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Message-ID: <bc57fe7d-2865-47d5-8add-cb57787d9000@gmail.com>
Date: Thu, 18 Sep 2025 13:59:14 +0200
From: Richard Gobert <richardbgobert@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
netdev@...r.kernel.org, pabeni@...hat.com, ecree.xilinx@...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
Subject: Re: [PATCH net-next v5 4/5] net: gro: remove unnecessary df checks
Willem de Bruijn wrote:
> 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.
>
Sorry about that. I'll explicitly mention ip_finish_output_gso in the commit
message.
>> 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@...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
>>>>
>>>
>>>
>>
>
>
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