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

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Message-ID: <willemdebruijn.kernel.d5fd7a312fe9@gmail.com>
Date: Mon, 15 Sep 2025 20:32:55 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Richard Gobert <richardbgobert@...il.com>, 
 netdev@...r.kernel.org, 
 pabeni@...hat.com, 
 ecree.xilinx@...il.com, 
 willemdebruijn.kernel@...il.com
Cc: davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 horms@...nel.org, 
 corbet@....net, 
 saeedm@...dia.com, 
 tariqt@...dia.com, 
 mbloch@...dia.com, 
 leon@...nel.org, 
 dsahern@...nel.org, 
 ncardwell@...gle.com, 
 kuniyu@...gle.com, 
 shuah@...nel.org, 
 sdf@...ichev.me, 
 aleksander.lobakin@...el.com, 
 florian.fainelli@...adcom.com, 
 alexander.duyck@...il.com, 
 linux-kernel@...r.kernel.org, 
 linux-net-drivers@....com, 
 Richard Gobert <richardbgobert@...il.com>
Subject: Re: [PATCH net-next v5 4/5] net: gro: remove unnecessary df checks

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?

> 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?

> Clean up the code by removing the unnecessary don't-fragment checks.
> 
> Signed-off-by: Richard Gobert <richardbgobert@...il.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
>