netdev - Re: [PATCH net-next] tcp: tcp

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Date:	Sun, 04 Dec 2011 19:27:18 +0100
From:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
To:	Joe Perches <joe@...ches.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: tcp_sendmsg() page recycling

Le 04/12/2011 18:20, Joe Perches a écrit :
> On Sun, 2011-12-04 at 18:05 +0100, Eric Dumazet wrote:
>> If our TCP_PAGE(sk) is not shared (page_count() == 1), we can set page
>> offset to 0.
> []
>> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> []
>> @@ -1009,7 +1009,12 @@ new_segment:
>>   				int merge = 0;
>>   				int i = skb_shinfo(skb)->nr_frags;
>>   				struct page *page = TCP_PAGE(sk);
>> -				int off = TCP_OFF(sk);
>> +				int off;
>> +
>> +				if (page&&  page_count(page) == 1)
>> +					TCP_OFF(sk) = 0;
>> +
>> +				off = TCP_OFF(sk);
>
> This might be clearer and take one less indirection as
>
> 				if (page&&  page_count(page) == 1) {
> 					TCP_OFF(sk) = 0;
> 					off = 0;
> 				} else {
> 					off = 0;
> 				}
>
> or maybe
>
> 				if (page&&  page_count(page) == 1)
> 					off = TCP_OFF(sk) = 0;
> 				else
> 					off = 0;

Well, those two fragments don't look equivalent to the original one, unless TCP_OFF(sk) happens to 
be 0 before the test. But if this is true, then the whole thing seems useless. Do I miss something?

For as far as I understand, the following should be equivalent to Eric's original, but I don't 
consider it better than the original.

  				if (page&&  page_count(page) == 1)
  					off = TCP_OFF(sk) = 0;
  				else
  					off = TCP_OFF(sk);

	Nicolas.
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