[PATCH 5.15.y] NTB: ntb_netdev: Preserve RX queue depth on allocation failure

[Sasha Levin <sashal@kernel.org>]

From: Koichiro Den <den@valinux.co.jp>

[ Upstream commit d2121faf133ac3bf9531b53a7e21273649a08517 ]

ntb_netdev_rx_handler() hands the received skb to the network stack
before allocating its replacement. If the allocation fails, nothing is
reposted. Every failure therefore takes one buffer out of the RX queue
while the interface remains up, and enough failures eventually stall
reception.

A retry path could refill the queue later, but ntb_netdev has none.
Allocate the replacement first instead. If that fails, drop the packet
and repost the same skb. This keeps the queue full and lets packet
delivery resume as soon as memory is available again.

Fixes: 548c237c0a99 ("net: Add support for NTB virtual ethernet device")
Cc: stable@vger.kernel.org
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260806032537.3526498-1-den@valinux.co.jp
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ kept HEAD's `struct net_device *ndev = qp_data;` declaration instead of the per-queue context variables, adding only `new_skb` to the existing `skb` declaration ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/net/ntb_netdev.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ntb_netdev.c b/drivers/net/ntb_netdev.c
index b701ee83e64a8..7e562751d6ddc 100644
--- a/drivers/net/ntb_netdev.c
+++ b/drivers/net/ntb_netdev.c
@@ -100,7 +100,7 @@ static void ntb_netdev_rx_handler(struct ntb_transport_qp *qp, void *qp_data,
 				  void *data, int len)
 {
 	struct net_device *ndev = qp_data;
-	struct sk_buff *skb;
+	struct sk_buff *skb, *new_skb;
 	int rc;
 
 	skb = data;
@@ -115,6 +115,12 @@ static void ntb_netdev_rx_handler(struct ntb_transport_qp *qp, void *qp_data,
 		goto enqueue_again;
 	}
 
+	new_skb = netdev_alloc_skb(ndev, ndev->mtu + ETH_HLEN);
+	if (!new_skb) {
+		ndev->stats.rx_dropped++;
+		goto enqueue_again;
+	}
+
 	skb_put(skb, len);
 	skb->protocol = eth_type_trans(skb, ndev);
 	skb->ip_summed = CHECKSUM_NONE;
@@ -127,12 +133,7 @@ static void ntb_netdev_rx_handler(struct ntb_transport_qp *qp, void *qp_data,
 		ndev->stats.rx_bytes += len;
 	}
 
-	skb = netdev_alloc_skb(ndev, ndev->mtu + ETH_HLEN);
-	if (!skb) {
-		ndev->stats.rx_errors++;
-		ndev->stats.rx_frame_errors++;
-		return;
-	}
+	skb = new_skb;
 
 enqueue_again:
 	rc = ntb_transport_rx_enqueue(qp, skb, skb->data, ndev->mtu + ETH_HLEN);
-- 
2.53.0