Subject: Re: [PATCH 1/4 net-next] net: allow skb->head to be a page fragment

From: Li Yu <[email-masked]>
Date: Fri, 09 Nov 2012 10:31:11 +0800

于 2012年11月08日 21:30, Eric Dumazet 写道:
> On Thu, 2012-11-08 at 15:46 +0800, Li Yu wrote:
>
>> I think that all kmalloced() objects are continuous on both virtual
>> and physical address space, so such page fragment converting should be
>> safe until we use slab allocator to free them.
>>
>
> Its not safe at all. Good luck with coping with SLAB, SLUB, SLOB, and
> various debugging facilities on top of them.
>

The slab class API semantic should not change among them. the debugging
facilities should only add or modify metadata around user bytes too.
However, I also decided giving up use kmalloced() objects since it is 
hard to free them safely them without some extra efforts after doing
page fragmentation on them.

I think that adding skb->frag_head indeed is a better choice, this means 
I have to rework some NIC drivers :(

>> I ask this question since I am working on a patch to try to directly
>> forward the data from rx queue of a socket to tx queue of another
>> socket. We noticed that the splice itself still has visible costs when
>> forwarding small messages, and hope such direct forwarding optimization
>> can improve this, this idea comes from TCP friends.
>
> If not using splice, you directly transfert one skb coming from one
> side, to the other side, so you dont care skb->head being a frag or not.
>
> If messages are small, there is probably no hope trying to coalesce them
> (or else fixing the application would be much easier)
>

This patch is hoped that can work well for big messages too, so I
selected moving page fragment, and I think this should be easier a bit
because I can directly reuse tcp_sendpage() interface or just need a bit 
of changes on it :)

If it can work well, I will send it out.

Thans very much for your time, :)

Yu