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Re: [RFC net-next 10/15] ipxlat: add 4to6 pre-fragmentation path To: ralf@mandelbit.com Cc: andrew+netdev@lunn.ch, antonio@mandelbit.com, davem@davemloft.net, dxld@darkboxed.org, edumazet@google.com, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com References: <20260319151230.655687-11-ralf@mandelbit.com> In-Reply-To: <20260319151230.655687-11-ralf@mandelbit.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SEWP216CA0142.KORP216.PROD.OUTLOOK.COM (2603:1096:101:2be::10) To TYRPR01MB12666.jpnprd01.prod.outlook.com (2603:1096:405:1b4::11) X-Microsoft-Original-Message-ID: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: TYRPR01MB12666:EE_|TY4PR01MB15938:EE_ X-MS-Office365-Filtering-Correlation-Id: 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=?utf-8?B?a1J4VDJrVTE0YVpKYUNRTlZHOXVyTnhEMVo2Y2hBemhodjhCaUlBWE1ZanV6?= =?utf-8?B?dFhacWVnWEc4eUY0VjR5dzFNYW5jelI1QkNla3UvTWVJR2xaeE1RTWZtZTly?= =?utf-8?B?TldVT1RJUVJpaTFGcUI0NGwvdGUxRUJqbzNxc0QwR1poUDFkeEoxQ0k2YjFU?= =?utf-8?B?R051YVlxdmxWcVB0L1hpQkJqUFVybXpsT3RuRnpMVDdnZFZSN3BBbkE2M3lw?= =?utf-8?B?a3F3d211eEtnelJSK3BGaXpUU2tLcWJjQmU2bm5qQjNWSDRzSFR4ZkNsSGtq?= =?utf-8?B?OUtkSGRRSzF1d1ZFVXEwMnYxWFcwZW4zYW1QS1J0NlRkSU81bkNIMWY1Tm9H?= =?utf-8?B?S3I0ZDdWdVZZRmhVNjl2WVdFd0syeDZ5eXhQVmJzR1BLS2NyZkE3d0owRUhP?= =?utf-8?B?YnkzeEZSSGs0RHFYRityQStkaktid25nVzFsaTBPWnJWRmtEVHZUR3dERHRp?= =?utf-8?B?WDJRU0ZoSWlnPT0=?= X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: dcc7106f-a3c4-4b45-aaef-08deb4da1959 X-MS-Exchange-CrossTenant-AuthSource: TYRPR01MB12666.jpnprd01.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 May 2026 12:36:38.5236 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: TY4PR01MB15938 Hi Ralf, >+int ipxlat_46_plan_prefrag(struct ipxlat_priv *ipxlat, struct sk_buff *skb) >+{ >+ unsigned int pkt_len6, pmtu6, threshold6, frag_max_size, pkt_len4, >+ old_l3_len, new_l3_len; >+ struct ipxlat_cb *cb = ipxlat_skb_cb(skb); >+ const struct iphdr *in4 = ip_hdr(skb); >+ int l3_delta, frag_l3_delta; >+ >+ if (unlikely(cb->frag_max_size)) { >+ DEBUG_NET_WARN_ON_ONCE(1); >+ cb->frag_max_size = 0; >+ } >+ >+ pkt_len4 = iph_totlen(skb, in4); >+ old_l3_len = cb->l3_hdr_len; >+ new_l3_len = sizeof(struct ipv6hdr) + >+ (ip_is_fragment(in4) ? sizeof(struct frag_hdr) : 0); >+ l3_delta = (int)new_l3_len - (int)old_l3_len; >+ pkt_len6 = pkt_len4 + l3_delta; >+ >+ pmtu6 = ipxlat_46_lookup_pmtu6(ipxlat, skb, in4); >+ threshold6 = min(pmtu6, READ_ONCE(ipxlat->lowest_ipv6_mtu)); >+ >+ if (likely(pkt_len6 <= threshold6)) >+ return 0; >+ >+ /* df packets are never locally pre-fragmented */ >+ if (likely(be16_to_cpu(in4->frag_off) & IP_DF)) { >+ /* Let the IPv6 forwarding path raise PTB when needed and rely >+ * on the reverse 6->4 ICMP translation path for feedback. >+ */ >+ return 0; >+ } How about putting the DF check before the PMTU lookup? ipxlat_46_lookup_pmtu6() requires ip6_route_output() and dst_release(), but the DF bit is much cheaper to test and is typically set on most TCP/QUIC packets. It looks like a pure reorder but I have not verified it carefully. Best regards, Xavier