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

[Yunsheng Lin]

Hi, Alexander

On 2017/12/23 0:32, Alexander Duyck wrote:
> On Fri, Dec 22, 2017 at 12:49 AM, Yunsheng Lin  wrote:
>> Hi, Alexander
>>
>> On 2017/12/22 0:29, Alexander Duyck wrote:
>>> On Thu, Dec 21, 2017 at 1:16 AM, Yunsheng Lin  
>>> wrote:
 Hi, Alexander

 On 2017/12/21 0:24, Alexander Duyck wrote:
> On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  
> wrote:
>> 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):
>>
>> Sorry, a typo error here. I meant the inner header does *not* have DF flag 
>> set here.
>>

 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 0x, 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.
>>
>> For the new skb, NAPI_GRO_CB(skb)->is_atomic is indeed applied to the
>> inner-most header.
>>
>> What about the NAPI_GRO_CB(p)->is_atomic, p is the same skb flow already
>> merged by gro.
>>
>> Let me try if I understand it correctly:
>> when there is only one skb merged in p, then NAPI_GRO_CB(p)->is_atomic
>> is set according to the first skb' inner-most ip DF flags.
>>
>> When the second skb comes, and inet_gro_receive is processing the
>> outer-most ip, for the below code, NAPI_GRO_CB(p)->is_atomic
>> is for first skb's inner-most ip DF flags, and "iph->frag_off & htons(IP_DF)"
>> is for second skb' outer-most ip DF flags.
>> Why don't we use the first skb's outer-most ip DF flags here? I think it is
>> more logical to use first skb's outer-most ip DF flags here. But the first
>> skb's outer-most ip DF flags is lost when we get here, right?
>>
>> if (!NAPI_GRO_CB(p)->is_atomic ||
>> !(iph->frag_off & htons(IP_DF))) {
>> flush_id ^= 

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

[Alexander Duyck]

On Fri, Dec 22, 2017 at 12:49 AM, Yunsheng Lin  wrote:
> Hi, Alexander
>
> On 2017/12/22 0:29, Alexander Duyck wrote:
>> On Thu, Dec 21, 2017 at 1:16 AM, Yunsheng Lin  wrote:
>>> Hi, Alexander
>>>
>>> On 2017/12/21 0:24, Alexander Duyck wrote:
 On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  
 wrote:
> 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):
>
> Sorry, a typo error here. I meant the inner header does *not* have DF flag 
> set here.
>
>>>
>>> 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 0x, 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.
>
> For the new skb, NAPI_GRO_CB(skb)->is_atomic is indeed applied to the
> inner-most header.
>
> What about the NAPI_GRO_CB(p)->is_atomic, p is the same skb flow already
> merged by gro.
>
> Let me try if I understand it correctly:
> when there is only one skb merged in p, then NAPI_GRO_CB(p)->is_atomic
> is set according to the first skb' inner-most ip DF flags.
>
> When the second skb comes, and inet_gro_receive is processing the
> outer-most ip, for the below code, NAPI_GRO_CB(p)->is_atomic
> is for first skb's inner-most ip DF flags, and "iph->frag_off & htons(IP_DF)"
> is for second skb' outer-most ip DF flags.
> Why don't we use the first skb's outer-most ip DF flags here? I think it is
> more logical to use first skb's outer-most ip DF flags here. But the first
> skb's outer-most ip DF flags is lost when we get here, right?
>
> if (!NAPI_GRO_CB(p)->is_atomic ||
> !(iph->frag_off & htons(IP_DF))) {
> flush_id ^= NAPI_GRO_CB(p)->count;
> flush_id = flush_id ? 0x : 0;
> }

We already did in a way. If you look earlier up we will set flush if
the DF bit of this packet and the 

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

[Yunsheng Lin]

Hi, Alexander

On 2017/12/22 0:29, Alexander Duyck wrote:
> On Thu, Dec 21, 2017 at 1:16 AM, Yunsheng Lin  wrote:
>> Hi, Alexander
>>
>> On 2017/12/21 0:24, Alexander Duyck wrote:
>>> On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  
>>> wrote:
 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):

Sorry, a typo error here. I meant the inner header does *not* have DF flag set 
here.

>>
>> 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 0x, 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.

For the new skb, NAPI_GRO_CB(skb)->is_atomic is indeed applied to the
inner-most header.

What about the NAPI_GRO_CB(p)->is_atomic, p is the same skb flow already
merged by gro.

Let me try if I understand it correctly:
when there is only one skb merged in p, then NAPI_GRO_CB(p)->is_atomic
is set according to the first skb' inner-most ip DF flags.

When the second skb comes, and inet_gro_receive is processing the
outer-most ip, for the below code, NAPI_GRO_CB(p)->is_atomic
is for first skb's inner-most ip DF flags, and "iph->frag_off & htons(IP_DF)"
is for second skb' outer-most ip DF flags.
Why don't we use the first skb's outer-most ip DF flags here? I think it is
more logical to use first skb's outer-most ip DF flags here. But the first
skb's outer-most ip DF flags is lost when we get here, right?

if (!NAPI_GRO_CB(p)->is_atomic ||
!(iph->frag_off & htons(IP_DF))) {
flush_id ^= NAPI_GRO_CB(p)->count;
flush_id = flush_id ? 0x : 0;
}


 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 

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

[Alexander Duyck]

On Thu, Dec 21, 2017 at 1:16 AM, Yunsheng Lin  wrote:
> Hi, Alexander
>
> On 2017/12/21 0:24, Alexander Duyck wrote:
>> On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  wrote:
>>> 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 0x, 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.

>>
>> 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

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

[Yunsheng Lin]

Hi, Alexander

On 2017/12/21 0:24, Alexander Duyck wrote:
> On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  wrote:
>> 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 0x, then the second packet will not
be merged to first packet in tcp_gro_receive.


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


> 
> 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 "&="?


Thank very much for your time reqlying.
Yunsheng Lin

 I
> don't know if there has been much testing of multiple levels of tunnel
> header.
>> Thanks.
> 
> - Alex
> 
> .
> 

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

[Alexander Duyck]

On Wed, Dec 20, 2017 at 1:09 AM, Yunsheng Lin  wrote:
> 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.

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
don't know if there has been much testing of multiple levels of tunnel
header.

Thanks.

- Alex

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

[Yunsheng Lin]

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.