Commit eea86af6 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by David S. Miller
Browse files

net: sock: adapt SOCK_MIN_RCVBUF and SOCK_MIN_SNDBUF

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

Further, as Eric Dumazet points out, the minimal skb truesize in transmit path is
SKB_TRUESIZE(2048) after commit f07d960d

 ("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

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: default avatarEric Dumazet <>
Signed-off-by: default avatarDaniel Borkmann <>
Signed-off-by: default avatarEric Dumazet <>
Signed-off-by: default avatarDavid S. Miller <>
parent dc25c676
......@@ -2047,18 +2047,21 @@ 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) + MTU + padding, unless net driver perform copybreak.
* Note: for send buffers, TCP works better if we can build two skbs at
* minimum.
#define SOCK_MIN_RCVBUF (2048 + sizeof(struct sk_buff))
#define TCP_SKB_MIN_TRUESIZE (2048 + sizeof(struct sk_buff))
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);
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