Re: [Xen-devel] compound skb frag pages appearing in start_xmit

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Tue, 2012-10-09 at 15:40 +0100, Ian Campbell wrote:
> On Tue, 2012-10-09 at 15:27 +0100, Eric Dumazet wrote:
> > On Tue, 2012-10-09 at 15:17 +0100, Ian Campbell wrote:
> > 
> > > Does the higher order pages effectively reduce the number of frags which
> > > are in use? e.g if MAX_SKB_FRAGS is 16, then for order-0 pages you could
> > > have 64K worth of frag data.
> > > 
> > > If we switch to order-3 pages everywhere then can the skb contain 512K
> > > of data, or does the effective maximum number of frags in an skb reduce
> > > to 2?
> > 
> > effective number of frags reduce to 2 or 3
> > 
> > (We still limit GSO packets to ~63536 bytes)
> 
> Great! Then I think the fix is more/less trivial...
> 
> As an aside, when the skb head is < 4096 bytes is that necessarily a
> compound page or might it just be a large kmalloc area?
> 

skb->head can be either allocated by kmalloc() (standard alloc_skb()) or
a page frag (if allocated in rx path)

Not sure its related to headlen/size...




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