Re: [PATCH v5 net-next 01/36] net: Introduce direct data placement tcp offload

[Boris Pismenny <borispismenny@gmail.com>]

On 22/07/2021 14:26, Eric Dumazet wrote:
> On Thu, Jul 22, 2021 at 1:04 PM Boris Pismenny <borisp@nvidia.com> wrote:
>>
>> From: Boris Pismenny <borisp@mellanox.com>
>>
>>
> ...
> 
>>  };
>>
>>  const char
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index e6ca5a1f3b59..4a7160bba09b 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -5149,6 +5149,9 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
>>                 memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
>>  #ifdef CONFIG_TLS_DEVICE
>>                 nskb->decrypted = skb->decrypted;
>> +#endif
>> +#ifdef CONFIG_ULP_DDP
>> +               nskb->ddp_crc = skb->ddp_crc;
> 
> Probably you do not want to attempt any collapse if skb->ddp_crc is
> set right there.
> 

Right.

>>  #endif
>>                 TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
>>                 if (list)
>> @@ -5182,6 +5185,11 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
>>  #ifdef CONFIG_TLS_DEVICE
>>                                 if (skb->decrypted != nskb->decrypted)
>>                                         goto end;
>> +#endif
>> +#ifdef CONFIG_ULP_DDP
>> +
>> +                               if (skb->ddp_crc != nskb->ddp_crc)
> 
> This checks only the second, third, and remaining skbs.
> 

Right, as we handle the head skb above. Could you clarify?

>> +                                       goto end;
>>  #endif
>>                         }
>>                 }
> 
> 
> tcp_collapse() is copying data from small skbs to pack it to bigger
> skb (one page of payload), in case
> of memory emergency/pressure (socket queues are full)
> 
> If your changes are trying to avoid 'needless'  copies, maybe you
> should reconsider and let the emergency packing be done.
> 
> If the copy is not _possible_, you should rephrase your changelog to
> clearly state the kernel _cannot_ access this memory in any way.
> 

The issue is that skb_condense also gets called on many skbs in
tcp_add_backlog and it will identify skbs that went through DDP as ideal
for packing, even though they are not small and packing is
counter-productive as data already resides in its destination.

As mentioned above, it is possible to copy, but it is counter-productive
in this case. If there was a real need to access this memory, then it is
allowed.