[PATCH net-next v3] tls: drop duplicate check_app_limited in tls_push_sg

[Geliang Tang <geliang@kernel.org>]

From: Geliang Tang <tanggeliang@kylinos.cn>

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, tls_push_sg(), 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
tls_push_sg() for KTLS 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 <tanggeliang@kylinos.cn>
---
Note:

The patch benefits my ongoing "MPTCP KTLS support" work [1]. In patch 9
of that series, I had defined a check_app_limited() interface in struct
tls_prot_ops, but this interface is unnecessary if we can drop this
redundant call.

v3:
 - send this patch separately to net-next as Jakub suggested.
 - update the commit log.

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 KTLS support
https://lore.kernel.org/netdev/cover.1782123118.git.tanggeliang@kylinos.cn/
---
 net/tls/tls_main.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/tls/tls_main.c b/net/tls/tls_main.c
index fbb274287aa5..8bdb78718a8d 100644
--- a/net/tls/tls_main.c
+++ b/net/tls/tls_main.c
@@ -185,8 +185,6 @@ int tls_push_sg(struct sock *sk,
 
 	ctx->splicing_pages = true;
 	while (1) {
-		/* is sending application-limited? */
-		tcp_rate_check_app_limited(sk);
 		p = sg_page(sg);
 retry:
 		bvec_set_page(&bvec, p, size, offset);
-- 
2.53.0