Re: [PATCH net-next v5 4/5] net: gro: remove unnecessary df checks

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Richard Gobert wrote:
> Willem de Bruijn wrote:
> > Richard Gobert wrote:
> >> Currently, packets with fixed IDs will be merged only if their
> >> don't-fragment bit is set. This restriction is unnecessary since packets
> >> without the don't-fragment bit will be forwarded as-is even if they were
> >> merged together.
> > 
> > Please expand why this is true.
> > 
> > Because either NETIF_F_TSO_MANGLEID is set or segmentation
> > falls back onto software GSO which handles the two FIXEDID
> > variants correctly now, I guess?
> > 
> 
> This is true because the merged packets will be segmented back to
> their original forms before being forwarded. As you already said, the IDs
> will either stay identical or potentially become incrementing if MANGLEID
> is set, either of which is fine.
> 
> >> If packets are merged together and then fragmented, they will first be
> >> re-split into segments before being further fragmented, so the behavior
> >> is identical whether or not the packets were first merged together.
> > 
> > I don't follow this scenario. Fragmentation of a GSO packet after GRO
> > and before GSO?
> > 
> 
> Yes. One could worry that merging packets with the same ID but without DF
> would cause issues if they are then fragmented by the host. What I'm saying
> is that if such packets are merged and then fragmented, they will first be
> segmented back to their original forms by GSO before being further fragmented
> (see ip_finish_output_gso). The fragmentation occurs as if the packets were
> never merged to begin with.

This explicit pointer that fragmentation for such GSO packets happens
in ip_finish_output_gso, which first calls skb_gso_segment, is
informative. It again turns an assertion into an explanation.

I think you jumped the gun a bit on sending a v6 right with these
answers. I'd like these clarifications recorded.

> IOW, fragmentation occurs the same way regardless
> of whether the packets were merged (GRO + GSO is transparent). I thought I'd
> mention this to clarify why this patch doesn't cause any issues.
> 
> >> Clean up the code by removing the unnecessary don't-fragment checks.
> >>
> >> Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
> >> ---
> >>  include/net/gro.h                 | 5 ++---
> >>  net/ipv4/af_inet.c                | 3 ---
> >>  tools/testing/selftests/net/gro.c | 9 ++++-----
> >>  3 files changed, 6 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/include/net/gro.h b/include/net/gro.h
> >> index 6aa563eec3d0..f14b7e88dbef 100644
> >> --- a/include/net/gro.h
> >> +++ b/include/net/gro.h
> >> @@ -448,17 +448,16 @@ static inline int inet_gro_flush(const struct iphdr *iph, const struct iphdr *ip
> >>  	const u32 id2 = ntohl(*(__be32 *)&iph2->id);
> >>  	const u16 ipid_offset = (id >> 16) - (id2 >> 16);
> >>  	const u16 count = NAPI_GRO_CB(p)->count;
> >> -	const u32 df = id & IP_DF;
> >>  
> >>  	/* All fields must match except length and checksum. */
> >> -	if ((iph->ttl ^ iph2->ttl) | (iph->tos ^ iph2->tos) | (df ^ (id2 & IP_DF)))
> >> +	if ((iph->ttl ^ iph2->ttl) | (iph->tos ^ iph2->tos) | ((id ^ id2) & IP_DF))
> >>  		return true;
> >>  
> >>  	/* When we receive our second frame we can make a decision on if we
> >>  	 * continue this flow as an atomic flow with a fixed ID or if we use
> >>  	 * an incrementing ID.
> >>  	 */
> >> -	if (count == 1 && df && !ipid_offset)
> >> +	if (count == 1 && !ipid_offset)
> >>  		NAPI_GRO_CB(p)->ip_fixedid |= 1 << inner;
> >>  
> >>  	return ipid_offset ^ (count * !(NAPI_GRO_CB(p)->ip_fixedid & (1 << inner)));
> >> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> >> index fc7a6955fa0a..c0542d9187e2 100644
> >> --- a/net/ipv4/af_inet.c
> >> +++ b/net/ipv4/af_inet.c
> >> @@ -1393,10 +1393,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
> >>  
> >>  	segs = ERR_PTR(-EPROTONOSUPPORT);
> >>  
> >> -	/* fixed ID is invalid if DF bit is not set */
> >>  	fixedid = !!(skb_shinfo(skb)->gso_type & (SKB_GSO_TCP_FIXEDID << encap));
> >> -	if (fixedid && !(ip_hdr(skb)->frag_off & htons(IP_DF)))
> >> -		goto out;
> >>  
> >>  	if (!skb->encapsulation || encap)
> >>  		udpfrag = !!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP);
> >> diff --git a/tools/testing/selftests/net/gro.c b/tools/testing/selftests/net/gro.c
> >> index d5824eadea10..3d4a82a2607c 100644
> >> --- a/tools/testing/selftests/net/gro.c
> >> +++ b/tools/testing/selftests/net/gro.c
> >> @@ -670,7 +670,7 @@ static void send_flush_id_case(int fd, struct sockaddr_ll *daddr, int tcase)
> >>  		iph2->id = htons(9);
> >>  		break;
> >>  
> >> -	case 3: /* DF=0, Fixed - should not coalesce */
> >> +	case 3: /* DF=0, Fixed - should coalesce */
> >>  		iph1->frag_off &= ~htons(IP_DF);
> >>  		iph1->id = htons(8);
> >>  
> >> @@ -1188,10 +1188,9 @@ static void gro_receiver(void)
> >>  			correct_payload[0] = PAYLOAD_LEN * 2;
> >>  			check_recv_pkts(rxfd, correct_payload, 1);
> >>  
> >> -			printf("DF=0, Fixed - should not coalesce: ");
> >> -			correct_payload[0] = PAYLOAD_LEN;
> >> -			correct_payload[1] = PAYLOAD_LEN;
> >> -			check_recv_pkts(rxfd, correct_payload, 2);
> >> +			printf("DF=0, Fixed - should coalesce: ");
> >> +			correct_payload[0] = PAYLOAD_LEN * 2;
> >> +			check_recv_pkts(rxfd, correct_payload, 1);
> >>  
> >>  			printf("DF=1, 2 Incrementing and one fixed - should coalesce only first 2 packets: ");
> >>  			correct_payload[0] = PAYLOAD_LEN * 2;
> >> -- 
> >> 2.36.1
> >>
> > 
> > 
>