Re: [QUESTION] Doubt about NAPI_GRO_CB(skb)->is_atomic in tcpv4 gro process

On Thu, Dec 21, 2017 at 1:16 AM, Yunsheng Lin [off-list ref] wrote:

Hi, Alexander

On 2017/12/21 0:24, Alexander Duyck wrote:

quoted

On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin [off-list ref] wrote:

quoted

Hi, all
        I have some doubt about NAPI_GRO_CB(skb)->is_atomic when
analyzing the tcpv4 gro process:

Firstly we set NAPI_GRO_CB(skb)->is_atomic to 1 in dev_gro_receive:
https://elixir.free-electrons.com/linux/v4.15-rc4/source/net/core/dev.c#L4838

And then in inet_gro_receive, we check the NAPI_GRO_CB(skb)->is_atomic
before setting NAPI_GRO_CB(skb)->is_atomic according to IP_DF bit in the ip header:
https://elixir.free-electrons.com/linux/v4.15-rc4/source/net/ipv4/af_inet.c#L1319

struct sk_buff **inet_gro_receive(struct sk_buff **head, struct sk_buff *skb)
{
.....................
        for (p = *head; p; p = p->next) {
........................

                /* If the previous IP ID value was based on an atomic
                 * datagram we can overwrite the value and ignore it.
                 */
                if (NAPI_GRO_CB(skb)->is_atomic)                      //we check it here
                        NAPI_GRO_CB(p)->flush_id = flush_id;
                else
                        NAPI_GRO_CB(p)->flush_id |= flush_id;
        }

        NAPI_GRO_CB(skb)->is_atomic = !!(iph->frag_off & htons(IP_DF));  //we set it here
        NAPI_GRO_CB(skb)->flush |= flush;
        skb_set_network_header(skb, off);
................................
}

My question is whether we should check the NAPI_GRO_CB(skb)->is_atomic or NAPI_GRO_CB(p)->is_atomic?
If we should check NAPI_GRO_CB(skb)->is_atomic, then maybe it is unnecessary because it is alway true.
If we should check NAPI_GRO_CB(p)->is_atomic, maybe there is a bug here.

So what is the logic here? I am just start analyzing the gro, maybe I miss something obvious here.

The logic there is to address the multiple IP header case where there
are 2 or more IP headers due to things like VXLAN or GRE tunnels. So
what will happen is that an outer IP header will end up being sent
with DF not set and will clear the is_atomic value then we want to OR
in the next header that is applied. It defaults to assignment on
is_atomic because the first IP header will encounter flush_id with no
previous configuration occupying it.

I see your point now.

But for the same flow of tunnels packet, the outer and inner ip header must
have the same fixed id or increment id?

For example, if we have a flow of tunnels packet which has fixed id in outer
header and increment id in inner header(the inner header does have DF flag set):

1. For the first packet, NAPI_GRO_CB(skb)->is_atomic will be set to zero when
inet_gro_receive is processing the inner ip header.

2. For the second packet, when inet_gro_receive is processing the outer ip header
which has a fixed id, NAPI_GRO_CB(p)->is_atomic is zero according to [1], so
NAPI_GRO_CB(p)->flush_id will be set to 0xFFFF, then the second packet will not
be merged to first packet in tcp_gro_receive.

I'm not sure how valid your case here is. The is_atomic is only really
meant to apply to the inner-most header. In the case of TCP the
inner-most header should almost always have the DF bit set which means
the inner-most is almost always atomic.

I thought outer ip header could have a fixed id while inner ip header could
have a increment id. Do I miss something here?

You have it backwards. The innermost will have DF bit set so it can be
fixed, the outer-most will in many cases not since it is usually UDP
and as such it will likely need to increment.

quoted

The part I am not sure about is if we should be using assignment for
is_atomic or using an "&=" to clear the bit and leave it cleared.

I am not sure I understood you here. is_atomic is a bit field, why do you
want to use "&="?

Actually that was my mind kind of wandering. It has been a while since
I looked at this code and the use of &= wouldn't be appropriate since
is_atomic should only apply to the innermost header.

Basically the only acceptable combinations for is_atomic and flush_id
are false with 0, or true with 1. We can't have a fixed outer header
value if DF is not set.

Hope that helps to clarify things.

- Alex