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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8a4665df063sm12659367b3.41.2026.09.23.09.50.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Sep 2026 09:50:10 -0700 (PDT) Date: Wed, 23 Sep 2026 12:50:09 -0400 From: Willem de Bruijn To: Wang Zhan , netdev@vger.kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, keyong.sun@smartx.com, Ilya Maximets , Aaron Conole , Eelco Chaudron , dev@openvswitch.org, Andrew Lunn , Jason Wang , Willem de Bruijn , Neal Cardwell , Kuniyuki Iwashima , Alice Mikityanska , Wang Zhan Message-ID: In-Reply-To: <20260918084651.3022878-3-wang.zhan@smartx.com> References: <20260918084651.3022878-1-wang.zhan@smartx.com> <20260918084651.3022878-3-wang.zhan@smartx.com> Subject: Re: [PATCH net-next v2 2/4] net: gso: support bounded TCP segmentation 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 Wang Zhan wrote: > The bounded resegmentation added by the next patch splits an oversized TCP > GSO skb into several GSO skbs which fit the device limits. That needs the > GSO engine to group several MSS segments into one output skb, so let > callers bound the number of MSS segments each output skb carries and pass > the bound through the existing __skb_gso_segment() entry point. Ordinary > callers use zero for no limit. > > skb_segment() only groups several MSS into one output skb when the device > advertises NETIF_F_GSO_PARTIAL, or when the skb has a frag_list which can > be split into uniform pieces, and falls back to one segment per skb > otherwise. A caller which passes a bound asks for that grouping > regardless, so the frag_list check is skipped when max_segs is set. Every > other caller keeps it, and the bounded path is only used for skbs which do > not carry a frag_list. > > The output stays a GSO skb: gso_size is the original MSS and gso_segs is > the number of MSS it holds, so a downstream device can still perform > ordinary TSO. Store the bound in the existing skb_gso_cb scratch context, > alongside the call-local data_offset and mac_offset fields, so that the > segmentation methods keep their signature. A zero max_segs value means > that no bound is active; it is not a persistent skb flag. Clear the value > when each output skb copies the input header so the temporary limit is not > propagated to the next GSO call. > > Assisted-by: LLM > Signed-off-by: Wang Zhan > @@ -117,6 +119,7 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb, > > SKB_GSO_CB(skb)->mac_offset = skb_headroom(skb); > SKB_GSO_CB(skb)->encap_level = 0; > + SKB_GSO_CB(skb)->max_segs = min_t(unsigned int, max_segs, U16_MAX); Can this use GSO_MAX_SEGS rather than U16_MAX. > > skb_reset_mac_header(skb); > skb_reset_mac_len(skb); > diff --git a/net/core/skbuff.c b/net/core/skbuff.c > index dbbe10277d51d..9c0d140236bc6 100644 > --- a/net/core/skbuff.c > +++ b/net/core/skbuff.c > @@ -4793,6 +4793,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > struct sk_buff *segs = NULL; > struct sk_buff *tail = NULL; > struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list; > + unsigned int max_segs = SKB_GSO_CB(head_skb)->max_segs; > unsigned int mss = skb_shinfo(head_skb)->gso_size; > bool gso_by_frags = mss == GSO_BY_FRAGS; > unsigned int doffset = head_skb->data - skb_mac_header(head_skb); > @@ -4839,7 +4840,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > csum = !!can_checksum_protocol(features, proto); > > if (sg && csum && !gso_by_frags) { > - if (!(features & NETIF_F_GSO_PARTIAL)) { > + if (!max_segs && !(features & NETIF_F_GSO_PARTIAL)) { > struct sk_buff *iter; > unsigned int frag_len; > > @@ -4874,7 +4875,10 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > * now. > */ > DEBUG_NET_WARN_ON_ONCE(len / mss > GSO_MAX_SEGS); > - partial_segs = min(len / mss, GSO_MAX_SEGS); > + if (max_segs) > + partial_segs = min(len / mss, max_segs); > + else > + partial_segs = min(len / mss, GSO_MAX_SEGS); > if (partial_segs > 1) > mss *= partial_segs; > else > @@ -4975,6 +4979,12 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb, > > __copy_skb_header(nskb, head_skb); > > + /* > + * max_segs is a per-call limit, so output skbs must not > + * inherit it from the input skb. > + */ > + SKB_GSO_CB(nskb)->max_segs = 0; > + cb has to be treated as uninitialized between layers. All callers of __skb_gso_segment should (re)initialize this field. Is it necessary to clear it here. Note that we do not do that for any other cb fields.