[RFC PATCH v2 0/3] send credit update during setting SO_RCVLOWAT

[RFC PATCH v2 0/3] send credit update during setting SO_RCVLOWAT

[Arseniy Krasnov]

Hello,

                               DESCRIPTION

This patchset fixes old problem with hungup of both rx/tx sides and adds
test for it. This happens due to non-default SO_RCVLOWAT value and
deferred credit update in virtio/vsock. Link to previous old patchset:
https://lore.kernel.org/netdev/39b2e9fd-601b-189d-39a9-914e5574524c@sberdevices.ru/

Here is what happens step by step:

                                  TEST

                            INITIAL CONDITIONS

1) Vsock buffer size is 128KB.
2) Maximum packet size is also 64KB as defined in header (yes it is
   hardcoded, just to remind about that value).
3) SO_RCVLOWAT is default, e.g. 1 byte.


                                 STEPS

            SENDER                              RECEIVER
1) sends 128KB + 1 byte in a
   single buffer. 128KB will
   be sent, but for 1 byte
   sender will wait for free
   space at peer. Sender goes
   to sleep.


2)                                     reads 64KB, credit update not sent
3)                                     sets SO_RCVLOWAT to 64KB + 1
4)                                     poll() -> wait forever, there is
                                       only 64KB available to read.

So in step 4) receiver also goes to sleep, waiting for enough data or
connection shutdown message from the sender. Idea to fix it is that rx
kicks tx side to continue transmission (and may be close connection)
when rx changes number of bytes to be woken up (e.g. SO_RCVLOWAT) and
this value is bigger than number of available bytes to read.

I've added small test for this, but not sure as it uses hardcoded value
for maximum packet length, this value is defined in kernel header and
used to control deferred credit update. And as this is not available to
userspace, I can't control test parameters correctly (if one day this
define will be changed - test may become useless). 

Head for this patchset is:
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=18de1e517ed37ebaf33e771e46faf052e966e163

Link to v1:
https://lore.kernel.org/netdev/20231108072004.1045669-1-avkrasnov@salutedevices.com/

Changelog:
 * Patchset rebased and tested on new HEAD of net-next (see hash above).
 * New patch is added as 0001 - it removes return from SO_RCVLOWAT set
   callback in 'af_vsock.c' when transport callback is set - with that
   we can set 'sk_rcvlowat' only once in 'af_vsock.c' and in future do
   not copy-paste it to every transport. It was discussed in v1.
 * See per-patch changelog after ---.

Arseniy Krasnov (3):
  vsock: update SO_RCVLOWAT setting callback
  virtio/vsock: send credit update during setting SO_RCVLOWAT
  vsock/test: SO_RCVLOWAT + deferred credit update test

 drivers/vhost/vsock.c                   |   2 +
 include/linux/virtio_vsock.h            |   1 +
 net/vmw_vsock/af_vsock.c                |   9 +-
 net/vmw_vsock/virtio_transport.c        |   2 +
 net/vmw_vsock/virtio_transport_common.c |  28 +++++
 net/vmw_vsock/vsock_loopback.c          |   2 +
 tools/testing/vsock/vsock_test.c        | 136 ++++++++++++++++++++++++
 7 files changed, 178 insertions(+), 2 deletions(-)

-- 
2.25.1

[RFC PATCH v2 1/3] vsock: update SO_RCVLOWAT setting callback

[Arseniy Krasnov]

Do not return if transport callback for SO_RCVLOWAT is set (only in
error case). In this case we don't need to set 'sk_rcvlowat' field in
each transport - only in 'vsock_set_rcvlowat()'.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
---
 net/vmw_vsock/af_vsock.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 816725af281f..af0058037f72 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -2264,8 +2264,13 @@ static int vsock_set_rcvlowat(struct sock *sk, int val)
 
 	transport = vsk->transport;
 
-	if (transport && transport->set_rcvlowat)
-		return transport->set_rcvlowat(vsk, val);
+	if (transport && transport->set_rcvlowat) {
+		int err;
+
+		err = transport->set_rcvlowat(vsk, val);
+		if (err)
+			return err;
+	}
 
 	WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 	return 0;
-- 
2.25.1

[RFC PATCH v2 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT

[Arseniy Krasnov]

Send credit update message when SO_RCVLOWAT is updated and it is bigger
than number of bytes in rx queue. It is needed, because 'poll()' will
wait until number of bytes in rx queue will be not smaller than
SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup
for tx/rx is possible: sender waits for free space and receiver is
waiting data in 'poll()'.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
---
 Changelog:
 v1 -> v2:
  * Update commit message by removing 'This patch adds XXX' manner.
  * Do not initialize 'send_update' variable - set it directly during
    first usage.

