Re: Non-linear SKBs

From: Kristian Evensen
Date: Sat Oct 13 2007 - 10:49:19 EST


Reading through my mail again, I see that I was a bit unclear. What I want to achieve is to share a frag between to skbs (where one has no earlier referance to it). Sorry.

kristrev@xxxxxxxxxxxxxxxxxxxxx wrote:
If the underlying device can do scatter-gather and checksumming,
the TCP code builds outgoing packets by copying user date into
full system pages, and then attaching those pages into the SKB.
The protocol headers sit under the skb->data linear area, and
the user data mostly sits in the user pages under
skb_shinfo(skb)->frags[]

This increases the density of data packed into the memory allocated
compared to using skb->data for it. It also enormously simplifies
the logic necessary to support TSO.

Thank you very much, I think I am starting to get it now and coming to
think of it this will make my patch much more elegant. I have spent the
day reading more code, and am wondering if something along the likes of
this piece of code will do what I want ("copy" the data from the next skb
in the retransmission queue into this skb):

//Do preliminary checks to see if the "new" packet will be within mss,
that this_skb->nr_frags + next_skb->nr_frags < MAX_SKB_FRAGS and so on

int i;
int this_frags = this_skb->nr_frags;

for(i=0; i<next_skb->nr_frags; i++)
//Does the "copy"
this_skb->frags[this_frags + i] = next_skb->frags[i];

this_skb->data_len += next_skb->data_len;
this_skb->truesize += next_skb->data_len;
this_skb->nr_frags += next_skb->nr_frags;

//Update TSO?

By the way, am I correct in my assumption that one SKB's frags is stored
linearly in the frags-array? Or have I made a horrible misunderstanding?
:)

One of the things that I have yet to understand is the frag_list in the
skb_shared_info-struct. Does this contain all skb's that "use" this frag
and works as a sort of referance counter (frag won't be removed until the
variable is NULL and I have to append this_skb to the list), or is it
something else?

Thanks again for all help.

-Kristian


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