Re: [PATCH net-next 4/4] net: skbuff: fix l4_hash comment

[Dumitru Ceara <dceara@redhat.com>]

On 5/11/23 14:33, Eric Dumazet wrote:
> On Thu, May 11, 2023 at 2:10 PM Dumitru Ceara <dceara@redhat.com> wrote:
>>
>> Hi Antoine,
>>
>> On 5/11/23 11:34, Antoine Tenart wrote:
>>> Since commit 877d1f6291f8 ("net: Set sk_txhash from a random number")
>>> sk->sk_txhash is not a canonical 4-tuple hash. sk->sk_txhash is
>>> used in the TCP Tx path to populate skb->hash, with skb->l4_hash=1.
>>> With this, skb->l4_hash does not always indicate the hash is a
>>> "canonical 4-tuple hash over transport ports" but rather a hash from L4
>>> layer to provide a uniform distribution over flows. Reword the comment
>>> accordingly, to avoid misunderstandings.
>>
>> But AFAIU the hash used to be a canonical 4-tuple hash and was used as
>> such by other components, e.g., OvS:
>>
>> https://elixir.bootlin.com/linux/latest/source/net/openvswitch/actions.c#L1069
>>
>> It seems to me at least unfortunate that semantics change without
>> considering other users.  The fact that we now fix the documentation
>> makes it seem like OvS was wrong to use the skb hash.  However, before
>> 877d1f6291f8 ("net: Set sk_txhash from a random number") it was OK for
>> OvS to use the skb hash as a canonical 4-tuple hash.
>>
> 
> I do not think we can undo stuff that was done back in 2015
> 

I understand.  I guess I was kind of grasping at straws in the hope of
getting a canonical 4-tuple hash.

> Has anyone complained ?
> 

It did go unnoticed for a while but recently we started getting
(indirect) reports due to the hash changing.

This one is from an upstream OVN (OvS) user:
https://github.com/ovn-org/ovn/issues/112

This is from an OpenShift (also running OVN/OvS) user:
https://issues.redhat.com/browse/OCPBUGS-7406

> Note that skb->hash has never been considered as canonical, for obvious reasons.
> 
> 
>> Best regards,
>> Dumitru
>>
>>>
>>> Signed-off-by: Antoine Tenart <atenart@kernel.org>
>>> ---
>>>  include/linux/skbuff.h | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>> index 738776ab8838..f54c84193b23 100644
>>> --- a/include/linux/skbuff.h
>>> +++ b/include/linux/skbuff.h
>>> @@ -791,8 +791,8 @@ typedef unsigned char *sk_buff_data_t;
>>>   *   @active_extensions: active extensions (skb_ext_id types)
>>>   *   @ndisc_nodetype: router type (from link layer)
>>>   *   @ooo_okay: allow the mapping of a socket to a queue to be changed
>>> - *   @l4_hash: indicate hash is a canonical 4-tuple hash over transport
>>> - *           ports.
>>> + *   @l4_hash: indicate hash is from layer 4 and provides a uniform
>>> + *           distribution over flows.
>>>   *   @sw_hash: indicates hash was computed in software stack
>>>   *   @wifi_acked_valid: wifi_acked was set
>>>   *   @wifi_acked: whether frame was acked on wifi or not
>>
>