On Wed, Apr 22, 2015 at 8:55 PM, Pravin Shelar <pshe...@nicira.com> wrote: > On Wed, Apr 22, 2015 at 8:50 PM, Jesse Gross <je...@nicira.com> wrote: >> On Wed, Apr 22, 2015 at 8:37 PM, Pravin Shelar <pshe...@nicira.com> wrote: >>> On Wed, Apr 22, 2015 at 8:29 PM, Jesse Gross <je...@nicira.com> wrote: >>>> On Wed, Apr 22, 2015 at 8:22 PM, Pravin Shelar <pshe...@nicira.com> wrote: >>>>> On Wed, Apr 22, 2015 at 7:48 PM, Jesse Gross <je...@nicira.com> wrote: >>>>>> On Wed, Apr 22, 2015 at 3:38 PM, Pravin Shelar <pshe...@nicira.com> >>>>>> wrote: >>>>>>> On Wed, Apr 22, 2015 at 3:24 PM, Jesse Gross <je...@nicira.com> wrote: >>>>>>>> On Wed, Apr 22, 2015 at 3:19 PM, Pravin Shelar <pshe...@nicira.com> >>>>>>>> wrote: >>>>>>>>> On Wed, Apr 22, 2015 at 11:31 AM, Jesse Gross <je...@nicira.com> >>>>>>>>> wrote: >>>>>>>>>> On Wed, Apr 22, 2015 at 11:09 AM, Pravin Shelar <pshe...@nicira.com> >>>>>>>>>> wrote: >>>>>>>>>>> On Wed, Apr 22, 2015 at 10:52 AM, Jesse Gross <je...@nicira.com> >>>>>>>>>>> wrote: >>>>>>>>>>>> On Wed, Apr 22, 2015 at 10:03 AM, Pravin Shelar >>>>>>>>>>>> <pshe...@nicira.com> wrote: >>>>>>>>>>>>> On Mon, Apr 20, 2015 at 9:54 PM, Jesse Gross <je...@nicira.com> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 7:33 PM, Pravin Shelar >>>>>>>>>>>>>> <pshe...@nicira.com> wrote: >>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 5:56 PM, Jesse Gross <je...@nicira.com> >>>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:54 PM, Pravin Shelar >>>>>>>>>>>>>>>> <pshe...@nicira.com> wrote: >>>>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 3:36 PM, Jesse Gross >>>>>>>>>>>>>>>>> <je...@nicira.com> wrote: >>>>>>>>>>>>>>>>>> On Mon, Apr 20, 2015 at 1:29 PM, Pravin B Shelar >>>>>>>>>>>>>>>>>> <pshe...@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. >>> >>> If destructor is replicated on the individual skbs then it will be >>> called for each of those skbs when it is freed. >> >> Right; if we've correctly apportioned truesize among the individual >> skbs isn't that the goal? > > It works for destructors which only care for skb-truesize. But > destructor callback also takes argument, we also need to replicate > that and we do not have enough information to do it.
I'm not quite sure what you mean by an argument (I don't see anything really in either tcp_wfree or sock_wfree) but I agree that this could be dangerous. There's no guarantee that total truesize on the head packet accurately accounts for all of the individual frag_list members (although for the main users like IP fragment reassembly this appears to be the case). I'm a little nervous about orphaning the packets in this case since it can potentially affect socket accounting and from a high level, there's no logical reason to do so (i.e. we're not crossing a boundary or something like that). I guess that in many cases (particularly on the transmit side), moving the truesize around in normalize_frag_list() and skb_try_coalesce() will effectively cancel each other out if we can merge things together (although I guess that depends on the source of the frag_list, IP fragment reassembly first tries to coalesce and then falls back on creating frag_lists, in which case us trying to merge things is a waste of time). It would be nice if we could relegate this to a corner case and just linearize in those situations. Or better yet, more deeply understand the conditions where we get the frag_list on transmit and see if it's really worth trying to coalesce them at all. grepping drivers/net I don't see a lot of frag_list creation. Even on receive, I'm not totally sure that breaking up frag_lists makes sense if skb_segment() can handle it now... _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev