selftests: fix OOM in msg_zerocopy selftest

In selftests/net/msg_zerocopy.c, it has a while loop keeps calling sendmsg
on a socket with MSG_ZEROCOPY flag, and it will recv the notifications
until the socket is not writable. Typically, it will start the receiving
process after around 30+ sendmsgs. However, as the introduction of commit
dfa2f048 ("tcp: get rid of sysctl_tcp_adv_win_scale"), the sender is
always writable and does not get any chance to run recv notifications.
The selftest always exits with OUT_OF_MEMORY because the memory used by
opt_skb exceeds the net.core.optmem_max. Meanwhile, it could be set to a
different value to trigger OOM on older kernels too.

Thus, we introduce "cfg_notification_limit" to force sender to receive
notifications after some number of sendmsgs.

Fixes: 07b65c5b ("test: add msg_zerocopy test")
Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
Signed-off-by: Xiaochun Lu <xiaochun.lu@bytedance.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240701225349.3395580-2-zijianzhang@bytedance.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/msg_zerocopy.c

@@ -85,6 +85,7 @@ static bool cfg_rx;
 static int  cfg_runtime_ms	= 4200;
 static int  cfg_verbose;
 static int  cfg_waittime_ms	= 500;
+static int  cfg_notification_limit = 32;
 static bool cfg_zerocopy;
 
 static socklen_t cfg_alen;
@@ -95,6 +96,7 @@ static char payload[IP_MAXPACKET];
 static long packets, bytes, completions, expected_completions;
 static int  zerocopied = -1;
 static uint32_t next_completion;
+static uint32_t sends_since_notify;
 
 static unsigned long gettimeofday_ms(void)
 {
@@ -208,6 +210,7 @@ static bool do_sendmsg(int fd, struct msghdr *msg, bool do_zerocopy, int domain)
 		error(1, errno, "send");
 	if (cfg_verbose && ret != len)
 		fprintf(stderr, "send: ret=%u != %u\n", ret, len);
+	sends_since_notify++;
 
 	if (len) {
 		packets++;
@@ -460,6 +463,7 @@ static bool do_recv_completion(int fd, int domain)
 static void do_recv_completions(int fd, int domain)
 {
 	while (do_recv_completion(fd, domain)) {}
+	sends_since_notify = 0;
 }
 
 /* Wait for all remaining completions on the errqueue */
@@ -549,6 +553,9 @@ static void do_tx(int domain, int type, int protocol)
 		else
 			do_sendmsg(fd, &msg, cfg_zerocopy, domain);
 
+		if (cfg_zerocopy && sends_since_notify >= cfg_notification_limit)
+			do_recv_completions(fd, domain);
+
 		while (!do_poll(fd, POLLOUT)) {
 			if (cfg_zerocopy)
 				do_recv_completions(fd, domain);
@@ -708,7 +715,7 @@ static void parse_opts(int argc, char **argv)
 
 	cfg_payload_len = max_payload_len;
 
-	while ((c = getopt(argc, argv, "46c:C:D:i:mp:rs:S:t:vz")) != -1) {
+	while ((c = getopt(argc, argv, "46c:C:D:i:l:mp:rs:S:t:vz")) != -1) {
 		switch (c) {
 		case '4':
 			if (cfg_family != PF_UNSPEC)
@@ -736,6 +743,9 @@ static void parse_opts(int argc, char **argv)
 			if (cfg_ifindex == 0)
 				error(1, errno, "invalid iface: %s", optarg);
 			break;
+		case 'l':
+			cfg_notification_limit = strtoul(optarg, NULL, 0);
+			break;
 		case 'm':
 			cfg_cork_mixed = true;
 			break;