 drivers/vhost/vsock.c                   |  2 ++
 include/linux/virtio_vsock.h            |  1 +
 net/vmw_vsock/virtio_transport.c        |  2 ++
 net/vmw_vsock/virtio_transport_common.c | 28 +++++++++++++++++++++++++
 net/vmw_vsock/vsock_loopback.c          |  2 ++
 5 files changed, 35 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index f75731396b7e..ecfa5c11f5ee 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -451,6 +451,8 @@ static struct virtio_transport vhost_transport = {
 		.notify_buffer_size       = virtio_transport_notify_buffer_size,
 
 		.read_skb = virtio_transport_read_skb,
+
+		.set_rcvlowat             = virtio_transport_set_rcvlowat
 	},
 
 	.send_pkt = vhost_transport_send_pkt,
diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index ebb3ce63d64d..97dc1bebc69c 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit);
 void virtio_transport_deliver_tap_pkt(struct sk_buff *skb);
 int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list);
 int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor);
+int virtio_transport_set_rcvlowat(struct vsock_sock *vsk, int val);
 #endif /* _LINUX_VIRTIO_VSOCK_H */
diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index af5bab1acee1..cf3431189d0c 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -539,6 +539,8 @@ static struct virtio_transport virtio_transport = {
 		.notify_buffer_size       = virtio_transport_notify_buffer_size,
 
 		.read_skb = virtio_transport_read_skb,
+
+		.set_rcvlowat             = virtio_transport_set_rcvlowat
 	},
 
 	.send_pkt = virtio_transport_send_pkt,
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index f6dc896bf44c..4acee21b4350 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1684,6 +1684,34 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
 }
 EXPORT_SYMBOL_GPL(virtio_transport_read_skb);
 
+int virtio_transport_set_rcvlowat(struct vsock_sock *vsk, int val)
+{
+	struct virtio_vsock_sock *vvs = vsk->trans;
+	bool send_update;
+
+	spin_lock_bh(&vvs->rx_lock);
+
+	/* If number of available bytes is less than new
+	 * SO_RCVLOWAT value, kick sender to send more
+	 * data, because sender may sleep in its 'send()'
+	 * syscall waiting for enough space at our side.
+	 */
+	send_update = vvs->rx_bytes < val;
+
+	spin_unlock_bh(&vvs->rx_lock);
+
+	if (send_update) {
+		int err;
+
+		err = virtio_transport_send_credit_update(vsk);
+		if (err < 0)
+			return err;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(virtio_transport_set_rcvlowat);
+
 MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Asias He");
 MODULE_DESCRIPTION("common code for virtio vsock");
diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
index 048640167411..388c157f6633 100644
--- a/net/vmw_vsock/vsock_loopback.c
+++ b/net/vmw_vsock/vsock_loopback.c
@@ -98,6 +98,8 @@ static struct virtio_transport loopback_transport = {
 		.notify_buffer_size       = virtio_transport_notify_buffer_size,
 
 		.read_skb = virtio_transport_read_skb,
+
+		.set_rcvlowat             = virtio_transport_set_rcvlowat
 	},
 
 	.send_pkt = vsock_loopback_send_pkt,
-- 
2.25.1

[RFC PATCH v2 3/3] vsock/test: SO_RCVLOWAT + deferred credit update test

[Arseniy Krasnov]

Test which checks, that updating SO_RCVLOWAT value also sends credit
update message. Otherwise mutual hungup may happen when receiver didn't
send credit update and then calls 'poll()' with non default SO_RCVLOWAT
value (e.g. waiting enough bytes to read), while sender waits for free
space at receiver's side. Important thing is that this test relies on
kernel's define for maximum packet size for virtio transport and this
value is not exported to user: VIRTIO_VSOCK_MAX_PKT_BUF_SIZE (this
define is used to control moment when to send credit update message).
If this value or its usage will be changed in kernel - this test may
become useless/broken.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
---
 Changelog:
 v1 -> v2:
  * Update commit message by removing 'This patch adds XXX' manner.
  * Update commit message by adding details about dependency for this
    test from kernel internal define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.
  * Add comment for this dependency in 'vsock_test.c' where this define
    is duplicated.

 tools/testing/vsock/vsock_test.c | 136 +++++++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)

diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 5b0e93f9996c..f5623b8d76b7 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -1225,6 +1225,137 @@ static void test_double_bind_connect_client(const struct test_opts *opts)
 	}
 }
 
