Re: [PATCH net,v2] net: sock: adapt SOCK_MIN_RCVBUF and SOCK_MIN_SNDBUF

[Eric Dumazet <eric.dumazet@gmail.com>]

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@gmail.com>
> Signed-off-by: Daniel Borkmann <dborkman@redhat.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@google.com>