[PATCH bpf-next v3] bpf: drop duplicate check_app_limited in tcp_bpf_push

From: Geliang Tang <redacted>

Before commit c5c37af6ecad9 ("tcp: Convert do_tcp_sendpages() to use
MSG_SPLICE_PAGES"), do_tcp_sendpages() did not call
tcp_rate_check_app_limited() internally, so callers needed an explicit
tcp_rate_check_app_limited() to cover it. That commit replaced
do_tcp_sendpages() with direct tcp_sendmsg_locked() calls, which perform
the check on every path that queues data. The outer call became redundant
but was left in place.

The site changed here, tcp_bpf_push(), holds the socket lock and invokes
tcp_sendmsg_locked() on every iteration. The early-return paths in
tcp_sendmsg_locked() that skip tcp_rate_check_app_limited() -- the
MSG_ZEROCOPY allocation failure and MSG_FASTOPEN branches -- return
without queueing any MSG_SPLICE_PAGES data, so there is no functional
consequence from omitting the outer check.

A potential benefit of this change is that it facilitates future reuse of
tcp_bpf_push() for sockmap support in protocols beyond TCP, such as MPTCP.
Since tcp_rate_check_app_limited() is TCP-specific while sendmsg_locked()
is a generic interface in struct proto_ops, this change allows us to switch
to different protocols via sk->sk_socket->ops->sendmsg_locked() without
carrying protocol-specific assumptions.

Signed-off-by: Geliang Tang <redacted>
---
Note:

This patch was originally part of my ongoing "MPTCP sockmap support"
series [1] (patch 5). Matthieu suggested converting it to a fix and sending
it directly to netdev. Removing the redundant tcp_rate_check_app_limited()
call benefits my subsequent MPTCP work: in patch 3 of that series, I
implement an MPTCP-specific mptcp_rate_check_app_limited() function and
call it in mptcp_sendmsg_locked(). This allows me to reuse tcp_bpf_push()
by simply replacing tcp_sendmsg_locked() with

		sk->sk_socket->ops->sendmsg_locked(),

without carrying protocol-specific assumptions.

v3:
 - send this patch separately to bpf as Jakub suggested.
 - update the commit log as bot+bpf-ci suggested.

v2:
 - drop the "Fixes" tags and update the prefixes and commit logs as Jiayuan
   suggested.
 - https://patchwork.kernel.org/project/netdevbpf/cover/cover.1789469930.git.tanggeliang@kylinos.cn/

v1:
 - https://patchwork.kernel.org/project/netdevbpf/cover/cover.1789368148.git.tanggeliang@kylinos.cn/

[1]
MPTCP sockmap support
https://lore.kernel.org/mptcp/b5f9e8d7-b738-1df6-3b5e-1d54cbbc663c@gmail.com/T/#t (local)
---
 net/ipv4/tcp_bpf.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index 2e234d155b5e..d5fcf3ce4861 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c

@@ -108,7 +108,6 @@ static int tcp_bpf_push(struct sock *sk, struct sk_msg *msg, u32 apply_bytes,
 		off  = sge->offset;
 		page = sg_page(sge);
 
-		tcp_rate_check_app_limited(sk);
 retry:
 		msghdr.msg_flags = flags | MSG_SPLICE_PAGES;
 		has_tx_ulp = tls_sw_has_ctx_tx(sk);

-- 
2.53.0