On Fri, 2008-12-12 at 22:25 +0300, Vladislav Bolkhovitin wrote:

quoted

One thing that leaps immediately to mind is that you could isolate this
to the net layer by putting it in skb_frag_struct.  However, such a move
would require a proper API for this in net ...

To have net_priv analog in skb was the first idea I was tried. But I 
quickly gave up, because it would required that all the pages in each 
skb_frag_struct be from the same originator, i.e. with the same 
net_priv. It is unpractical to change all the operations with skb's to 
forbid merging them, if they have different net_priv. There are too many 
such places in very not obvious code pieces.

Actually, I said carry in skb_frag_struct not skb ... that allows for
merging of skbs with different page sources.  The API changes would have
to allow setting at this level.

quoted

right now it looks like
you're using the struct page addition to carry this information from
SCSI to net, which is a bit of a layering violation.

I don't think there is any layering violation here. Just lower layer 
notifies upper layer that transmission of a page has finished. It's done 
a bit not straightforward, but still basically the same as, for 
instance, on_free_cmd() callbacks which SCST core uses to notify target 
drivers and dev handlers that the corresponding command is about to be 
freed, so they can free associated with it data as well.

The way you transmit the information you want notification is done by a
private tag in struct page, so you're carrying the information on an
object that belongs to neither layer ... that's the violation.  It's
essentially an extension of the net API that goes via the mm layer.

James