netdev - Re: optimizations to sk_buff handling in rds_tcp_data

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Message-ID: <1460332480.6473.536.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Sun, 10 Apr 2016 16:54:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	netdev@...r.kernel.org
Subject: Re: optimizations to sk_buff handling in rds_tcp_data_ready

On Sat, 2016-04-09 at 20:18 -0400, Sowmini Varadhan wrote:
> On (04/07/16 07:16), Eric Dumazet wrote:
> > Use skb split like TCP in output path ?
> > Really, pskb_expand_head() is not supposed to copy payload ;)
> 
> Question- how come skb_split doesnt have to deal with frag_list
> and do a skb_walk_frags()? Couldn't the split-line be somewhere
> in the frag_list? Also even for the skb_split_inside_header,
> dont we have to set
>   skb_shinfo(skb1)->frag_list = skb_shinfo(skb)->frag_list;
> and cut loose the skb_shinfo(skb)->frag_list?
> 
> As I try to mimic skb_split in some new set of "skb_carve"
> funtions, I'm running into all  the various frag_list cases. I'm 
> afraid I might end up needing most of the stuff under the "Pure 
> masohism" (sic) comment in __pskb_pull_tail(). 
> 

The helper was written for TCP I guess. TCP in output path never uses
skb_shinfo(skb)->frag_list : It is guaranteed to be NULL for all skbs.