netdev - Re: [PATCH net v2 3/4] udp: do not transition UDP fraglist to unnecessary checksum

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Date: Thu, 21 Mar 2024 09:48:57 +0100
From: Antoine Tenart <atenart@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc: steffen.klassert@...unet.com, willemdebruijn.kernel@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 3/4] udp: do not transition UDP fraglist to unnecessary checksum

Quoting Willem de Bruijn (2024-03-20 21:43:55)
> Antoine Tenart wrote:
> > Quoting Willem de Bruijn (2024-03-20 14:00:48)
> > > Antoine Tenart wrote:
> > > > Quoting Willem de Bruijn (2024-03-19 14:38:20)
> > > > > 
> > > > > The original patch converted to CHECKSUM_UNNECESSARY for a reason.
> > > > > The skb->csum of the main gso_skb is not valid?
> > > > > 
> > > > > Should instead only the csum_level be adjusted, to always keep
> > > > > csum_level == 0?
> > > > 
> > > > The above trace is an ICMPv6 packet being tunneled and GROed at the UDP
> > > > level, thus we have:
> > > >   UDP(CHECKSUM_PARTIAL)/Geneve/ICMPv6(was CHECKSUM_NONE)
> > > > csum_level would need to be 1 here; but we can't know that.
> > > 
> > > Is this a packet looped internally? Else it is not CHECKSUM_PARTIAL.
> > 
> > I'm not sure to follow, CHECKSUM_NONE packets going in a tunnel will be
> > encapsulated and the outer UDP header will be CHECKSUM_PARTIAL. The
> > packet can be looped internally or going to a remote host.
> 
> That is on transmit. To come into contact with UDP_GRO while having
> CHECKSUM_PARTIAL the packet will have to loop into the receive path,
> in some way that triggers GRO. Perhaps through gro_cells, as other
> GRO paths are hardware NIC drivers.

I get what you meant now, thanks. Yes, those Tx packets loop into the Rx
path. One easy way is through veth pairs, eg. packet get tunneled in a
netns, connected to another one via a veth pair.

> > > > There is another issue (no kernel trace): if a packet has partial csum
> > > > and is being GROed that information is lost and the packet ends up with
> > > > an invalid csum.
> > > 
> > > CHECKSUM_PARTIAL should be converted to CHECKSUM_UNNECESSARY for this
> > > reason. CHECKSUM_PARTIAL implies the header is prepared with pseudo
> > > header checksum. Similarly CHECKSUM_COMPLETE implies skb csum is valid.
> > > CHECKSUM_UNNECESSARY has neither expectations.
> > 
> > But not if the packet is sent to a remote host. Otherwise an inner
> > partial csum is never fixed by the stack/NIC before going out.
> 
> The stack will only offload a single checksum. With local checksum
> offload, this can be the inner checksum and the outer can be cheaply
> computed in software. udp_set_csum() handles this. It indeed sets lco
> if the inner packet has CHECKSUM_PARTIAL. Otherwise it sets ip_summed
> to CHECKSUM_PARTIAL, now pointing to the outer UDP header.
> 
> You're right. Regardless of whether it points to the inner or outer
> checksum, a conversion of CHECKSUM_PARTIAL to CHECKSUM_UNNECESSARY
> will break checksum offload in the forwarding case.
> 
> > > > Packets with CHECKSUM_UNNECESSARY should end up with the same info. My
> > > > impression is this checksum conversion is at best setting the same info
> > > > and otherwise is overriding valuable csum information.
> > > > 
> > > > Or would packets with CSUM_NONE being GROed would benefit from the
> > > > CHECKSUM_UNNECESSARY conversion?
> > > 
> > > Definitely. If the packet has CHECKSUM_NONE and GRO checks its
> > > validity in software, converting it to CHECKSUM_UNNECESSARY avoids
> > > potential additional checks at later stages in the packet path.
> > 
> > Makes sense. The current code really looks like
> > __skb_incr_checksum_unnecessary, w/o the CHECKSUM_NONE check to only
> > convert those packets.

If I sum up our discussion CHECKSUM_NONE conversion is wanted,
CHECKSUM_UNNECESSARY conversion is a no-op and CHECKSUM_PARTIAL
conversion breaks things. What about we just convert CHECKSUM_NONE to
CHECKSUM_UNNECESSARY?

diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index 50a8a65fad23..44779d4c538b 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -174,7 +174,7 @@ INDIRECT_CALLABLE_SCOPE int udp6_gro_complete(struct sk_buff *skb, int nhoff)
                if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
                        if (skb->csum_level < SKB_MAX_CSUM_LEVEL)
                                skb->csum_level++;
-               } else {
+               } else if (skb->ip_summed == CHECKSUM_NONE) {
                        skb->ip_summed = CHECKSUM_UNNECESSARY;
                        skb->csum_level = 0;
                }

Or directly call __skb_incr_checksum_unnecessary.

Thanks,
Antoine