[PATCH net-next v4 3/4] vsock/virtio: coalesce RX write-space notifications in lock batches

[Jia Jia <physicalmtea@gmail.com>]

Each received packet updates peer credit and may call sk_write_space().
A writer cannot use that credit until the batch releases the socket lock,
so one notification at batch finish is sufficient.

Save the initial sk_write_space callback when the AF_VSOCK socket is
created and require it to remain installed for batching. This lets the
transport module deliver the deferred notification without referring to
the unexported default callback. Sockets with replaced callbacks retain
their per-packet behavior.

Record the batch socket before processing its first packet so a batch that
ends on that packet cannot lose the pending notification.

Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
 include/linux/virtio_vsock.h            |  1 +
 include/net/af_vsock.h                  |  5 +++
 net/vmw_vsock/af_vsock.c                |  1 +
 net/vmw_vsock/virtio_transport_common.c | 53 +++++++++++++++++--------
 4 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index 26dde6909c4ed..bf077ee0ded88 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -287,6 +287,7 @@ struct virtio_transport_rx_batch {
 	struct sock *sk;
 	unsigned int pkts;
 	size_t bytes;
+	bool write_space_pending;
 };
 
 void virtio_transport_recv_pkt_batch(struct virtio_transport *t,
diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 5549298c1ec6b..9a40960d10cbe 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -63,6 +63,11 @@ struct vsock_sock {
 	u32 peer_shutdown;
 	bool sent_request;
 	bool ignore_connecting_rst;
+	/*
+	 * Initial callback, used to identify replacements and deliver deferred
+	 * write-space notifications.
+	 */
+	void (*default_write_space)(struct sock *sk);
 
 	/* Protected by lock_sock(sk) */
 	u64 buffer_size;
diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 44cee74519555..fe7f49da6c4bc 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -958,6 +958,7 @@ static struct sock *__vsock_create(struct net *net,
 		sk->sk_type = type;
 
 	vsk = vsock_sk(sk);
+	vsk->default_write_space = sk->sk_write_space;
 	vsock_addr_init(&vsk->local_addr, VMADDR_CID_ANY, VMADDR_PORT_ANY);
 	vsock_addr_init(&vsk->remote_addr, VMADDR_CID_ANY, VMADDR_PORT_ANY);
 
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index ff74e805527a5..f4b5f0230493e 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1820,6 +1820,7 @@ virtio_transport_recv_pkt_batchable(struct virtio_transport *t,
 	return sk->sk_state == TCP_ESTABLISHED &&
 	       sk->sk_type == SOCK_STREAM &&
 	       READ_ONCE(sk->sk_prot) == sk->sk_prot_creator &&
+	       READ_ONCE(sk->sk_write_space) == vsk->default_write_space &&
 	       !sock_flag(sk, SOCK_DONE) &&
 	       vsk->transport == &t->transport &&
 	       vvs && virtio_transport_rx_skb_has_headroom(vvs) &&
@@ -1828,13 +1829,15 @@ virtio_transport_recv_pkt_batchable(struct virtio_transport *t,
 
 /*
  * The caller holds sk's socket lock.  Set @batchable if the socket can remain
- * locked for another ordinary STREAM/RW packet.  Return true if the caller
- * must free @skb.
+ * locked for another ordinary STREAM/RW packet.  When @batch is non-NULL,
+ * record a default write-space notification for batch finish instead of
+ * delivering it immediately.  Return true if the caller must free @skb.
  */
 static bool
 virtio_transport_recv_pkt_locked(struct virtio_transport *t,
 				 struct sk_buff *skb, struct sock *sk,
-				 bool *batchable)
+				 bool *batchable,
+				 struct virtio_transport_rx_batch *batch)
 {
 	struct virtio_vsock_hdr *hdr = virtio_vsock_hdr(skb);
 	struct sockaddr_vm src;
@@ -1863,8 +1866,12 @@ virtio_transport_recv_pkt_locked(struct virtio_transport *t,
 	if (vsk->local_addr.svm_cid != VMADDR_CID_ANY)
 		vsk->local_addr.svm_cid = le64_to_cpu(hdr->dst_cid);
 
-	if (space_available)
-		sk->sk_write_space(sk);
+	if (space_available) {
+		if (batch)
+			batch->write_space_pending = true;
+		else
+			sk->sk_write_space(sk);
+	}
 
 	switch (sk->sk_state) {
 	case TCP_LISTEN:
@@ -1931,7 +1938,7 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 	}
 
 	lock_sock(sk);
-	free_pkt = virtio_transport_recv_pkt_locked(t, skb, sk, NULL);
+	free_pkt = virtio_transport_recv_pkt_locked(t, skb, sk, NULL, NULL);
 	release_sock(sk);
 
 	/* Release refcnt obtained when we fetched this socket out of the
@@ -1949,22 +1956,27 @@ EXPORT_SYMBOL_GPL(virtio_transport_recv_pkt);
 
 /*
  * Finish the RX batch.
- * For a non-empty batch, the caller must hold the socket lock acquired with
- * lock_sock(). This function releases the lock and the batch's lookup
- * reference.
+ * For a non-empty batch, the caller holds the socket lock acquired with
+ * lock_sock(). Deliver any deferred write-space notification while the lock
+ * is held, then release the lock and the batch's lookup reference.
  * An empty batch is a no-op.
  */
 void virtio_transport_rx_batch_finish(struct virtio_transport_rx_batch *batch)
 {
+	bool write_space_pending = batch->write_space_pending;
 	struct sock *sk = batch->sk;
 
 	batch->sk = NULL;
 	batch->pkts = 0;
 	batch->bytes = 0;
+	batch->write_space_pending = false;
 
 	if (!sk)
 		return;
 
+	if (write_space_pending)
+		vsock_sk(sk)->default_write_space(sk);
+
 	release_sock(sk);
 	sock_put(sk);
 }
@@ -2019,7 +2031,8 @@ void virtio_transport_recv_pkt_batch(struct virtio_transport *t,
 			}
 
 			free_pkt = virtio_transport_recv_pkt_locked(t, skb, sk,
-								    &batchable);
+								    &batchable,
+								    batch);
 			if (!batchable)
 				virtio_transport_rx_batch_finish(batch);
 			if (free_pkt)
@@ -2059,23 +2072,31 @@ void virtio_transport_recv_pkt_batch(struct virtio_transport *t,
 
 	lock_sock(sk);
 	/*
-	 * Sockmap removal restores the native protocol under sk_callback_lock.
-	 * Serialize this initial eligibility check with that update.
+	 * Sockmap teardown restores callbacks under sk_callback_lock. Keep the
+	 * initial eligibility check consistent with that update.
 	 */
 	read_lock_bh(&sk->sk_callback_lock);
 	start_batch = virtio_transport_recv_pkt_batchable(t, sk);
 	read_unlock_bh(&sk->sk_callback_lock);
 
+	if (start_batch)
+		batch->sk = sk;
+
 	free_pkt = virtio_transport_recv_pkt_locked(t, skb, sk,
-						    start_batch ? &batchable : NULL);
+						    start_batch ? &batchable : NULL,
+						    start_batch ? batch : NULL);
 	if (start_batch && batchable) {
 		/* Keep the lookup reference until the batch is released. */
-		batch->sk = sk;
 		return;
 	}
 
-	release_sock(sk);
-	sock_put(sk);
+	if (start_batch) {
+		virtio_transport_rx_batch_finish(batch);
+	} else {
+		release_sock(sk);
+		sock_put(sk);
+	}
+
 	if (free_pkt)
 		kfree_skb(skb);
 }
-- 
2.53.0