tap/tun: add skb_zcopy_init() helper for initialization.

Replace direct assignments with skb_zcopy_init() for zerocopy
cases where a new skb is initialized, without changing the
reference counts.
Signed-off-by: NJonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

drivers/net/tap.c

@@ -722,8 +722,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 	tap = rcu_dereference(q->tap);
 	/* copy skb_ubuf_info for callback when skb has no error */
 	if (zerocopy) {
-		skb_shinfo(skb)->destructor_arg = msg_control;
-		skb_shinfo(skb)->flags |= SKBFL_ZEROCOPY_FRAG;
+		skb_zcopy_init(skb, msg_control);
 	} else if (msg_control) {
 		struct ubuf_info *uarg = msg_control;
 		uarg->callback(NULL, uarg, false);

drivers/net/tun.c

@@ -1814,8 +1814,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 
 	/* copy skb_ubuf_info for callback when skb has no error */
 	if (zerocopy) {
-		skb_shinfo(skb)->destructor_arg = msg_control;
-		skb_shinfo(skb)->flags |= SKBFL_ZEROCOPY_FRAG;
+		skb_zcopy_init(skb, msg_control);
 	} else if (msg_control) {
 		struct ubuf_info *uarg = msg_control;
 		uarg->callback(NULL, uarg, false);

drivers/vhost/net.c

@@ -903,6 +903,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
 			ubuf->callback = vhost_zerocopy_callback;
 			ubuf->ctx = nvq->ubufs;
 			ubuf->desc = nvq->upend_idx;
+			ubuf->flags = SKBFL_ZEROCOPY_FRAG;
 			refcount_set(&ubuf->refcnt, 1);
 			msg.msg_control = &ctl;
 			ctl.type = TUN_MSG_UBUF;

include/linux/skbuff.h

@@ -1448,6 +1448,12 @@ static inline void skb_zcopy_get(struct ubuf_info *uarg)
 	refcount_inc(&uarg->refcnt);
 }
 
+static inline void skb_zcopy_init(struct sk_buff *skb, struct ubuf_info *uarg)
+{
+	skb_shinfo(skb)->destructor_arg = uarg;
+	skb_shinfo(skb)->flags |= uarg->flags;
+}
+
 static inline void skb_zcopy_set(struct sk_buff *skb, struct ubuf_info *uarg,
 				 bool *have_ref)
 {
@@ -1456,8 +1462,7 @@ static inline void skb_zcopy_set(struct sk_buff *skb, struct ubuf_info *uarg,
 			*have_ref = false;
 		else
 			skb_zcopy_get(uarg);
-		skb_shinfo(skb)->destructor_arg = uarg;
-		skb_shinfo(skb)->flags |= uarg->flags;
+		skb_zcopy_init(skb, uarg);
 	}
 }