[ovs-dev] [PATCH v7] datapath: Add Stateless TCP Tunneling protocol.

[Jesse Gross]

On Wed, Apr 22, 2015 at 8:22 PM, Pravin Shelar <pshelar at nicira.com> wrote:
> On Wed, Apr 22, 2015 at 7:48 PM, Jesse Gross <jesse at nicira.com> wrote:
>> On Wed, Apr 22, 2015 at 3:38 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>> On Wed, Apr 22, 2015 at 3:24 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>> On Wed, Apr 22, 2015 at 3:19 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>> On Wed, Apr 22, 2015 at 11:31 AM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>> On Wed, Apr 22, 2015 at 11:09 AM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>> On Wed, Apr 22, 2015 at 10:52 AM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>> On Wed, Apr 22, 2015 at 10:03 AM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>> On Mon, Apr 20, 2015 at 9:54 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>> On Mon, Apr 20, 2015 at 7:33 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>> On Mon, Apr 20, 2015 at 5:56 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:54 PM, Pravin Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:36 PM, Jesse Gross <jesse at nicira.com> wrote:
>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 1:29 PM, Pravin B Shelar <pshelar at nicira.com> wrote:
>>>>>>>>>>>>>>> diff --git a/datapath/linux/compat/stt.c b/datapath/linux/compat/stt.c
>>>>>>>>>>>>>>> new file mode 100644
>>>>>>>>>>>>>>> index 0000000..209bf1a
>>>>>>>>>>>>>>> --- /dev/null
>>>>>>>>>>>>>>> +++ b/datapath/linux/compat/stt.c
>>>>>>>>>>>>>>> +static void update_headers(struct sk_buff *skb, bool head,
>>>>>>>>>>>>>>> +                              unsigned int l4_offset, unsigned int hdr_len,
>>>>>>>>>>>>>>> +                              bool ipv4, u32 tcp_seq)
>>>>>>>>>>>>>> [...]
>>>>>>>>>>>>>>> +       skb->truesize = SKB_TRUESIZE(skb_end_offset(skb)) + skb->data_len;
>>>>>>>>>>>>>>> +}
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I wonder if there are any possible edge cases with resetting truesize
>>>>>>>>>>>>>> where the packet is still in someone's transmit queue (such as if we
>>>>>>>>>>>>>> are looping back packet). Do we need to orphan it first?
>>>>>>>>>>>>>>
>>>>>>>>>>>>> ok, I will orphan it in update_headers.
>>>>>>>>>>>>
>>>>>>>>>>>> Just to clarify - I was mostly just thinking aloud on orphaning it.
>>>>>>>>>>>> I'm not totally sure if that is the right thing to do or if this is
>>>>>>>>>>>> the right place to do it. I'm not sure what the conceptual
>>>>>>>>>>>> justification would be for it and it could potentially result in the
>>>>>>>>>>>> sender's buffers not being properly limited. Perhaps not resetting the
>>>>>>>>>>>> truesize is the right thing too...
>>>>>>>>>>>>
>>>>>>>>>>> I have seen warning msg if we do no keep truesize update along with
>>>>>>>>>>> changes to skb.
>>>>>>>>>>
>>>>>>>>>> Hmm, interesting, what is the warning? I don't think that I have seen
>>>>>>>>>> that before.
>>>>>>>>>
>>>>>>>>> Actually skb_try_coalesce() is updating it correctly. so there no need
>>>>>>>>> to change truesize anymore. I will update patch accordingly.
>>>>>>>>
>>>>>>>> That's much nicer. I also checked and other receive side code (like
>>>>>>>> TCP input) doesn't worry about the case where a local sender may still
>>>>>>>> be accounting for the packet since any type of loopback device does
>>>>>>>> call skb_orphan() in some form.
>>>>>>>>
>>>>>>>> I hate to bring this up but what about on transmit? In cases where we
>>>>>>>> merge or split skbs (skb_try_coalesce() and normalize_frag_list()
>>>>>>>> respectively) we do track the truesize for correctly for the result
>>>>>>>> but the individual pieces might not have the right destructors or
>>>>>>>> might not have their truesize updated for the destructor they do have.
>>>>>>>
>>>>>>> How about about we update merged skb stats (len, data_len, truesize)
>>>>>>> according to *delta_truesize we get from skb_try_coalesce() and then
>>>>>>> free the skb?
>>>>>>
>>>>>> I think that would work for the skb_try_coalesce() case (although I
>>>>>> would only worry about truesize, not the lengths). For
>>>>>> normalize_frag_list() I think we would either have to add a destructor
>>>>>> or not update truesize. I'm not sure that the condition where we have
>>>>>> frag_lists and a destructor actually ever happens in practice though.
>>>>>
>>>>> I am not sure why do we need a destructor for normalize_frag_list
>>>>> changes. Even with the changes in the function truesize is consistent
>>>>> for given skb memory usage.
>>>>
>>>> If you have a packet with a frag_list, all of the memory for the
>>>> individual elements will be accounted for in the truesize in the top
>>>> level skb. This skb could also be accounted to some socket and have a
>>>> destructor. When we break apart the list, we remove the truesize from
>>>> the head because the memory is now part of individual packets.
>>>> However, the destructor is presumably only on the head and so only its
>>>> memory will be removed from the socket accounting when it is freed but
>>>> not each of the other skbs that came from it.
>>>
>>> ok. In that case we can not have our own destructor since there is one
>>> already (I am not sure if we can use skb->cb to restore original). not
>>> changing true size can complicate skb coalesce, since it does update
>>> truesize. Easy option would be orphan skb if we are going to coalesce
>>> its fragments.
>>
>> I'm not sure that we need our own destructor. What do you think about
>> just replicating the original destructor onto each of the newly
>> generated skbs?
>>
> In that case we assume there is no state associated with the skb, that
> might not be always true.

What state do you mean? If you mean a destructor on the individual
skbs, I think that is already true because only the top level
destructor will get called when the original skb is freed.