Re: [PATCH net-next] net: sock: adapt SOCK_MIN_RCVBUF and SOCK_MIN_SNDBUF

[Daniel Borkmann <dborkman@redhat.com>]

On 06/19/2013 11:51 AM, Eric Dumazet wrote:
> On Wed, 2013-06-19 at 11:18 +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 equal and adjusted
>> through the macro SKB_TRUESIZE(), which is also used elsewhere to adjust sk
>> buffer sizes. 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.
>>
>> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
>> ---
>>   include/net/sock.h | 11 ++++++-----
>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/net/sock.h b/include/net/sock.h
>> index ac8e181..189ef98 100644
>> --- a/include/net/sock.h
>> +++ b/include/net/sock.h
>> @@ -2045,18 +2045,19 @@ static inline void sk_wake_async(struct sock *sk, int how, int band)
>>   		sock_wake_async(sk->sk_socket, how, band);
>>   }
>>
>> -#define SOCK_MIN_SNDBUF 2048
>>   /*
>> - * Since sk_rmem_alloc sums skb->truesize, even a small frame might need
>> - * sizeof(sk_buff) + MTU + padding, unless net driver perform copybreak
>> + * Since sk_{r,w}mem_alloc sums skb->truesize, even a small frame might
>> + * need sizeof(sk_buff) + sizeof(skb_shared_info) + MTU + padding, unless
>> + * net driver perform copybreak.
>>    */
>> -#define SOCK_MIN_RCVBUF (2048 + sizeof(struct sk_buff))
>> +#define SOCK_MIN_RCVBUF		SKB_TRUESIZE(2048)
>> +#define SOCK_MIN_SNDBUF		SKB_TRUESIZE(2048)
>>
>>
>>   static inline void sk_stream_moderate_sndbuf(struct sock *sk)
>>   {
>>   	if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK)) {
>>   		sk->sk_sndbuf = min(sk->sk_sndbuf, sk->sk_wmem_queued >> 1);
>> -		sk->sk_sndbuf = max(sk->sk_sndbuf, SOCK_MIN_SNDBUF);
>> +		sk->sk_sndbuf = max_t(u32, sk->sk_sndbuf, SOCK_MIN_SNDBUF);
>>   	}
>>   }
>>
>
> Funny you send this patch, because I prepared a similar patch
> yesterday ;)

Hehe, that is indeed funny. :-)

> My motivation was a bit different, because we hit a (small) regression
> here in Google for some applications setting low SO_SNDBUF/SO_RCVBUF
> values, because of new TCP needs :
>
> Minimal skb truesize in transmit path is indeed SKB_TRUESIZE(2048) after
> commit f07d960df33c5aef ("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.
>
> So I believe that we need :
>
> /* TCP works better if we can build two skbs at minimum */
> #define SOCK_MIN_SNDBUF		(2 * SKB_TRUESIZE(2048))

Ok, if you prefer, I can send an update.

Thanks,

Daniel