Re: Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

Ian Campbell
Tue, 12 Mar 2013 11:40:34 -0000

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.

I''d worry about failing the higher order allocations needed to
linearize
in this way too -- better to coalesce as much as possible into single
page frags.

Ian.