Re: [Xen-devel] Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, 2013-03-13 at 06:22 +0000, annie li wrote:
> On 2013-3-12 23:25, Wei Liu wrote:
> > On Tue, 2013-03-12 at 15:07 +0000, ANNIE LI wrote:
> >> On 2013-3-12 20:18, Wei Liu wrote:
> >>> On Tue, 2013-03-12 at 11:40 +0000, Ian Campbell wrote:
> >>>> On Sun, 2013-03-10 at 19:18 +0000, Wei Liu wrote:
> >>>>> On Sat, Mar 09, 2013 at 02:57:16AM +0000, Ian Campbell wrote:
> >>>>>>>> - change MAX_SKB_FRAGS to 19 to accommodate all guests
> >>>>>> Changing MAX_SKB_FRAGS is *not* an option upstream. This might be a
> >>>>>> useful local hack but we need to drop the idea as a long term fix.
> >>>>>>
> >>>>>>> Ugh. The negotiations between host and guest is probably the best
> >>>>>>> choice. The issues you are going to hit are that you might need
> >>>>>>> to redo the skbs to match what the frontend's max is.
> >>>>>> IMHO the right fix is for netback to coalesce as it copies from the
> >>>>>> frontend if it needs to do so, it is copying anyway so it should be
> >>>>>> cheap enough. I thought we had discussed this and someone was working 
> >>>>>> on
> >>>>>> implementing it. If not Annie then perhaps it was Matt or Siva (both 
> >>>>>> now
> >>>>>> CC'd)
> >>>>>>
> >>>>> As a short term fix, can we use skb_linearize() if skb->nr_frags>=
> >>>>> MAX_SKB_FRAGS?
> >>>> No, because that would require changing the frontend, while this fix
> >>>> needs to be in the backend if older guests are to continue working.
> >>>>
> >>>> You can't use skb_linearize in netback as is because you would first
> >>>> need to be able to build the skb with nr_frags>= MAX_SKB_FRAGS in order
> >>>> to pass it to that function.
> >>>>
> >>> Yes, the idea is to define NETBK_MAX_SKB_FRAGS to some bigger number
> >>> (say 20) to accommodate the possible maximum number of frags in
> >>> frontend. The thing that truly matters it the skb->len, which should be
> >>> <64K, nr_frags is not important.
> >> I doubt this would work since you can not build out skb with nr_frags >=
> >> MAX_SKB_FRAGS. See following code in skbuff.h,
> >>
> >> #if (65536/PAGE_SIZE + 1) < 16
> >> #define MAX_SKB_FRAGS 16UL
> >> #else
> >> #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
> >> #endif
> >>
> >> and every skb contains MAX_SKB_FRAGS frags,
> >>
> >> struct skb_shared_info {
> >> ....
> >> skb_frag_t      frags[MAX_SKB_FRAGS];
> >> ...
> >> }
> >>
> > So I haven't gone this far to trigger this. :-(
> >
> > I just double checked with printk, nr_frags has not exceeded backend
> > MAX_SBK_FRAGS even if I have frontend MAX_SKB_FRAGS = 25.
> 
> You mean you changed MAX_SKB_FRAGS to 25 in skbuff.h? Did your frontend 

Yes. In frontend's kernel.

> send out skbs with 25 frags?
> 

No. The maximum number I see is 17.

I didn't spend much time on this though. It's just a little experiment
during my weekend.


Wei.



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