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

Wei Liu
Tue, 12 Mar 2013 12:18:24 -0000

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 wrote some code during weekend and it seemed to work, but I have not
verify it extensively.


Wei.