same datagram in different flows.
>>
> 
> It also fools bond_xmit_hash(), packets of the same datagram can be sent on
> two bonding slaves instead of one, meaning adding pressure on the defrag unit
> in receiver.
> 
> Reviewed-by: Eric Dumazet <edumazet@google.com>
> 

Also we might note that flow dissector itself is buggy as
found by Soukjin Bae ( https://patchwork.ozlabs.org/patch/994601/ )

I will send a v2 of his patch with a different changelog.

Defrag is fixed [1] but the bug in flow dissector is adding
extra work and hash inconsistencies.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=0d5b9311baf27bb545f187f12ecfd558220c607d