On 08/19/2015 11:39 PM, Eric Dumazet wrote:
> On Wed, 2015-08-19 at 19:17 +0000, Grumbach, Emmanuel wrote:
>
>> Hm.. how would net/core/tso.c avoid this?
>
> Because a driver using these helpers keep around the original LSO packet
> and frees it normally at TX completion time.
>
>> I can't see anything related to truesize there.
>> Note that this work since it is guaranteed that we release the skbs in
>> order.
>>
>>>
>>> (BTW TCP packets do not have sock_wfree as destructor but tcp_wfree(),
>>> yet we want backpressure mostly for TCP stack (TCP Small Queues))
>>>
>>>
>>
>> I am not sure I follow here.
>> You want me to test:
>> if (skb_gso->destructor == tcp_wfree) ?
>
>
> Yes.
>
> Look for example at tcp_gso_segment() (called from skb_gso_segment())
>
> copy_destructor = gso_skb->destructor == tcp_wfree;
> ...
> /* Following permits TCP Small Queues to work well with GSO :
> * The callback to TCP stack will be called at the time last frag
> * is freed at TX completion, and not right now when gso_skb
> * is freed by GSO engine
> */
> if (copy_destructor) {
> swap(gso_skb->sk, skb->sk);
> swap(gso_skb->destructor, skb->destructor);
> sum_truesize += skb->truesize;
> atomic_add(sum_truesize - gso_skb->truesize,
> &skb->sk->sk_wmem_alloc);
> }
>
>
>>
>> I checked that code using iperf and saw that I don't get into this if,
>> but I (probably wrongly) assumed that other applications would set a
>> flag on the socket (forgive my ignorance) that would make this if be taken.
>
> If you do not see skb->destructor == tcp_wfree, then something is
> definitely wrong on your setup.
>
tcp_wfree isn't exported. I can change that. It will be a challenge for
backport though. Hm....
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