Subject: Re: [PATCH][RFC] network splice receive

From: Jens Axboe <[email-masked]>
Date: Tue, 12 Jun 2007 14:40:05 +0200

On Tue, Jun 12 2007, Evgeniy Polyakov wrote:
> > > > > and putting cloned skb into private field instead of 
> > > > > original on in spd_fill_page() ends up without kernel hung.
> > > > 
> > > > Why? Seems pointless to allocate a clone just to hold on to the skb, a
> > > > reference should be equally good. I would not be opposed to doing it
> > > > this way, I just don't see what a clone buys us as compared to just
> > > > holding that reference to the skb.
> > > 
> > > Receiving code does not expect shared skbs - too many fields are changed
> > > with assumptions that it is a private copy.
> > 
> > Actually the main problem is that tcp_read_sock() unconditionally frees
> > the skb, so it wouldn't help if we grabbed a reference to it. I've yet
> > to receive an explanation of why it does so, seem awkward and violates
> > the whole principle of reference counted objects. Davem??
> > 
> > So for now, skb_splice_bits() clones the incoming skb to avoid that. I'd
> > hope we can get rid of that by fixing tcp_read_sock(), though.
> 
> It does that because it knows, that skb is not allowed to be shared
> there. Similar things are being done in udp for example - code changes
> internal mebers of skb, since it knows skb is not shared.
> 
> For example generic_make_request() is not allowed to change, say, 
> bio->bi_sector or bi_destructor, since it does not own a block request, 
> not matter what bi_cnt is. From another side, ->bi_destructor() can do
> whatever it wants with bio without any check for its reference counter.

But generic_make_request() DOES change ->bi_sector, that's how partition
remapping works :-). The destructor can of course do whatever it wants,
by definition the bio is not referenced at that point (or it would not
have been called). So while I think your analogy is quite poor, I do now
follow the code (even if I think it's ugly). There's quite a big
difference between changing parts of the elements of a structure to just
grabbing a reference to it. If the skb cannot be referenced, skb_get()
should return NULL.

But that aside, I see the issue. I'll just stick to the clone, it works
fine as-is (well almost, there's a leak there, but functionally it's
ok!).

-- 
Jens Axboe