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

From: Vladislav Bolkhovitin <[email-masked]>
Date: Tue, 23 Dec 2008 22:16:25 +0300

Evgeniy Polyakov, on 12/20/2008 01:32 PM wrote:
> On Sat, Dec 20, 2008 at 07:10:45PM +1100, Herbert Xu ([email-masked]) wrote:
>>> Hm.  So if I get a destructor call from the shared_info, can I go an  
>>> inspect the page refcounts to see if its really the last use?
>> The pages that were originally in the shared_info at creation
>> time may no longer be there by the time it's freed because of
>> pskb_pull_tail.
> 
> Things should work fine, since pskb_expand_head() copies whole shared
> info structure (and thus will copy destructor), get all pages and then
> copy all pointers into the new skb, and then release old skb's data.
> 
> So destructor for the pages should not rely on which skb it is called on
> and check if pages are about to be really freed (i.e. check theirs
> reference counter).
> 
> __pskb_pull_tail() is tricky, it just puts some pages it does not want
> to be present in the skb, but it could be possible to add there
> destructor callback from the original skb with partial flag (or just
> having destructor with two parameters: skb and page, and if page is not
> NULL, then actually only given page is freed, otherwise the whole skb).

Actually, there's another way, which seems to be a lot simpler. Alexey 
Kuznetsov privately suggested it to me.

In skb_shared_info new pointer transaction_token would be added, which 
would point on:

struct sk_transaction_token
{
	atomic_t			io_count;
	struct sk_transaction_token	*next;
	unsigned long			token;
	unsigned long			private;
	void				(*finish_callback)(struct sk_transaction_token *);
};

When skb is translated, transaction_token inherited. If 2 skb are merged 
(the same places where I put net_get_page's in my patch), the *older* 
token is inherited. This is the main point of this idea.

Before starting new asynchronous send a client would open a new token. 
Everything sent then would receive that token. Finish_callback() would 
be called and the corresponding token freed, when io_count == 0 *AND* 
all previous tokens closed.

This idea seems to be simpler, than even what Rusty implemented. Correct 
me, if I wrong. But, unfortunately, in the near future I will have no 
time to develop it.. :-(

Vlad