+#define RCVLOWAT_CREDIT_UPD_BUF_SIZE	(1024 * 128)
+/* This define is the same as in 'include/linux/virtio_vsock.h':
+ * it is used to decide when to send credit update message during
+ * reading from rx queue of a socket. Value and its usage in
+ * kernel is important for this test.
+ */
+#define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE	(1024 * 64)
+
+static void test_stream_rcvlowat_def_cred_upd_client(const struct test_opts *opts)
+{
+	size_t buf_size;
+	void *buf;
+	int fd;
+
+	fd = vsock_stream_connect(opts->peer_cid, 1234);
+	if (fd < 0) {
+		perror("connect");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Send 1 byte more than peer's buffer size. */
+	buf_size = RCVLOWAT_CREDIT_UPD_BUF_SIZE + 1;
+
+	buf = malloc(buf_size);
+	if (!buf) {
+		perror("malloc");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Wait until peer sets needed buffer size. */
+	control_expectln("SRVREADY");
+
+	if (send(fd, buf, buf_size, 0) != buf_size) {
+		perror("send failed");
+		exit(EXIT_FAILURE);
+	}
+
+	free(buf);
+	close(fd);
+}
+
+static void test_stream_rcvlowat_def_cred_upd_server(const struct test_opts *opts)
+{
+	size_t recv_buf_size;
+	struct pollfd fds;
+	size_t buf_size;
+	void *buf;
+	int fd;
+
+	fd = vsock_stream_accept(VMADDR_CID_ANY, 1234, NULL);
+	if (fd < 0) {
+		perror("accept");
+		exit(EXIT_FAILURE);
+	}
+
+	buf_size = RCVLOWAT_CREDIT_UPD_BUF_SIZE;
+
+	if (setsockopt(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+		       &buf_size, sizeof(buf_size))) {
+		perror("setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+		exit(EXIT_FAILURE);
+	}
+
+	buf = malloc(buf_size);
+	if (!buf) {
+		perror("malloc");
+		exit(EXIT_FAILURE);
+	}
+
+	control_writeln("SRVREADY");
+
+	/* Wait until there will be 128KB of data in rx queue. */
+	while (1) {
+		ssize_t res;
+
+		res = recv(fd, buf, buf_size, MSG_PEEK);
+		if (res == buf_size)
+			break;
+
+		if (res <= 0) {
+			fprintf(stderr, "unexpected 'recv()' return: %zi\n", res);
+			exit(EXIT_FAILURE);
+		}
+	}
+
+	/* There is 128KB of data in the socket's rx queue,
+	 * dequeue first 64KB, credit update is not sent.
+	 */
+	recv_buf_size = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
+	recv_buf(fd, buf, recv_buf_size, 0, recv_buf_size);
+	recv_buf_size++;
+
+	/* Updating SO_RCVLOWAT will send credit update. */
+	if (setsockopt(fd, SOL_SOCKET, SO_RCVLOWAT,
+		       &recv_buf_size, sizeof(recv_buf_size))) {
+		perror("setsockopt(SO_RCVLOWAT)");
+		exit(EXIT_FAILURE);
+	}
+
+	memset(&fds, 0, sizeof(fds));
+	fds.fd = fd;
+	fds.events = POLLIN | POLLRDNORM | POLLERR |
+		     POLLRDHUP | POLLHUP;
+
+	/* This 'poll()' will return once we receive last byte
+	 * sent by client.
+	 */
+	if (poll(&fds, 1, -1) < 0) {
+		perror("poll");
+		exit(EXIT_FAILURE);
+	}
+
+	if (fds.revents & POLLERR) {
+		fprintf(stderr, "'poll()' error\n");
+		exit(EXIT_FAILURE);
+	}
+
+	if (fds.revents & (POLLIN | POLLRDNORM)) {
+		recv_buf(fd, buf, recv_buf_size, 0, recv_buf_size);
+	} else {
+		/* These flags must be set, as there is at
+		 * least 64KB of data ready to read.
+		 */
+		fprintf(stderr, "POLLIN | POLLRDNORM expected\n");
+		exit(EXIT_FAILURE);
+	}
+
+	free(buf);
+	close(fd);
+}
+
 static struct test_case test_cases[] = {
 	{
 		.name = "SOCK_STREAM connection reset",
@@ -1335,6 +1466,11 @@ static struct test_case test_cases[] = {
 		.run_client = test_double_bind_connect_client,
 		.run_server = test_double_bind_connect_server,
 	},
+	{
+		.name = "SOCK_STREAM virtio SO_RCVLOWAT + deferred cred update",
+		.run_client = test_stream_rcvlowat_def_cred_upd_client,
+		.run_server = test_stream_rcvlowat_def_cred_upd_server,
+	},
 	{},
 };
 
-- 
2.25.1