netdev - Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

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Date:   Mon, 20 Jan 2020 11:35:30 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/4] net: Support GRO/GSO fraglist chaining.

On Mon, Jan 20, 2020 at 3:36 AM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> On Wed, Jan 15, 2020 at 10:43:08AM -0500, Willem de Bruijn wrote:
> > > > > Maybe we can be conservative here and do a full
> > > > > __copy_skb_header for now. The initial version
> > > > > does not necessarily need to be the most performant
> > > > > version. We could try to identify the correct subset
> > > > > of header fields later then.
> > > >
> > > > We should probably aim for the right set from the start. If you think
> > > > this set is it, let's keep it.
> > >
> > > I'd prefer to do a full __copy_skb_header for now and think a bit
> > > longer if that what I chose is really the correct subset.
> >
> > Ok
> >
> > > > > > > I had to set ip_summed to CHECKSUM_UNNECESSARY on GRO to
> > > > > > > make sure the noone touches the checksum of the head
> > > > > > > skb. Otherise netfilter etc. tries to touch the csum.
> > > > > > >
> > > > > > > Before chaining I make sure that ip_summed and csum_level is
> > > > > > > the same for all chained skbs and here I restore the original
> > > > > > > value from nskb.
> > > > > >
> > > > > > This is safe because the skb_gro_checksum_validate will have validated
> > > > > > already on CHECKSUM_PARTIAL? What happens if there is decap or encap
> > > > > > in the path? We cannot revert to CHECKSUM_PARTIAL after that, I
> > > > > > imagine.
> > > > >
> > > > > Yes, the checksum is validated with skb_gro_checksum_validate. If the
> > > > > packets are UDP encapsulated, they are segmented before decapsulation.
> > > > > Original values are already restored. If an additional encapsulation
> > > > > happens, the encap checksum will be calculated after segmentation.
> > > > > Original values are restored before that.
> > > >
> > > > I was wondering more about additional other encapsulation protocols.
> > > >
> > > > >From a quick read, it seems like csum_level is associated only with
> > > > CHECKSUM_UNNECESSARY.
> > > >
> > > > What if a device returns CHECKSUM_COMPLETE for packets with a tunnel
> > > > that is decapsulated before forwarding. Say, just VLAN. That gets
> > > > untagged in __netif_receive_skb_core with skb_vlan_untag calling
> > > > skb_pull_rcsum. After segmentation the ip_summed is restored, with
> > > > skb->csum still containing the unmodified csum that includes the VLAN
> > > > tag?
> > >
> > > Hm, that could be really a problem. So setting CHECKSUM_UNNECESSARY
> > > should be ok, but restoring the old values are not. Our checksum
> > > magic is rather complex, it's hard to get it right for all possible
> > > cases. Maybe we can just set CHECKSUM_UNNECESSARY for all packets
> > > and keep this value after segmentation.
> >
> > Note that I'm not 100% sure that the issue can occur. But it seems likely.
> >
> > Yes, inverse CHECKSUM_UNNECESSARY conversion after verifying the checksum is
> > probably the way to go. Inverse, because it is the opposite of
> > __skb_gro_checksum_convert.
>
> I'm not sure if I understand what you mean here.

I mean that I agree that it's okay to convert from CHECKSUM_COMPLETE
to CHECKSUM_UNNECESSARY if the UDP checksum has been validated.

> I'd do the following
> for fraglist GRO in udp4_gro_complete:
>
>         if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
>                 if (skb->csum_level < SKB_MAX_CSUM_LEVEL)
>                         skb->csum_level++;
>         } else {
>                 skb->ip_summed = CHECKSUM_UNNECESSARY;
>                 skb->csum_level = 0;
>         }

Akin to __skb_incr_checksum_unnecessary, but applying conversion both
in the case of CHECKSUM_NONE and CHECKSUM_COMPLETE. Makes sense.


> and then copy these values to the segments after segmentation.