Re: [PATCH V2] xen/netfront: handle compound page fragments on transmit

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2012-11-21 at 12:08 +0000, Ian Campbell wrote:
> The max-frag related limitation comes from the "wire" protocol used
> between front and back. As it stands either the frontend or the backend
> is more than likely going to drop the sort of pathalogical skbs you are
> worried.
> 
> I agree that this absolutely needs to be fixed in the protocol (and I've
> posted a call to arms on this topic on xen-devel) but I'd like to do it
> in a coordinated manner as part of a protocol extension (where the front
> and backend negotiate the maximum number of order-0 pages per Ethernet
> frame they are willing to handle) rather than as a side effect of this
> patch.
> 
> So right now I don't want to introduce frontends which default to
> sending increased numbers of pages in to the wild, since that makes
> things more complex when we come to extend the protocol.
> 
> Perhaps in the short term doing an skb_linearize when we hit this case
> would help, that will turn the pathalogical skb into a much more normal
> one. It'll be expensive but it should be rare. That assumes you can
> linearize such a large skb, which depends on the ability to allocate
> large order pages which isn't a given. Herm, maybe that doesn't work
> then.
> 

First of all, thanks a lot for all these detailed informations.

This now makes sense !


> AFAIK we don't have an existing skb_foo operation which copies an skb,
> including (or only) the frags, with the side effect of aligning and
> coalescing them. Do we?
> 

No, we only have the full linearize helper, and skb_try_coalesce()
helpers.

TCP stack uses an internal function to collapse several skbs so skbs
using a single page, I guess we could generalize this and make it
available to other uses.

Thanks !