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

[Eric Dumazet <erdnetdev@gmail.com>]

On Wed, 2013-06-19 at 02:51 -0700, 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 ;)
> 
> 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))

Or more exactly :