netdev - Re: [PATCH net,v2] net: sock: adapt SOCK_MIN_RCVBUF and SOCK_MIN

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Message-Id: <20130619.211721.1594350084165223337.davem@davemloft.net>
Date:	Wed, 19 Jun 2013 21:17:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	dborkman@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net,v2] net: sock: adapt SOCK_MIN_RCVBUF and
 SOCK_MIN_SNDBUF

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 19 Jun 2013 05:19:15 -0700

> On Wed, 2013-06-19 at 12:51 +0200, Daniel Borkmann wrote:
>> The current situation is that SOCK_MIN_RCVBUF is 2048 + sizeof(struct sk_buff))
>> while SOCK_MIN_SNDBUF is 2048. Since in both cases, skb->truesize is used for
>> sk_{r,w}mem_alloc accounting, we should have both sizes adjusted via defining a
>> TCP_SKB_MIN_TRUESIZE.
>> 
>> Further, as Eric Dumazet points out, the minimal skb truesize in transmit path is
>> SKB_TRUESIZE(2048) after commit f07d960df33c5 ("tcp: avoid frag allocation for
>> small frames"), and tcp_sendmsg() tries to limit skb size to half the congestion
>> window, meaning we try to build two skbs at minimum. Thus, having SOCK_MIN_SNDBUF
>> as 2048 can hit a small regression for some applications setting to low
>> SO_SNDBUF / SO_RCVBUF. Note that we define a TCP_SKB_MIN_TRUESIZE, because
>> SKB_TRUESIZE(2048) adds SKB_DATA_ALIGN(sizeof(struct skb_shared_info)), but in
>> case of TCP skbs, the skb_shared_info is part of the 2048 bytes allocation for
>> skb->head.
>> 
>> The minor adaption in sk_stream_moderate_sndbuf() is to silence a warning by
>> using a typed max macro, as similarly done in SOCK_MIN_RCVBUF occurences, that
>> would appear otherwise.
>> 
>> Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
>> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
>> ---
>>  v1 -> v2:
>>   - Applied Eric's feedback, fixed up commit message
>>   - Set subject to 'net' instead of 'net-next' due to the reported regression
> 
> I am fine with this patch (I already run it as a matter of fact), but
> I think its net-next material :
> Regression is not new, and concerns very pathological cases, where
> applications relied on some non documented behavior of network stack.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks guys.
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