netdev - Re: [PATCH net-next 4/4] net: skbuff: fix l4

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Date: Thu, 11 May 2023 14:33:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Dumitru Ceara <dceara@...hat.com>
Cc: Antoine Tenart <atenart@...nel.org>, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, netdev@...r.kernel.org, Ilya Maximets <i.maximets@....org>
Subject: Re: [PATCH net-next 4/4] net: skbuff: fix l4_hash comment

On Thu, May 11, 2023 at 2:10 PM Dumitru Ceara <dceara@...hat.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

Has anyone complained ?

Note that skb->hash has never been considered as canonical, for obvious reasons.


> Best regards,
> Dumitru
>
> >
> > Signed-off-by: Antoine Tenart <atenart@...nel.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
>