[PATCH net] tcp: update window_clamp when SO_RCVBUF is set

Jakub Kicinski
2026-04-08T00:14:46Z

Commit under Fixes moved recomputing the window clamp to
tcp_measure_rcv_mss() (when scaling_ratio changes).
I suspect it missed the fact that we don't recompute the clamp
when rcvbuf is set. Until scaling_ratio changes we are
stuck with the old window clamp which may be based on
the small initial buffer. scaling_ratio may never change.

Inspired by Eric's recent commit d1361840f8c5 ("tcp: fix
SO_RCVLOWAT and RCVBUF autotuning") plumb the user action
thru to TCP and have it update the clamp.

A smaller fix would be to just have tcp_rcvbuf_grow()
adjust the clamp even if SOCK_RCVBUF_LOCK is set.
But IIUC this is what we were trying to get away from
in the first place.

Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: ncardwell@google.com
CC: kuniyu@google.com
CC: willemb@google.com
CC: dsahern@kernel.org
CC: quic_subashab@quicinc.com
CC: quic_stranche@quicinc.com
---
include/linux/net.h | 1 +
include/net/tcp.h | 1 +
net/core/sock.c | 9 +++++++++
net/ipv4/af_inet.c | 1 +
net/ipv4/tcp.c | 5 +++++
net/ipv6/af_inet6.c | 1 +
6 files changed, 18 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index a8e818de95b3..ca6a7bc5c9ae 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -223,6 +223,7 @@ struct proto_ops {
int (*sendmsg_locked)(struct sock *sk, struct msghdr *msg,
size_t size);
int (*set_rcvlowat)(struct sock *sk, int val);
+ void (*set_rcvbuf)(struct sock *sk, int val);
};

#define DECLARE_SOCKADDR(type, dst, src) \
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6156d1d068e1..b9db447892dd 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -516,6 +516,7 @@ void tcp_syn_ack_timeout(const struct request_sock *req);
int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
int flags);
int tcp_set_rcvlowat(struct sock *sk, int val);
+void tcp_set_rcvbuf(struct sock *sk, int val);
int tcp_set_window_clamp(struct sock *sk, int val);

static inline void
diff --git a/net/core/sock.c b/net/core/sock.c
index fdaf66e6dc18..f3a186376bc5 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -973,6 +973,8 @@ EXPORT_SYMBOL(sock_set_keepalive);

static void __sock_set_rcvbuf(struct sock *sk, int val)
{
+ struct socket *sock = sk->sk_socket;
+
/* Ensure val * 2 fits into an int, to prevent max_t() from treating it
* as a negative value.
*/
@@ -990,6 +992,13 @@ static void __sock_set_rcvbuf(struct sock *sk, int val)
* we actually used in getsockopt is the most desirable behavior.
*/
WRITE_ONCE(sk->sk_rcvbuf, max_t(int, val * 2, SOCK_MIN_RCVBUF));
+
+ if (sock) {
+ const struct proto_ops *ops = READ_ONCE(sock->ops);
+
+ if (ops->set_rcvbuf)
+ ops->set_rcvbuf(sk, sk->sk_rcvbuf);
+ }
}

void sock_set_rcvbuf(struct sock *sk, int val)
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index f98e46ae3e30..0e62032e76b1 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1091,6 +1091,7 @@ const struct proto_ops inet_stream_ops = {
.compat_ioctl = inet_compat_ioctl,
#endif
.set_rcvlowat = tcp_set_rcvlowat,
+ .set_rcvbuf = tcp_set_rcvbuf,
};
EXPORT_SYMBOL(inet_stream_ops);

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e57eaffc007a..1a494d18c5fd 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1858,6 +1858,11 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
return 0;
}

+void tcp_set_rcvbuf(struct sock *sk, int val)
+{
+ tcp_set_window_clamp(sk, tcp_win_from_space(sk, val));
+}
+
#ifdef CONFIG_MMU
static const struct vm_operations_struct tcp_vm_ops = {
};
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index ee341a8254bf..0a88b376141d 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -690,6 +690,7 @@ const struct proto_ops inet6_stream_ops = {
.compat_ioctl = inet6_compat_ioctl,
#endif
.set_rcvlowat = tcp_set_rcvlowat,
+ .set_rcvbuf = tcp_set_rcvbuf,
};
EXPORT_SYMBOL_GPL(inet6_stream_ops);

--
2.53.0