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[34.145.139.141]) by smtp.gmail.com with UTF8SMTPSA id d75a77b69052e-4b639dab3e8sm84155821cf.24.2025.09.16.08.57.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Sep 2025 08:57:27 -0700 (PDT) Date: Tue, 16 Sep 2025 11:57:26 -0400 From: Willem de Bruijn To: Richard Gobert , Willem de Bruijn , netdev@vger.kernel.org, pabeni@redhat.com, ecree.xilinx@gmail.com Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, horms@kernel.org, corbet@lwn.net, saeedm@nvidia.com, tariqt@nvidia.com, mbloch@nvidia.com, leon@kernel.org, dsahern@kernel.org, ncardwell@google.com, kuniyu@google.com, shuah@kernel.org, sdf@fomichev.me, aleksander.lobakin@intel.com, florian.fainelli@broadcom.com, alexander.duyck@gmail.com, linux-kernel@vger.kernel.org, linux-net-drivers@amd.com Message-ID: In-Reply-To: <4b2c5770-ab53-43f6-8c68-7e2f4a912d8e@gmail.com> References: <20250915113933.3293-1-richardbgobert@gmail.com> <20250915113933.3293-5-richardbgobert@gmail.com> <4b2c5770-ab53-43f6-8c68-7e2f4a912d8e@gmail.com> Subject: Re: [PATCH net-next v5 4/5] net: gro: remove unnecessary df checks Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 > >> --- > >> 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 > >> > > > > >