Re: [Xen-devel] [PATCH] xen/netfront: handle compound page fragments on transmit

On Tue, 2012-11-20 at 15:28 +0000, Eric Dumazet wrote:

On Tue, 2012-11-20 at 15:06 +0000, Ian Campbell wrote:

quoted

In practice no because of the property that the number of pages backing
the frags is <= MAX_SKB_FRAGS even if you are using compound pages as
the frags.

Yes, but you can make this test trigger with some hacks from userland
(since the frag allocator is per task instead of per socket), so you
should remove the dump_stack() ?

Best way would be to count exact number of slots.

This could be something like 48 slots for a single skb

(if each frag is 4098 (1+4096+1)bytes, only the last one is around 4000
bytes)

MAX_SKB_FRAGS is really number of frags, while your driver needs a count
of 'order-0' 'frames'

The use of MAX_SKB_FRAGS is a bit misleading here, it's really the max
number of slots which the other end will be willing to receive as a
single frame (in the Ethernet sense), as defined by the PV protocol. It
happens to be the same as MAX_SKB_FRAGS (or it is at least
MAX_SKB_FRAGS, I'm not too sure).

I'll nuke the dump_stack() though -- it's not clear what sort of useful
context it would contain anyway.

Ian.