netdev - Re: [PATCH net] ip: hash fragments consistently

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Date:   Mon, 23 Jul 2018 09:26:55 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net] ip: hash fragments consistently



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@...gle.com>