Re: [PATCH][RFC 23/23]: Support for zero-copy TCP transmit of userspace data

[Jeremy Fitzhardinge]

Herbert Xu wrote:
> On Fri, Dec 19, 2008 at 05:56:11PM -0800, Jeremy Fitzhardinge wrote:
>   
> > Evgeniy Polyakov wrote:
> >     
> > > Shared info is freed when there are no skbs referring to the shared info
> > > in question. Skb holds all pages in shared info in the fraglist array,
> > > so when it is about to be freed, it means that network stack does not
> > > use it (particulary it will putpage every page in fraglist). Usually
> > > there are two skbs in the network stack per packet in TCP (allocated at
> > > once though via fastclone mechanims): one is provided to the device
> > > (and will be freed there) and another one is placed into retransmit
> > > queue, where it will be located and freed when ack has been received.
> > >
> > > There may be another layers which may clone skb, but its shared info
> > > structure (shared between the clones) will only be freed when all users
> > > freed appropriate cloned skbs.
> > >       
>
> This is all correct.  However, please note that that if any clone
> does a pskb_expand_head then it will get its own private copy of
> of the shared info.  So you can't use the shared info to ref count
> the pages in it.

Ah, so the lifetime of the shared_info structure doesn't match the 
lifetime of the underlying pages, and this mechanism would be 
insufficient for my purposes?  If so, how can it be solved?

Thanks,
   J
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