On Fri, May 07, 2021 at 09:50:03AM -0400, Willem de Bruijn wrote: > On Fri, May 7, 2021 at 4:25 AM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote: > > > > On Thu, May 06, 2021 at 09:53:45PM -0400, Willem de Bruijn wrote: > > > On Thu, May 6, 2021 at 9:45 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: > > > > > > > > On 2021/5/7 9:25, Willem de Bruijn wrote: > > > > >>>> head_skb's data_len is the sum of skb_gro_len for each skb of the frags. > > > > >>>> data_len could be 8 if server sent a small size packet and it is GROed > > > > >>>> to head_skb. > > > > >>>> > > > > >>>> Please let me know if I am missing something. > > > > >>> > > > > >>> This is my understanding of the data path. This is a forwarding path > > > > >>> for TCP traffic. > > > > >>> > > > > >>> GRO is enabled and will coalesce multiple segments into a single large > > > > >>> packet. In bad cases, the coalesced packet payload is > MSS, but < MSS > > > > >>> + 20. > > > > >>> > > > > >>> Somewhere between GRO and GSO you have a BPF program that converts the > > > > >>> IPv6 address to IPv4. > > > > >> > > > > >> Your understanding is right. The data path is GRO -> BPF 6 to 4 -> > > > > >> GSO. > > > > >> > > > > >>> > > > > >>> There is no concept of head_skb at the time of this BPF program. It is > > > > >>> a single SKB, with an skb linear part and multiple data items in the > > > > >>> frags (no frag_list). > > > > >> > > > > >> Sorry for the confusion. head_skb what I mentioned was a skb linear > > > > >> part. I'm considering a single SKB with frags too. > > > > >> > > > > >>> > > > > >>> When entering the GSO stack, this single skb now has a payload length > > > > >>> < MSS. So it would just make a valid TCP packet on its own? > > > > >>> > > > > >>> skb_gro_len is only relevant inside the GRO stack. It internally casts > > > > >>> the skb->cb[] to NAPI_GRO_CB. This field is a scratch area that may be > > > > >>> reused for other purposes later by other layers of the datapath. It is > > > > >>> not safe to read this inside bpf_skb_proto_6_to_4. > > > > >> > > > > >> The condition what I made uses skb->data_len not skb_gro_len. Does > > > > >> skb->data_len have a different meaning on each layer? As I know, > > > > >> data_len indicates the amount of frags or frag_list. skb->data_len > > > > >> should be > 20 in the sample case because the payload size of the skb > > > > >> linear part is the same with mss. > > > > > > > > > > Ah, got it. > > > > > > > > > > data_len is the length of the skb minus the length in the skb linear > > > > > section (as seen in skb_headlen). > > > > > > > > > > So this gso skb consists of two segments, the first one entirely > > > > > linear, the payload of the second is in skb_shinfo(skb)->frags[0]. > > > > > > > > > > It is not guaranteed that gso skbs built from two individual skbs end > > > > > up looking like that. Only protocol headers in the linear segment and > > > > > the payload of both in frags is common. > > > > > > > > > >> We can modify netif_needs_gso as another option to hit > > > > >> skb_needs_linearize in validate_xmit_skb. But I think we should compare > > > > >> skb->gso_size and skb->data_len too to check if mss exceed a payload > > > > >> size. > > > > > > > > > > The rest of the stack does not build such gso packets with payload len > > > > > < mss, so we should not have to add workarounds in the gso hot path > > > > > for this. > > > > > > > > > > Also no need to linearize this skb. I think that if the bpf program > > > > > would just clear the gso type, the packet would be sent correctly. > > > > > Unless I'm missing something. > > > > > > > > Does the checksum/len field in ip and tcp/udp header need adjusting > > > > before clearing gso type as the packet has became bigger? > > > > > > gro takes care of this. see for instance inet_gro_complete for updates > > > to the ip header. > > > > I think clearing the gso type will get an error at tcp4_gso_segment > > because netif_needs_gso returns true in validate_xmit_skb. > > Oh right. Whether a packet is gso is defined by gso_size being > non-zero, not by gso_type. > > > > > > > > Also, instead of testing skb->data_len, may test the skb->len? > > > > > > > > skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff > > > > > > Yes. Essentially doing the same calculation as the gso code that is > > > causing the packet to be dropped. > > > > BPF program is usually out of control. Can we take a general approach? > > The below 2 cases has no issue when mss upgrading. > > 1) skb->data_len > mss + 20 > > 2) skb->data_len < mss && skb->data_len > 20 > > The corner case is when > > 3) skb->data_len > mss && skb->data_len < mss + 20 > > Again, you cannot use skb->data_len alone to make inferences about the > size of the second packet. This approach is oriented a general way that does not make inferences about the size of the second packet. We can obviously increase the mss size when 1) skb->data_len > mss + 20 The issue will be fixed even if we consider the #1 condition. But there is a precondition that mss < skb payload. If skb->data_len < mss then skb_headlen(skb) contains the size of mss. So, we can check the #2 condition too. 2) skb->data_len < mss && skb->data_len > 20 > > > > > But to cover #3 case, we should check the condition Yunsheng Lin said. > > What if we do mss upgrading for both #1 and #2 cases only? > > > > + unsigned short off_len = skb->data_len > shinfo->gso_size ? > > + shinfo->gso_size : 0; > > [...] > > /* Due to IPv4 header, MSS can be upgraded. */ > > - skb_increase_gso_size(shinfo, len_diff); > > + if (skb->data_len - off_len > len_diff) > > + skb_increase_gso_size(shinfo, len_diff); > > That generates TCP packets with different MSS within the same stream. > > My suggestion remains to just not change MSS at all. But this has to > be a new flag to avoid changing established behavior. I don't understand why the mss size should be kept in GSO step. Will there be any issue with different mss? In general, upgrading mss make sense when 6 to 4. The new flag would be set by user to not change mss. What happened if user does not set the flag? I still think we should fix the issue with a general approach. Or can we remove the skb_increase_gso_size line?
