FAILED: patch "[PATCH] NTB: ntb_netdev: Preserve RX queue depth on allocation" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x d2121faf133ac3bf9531b53a7e21273649a08517
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026081711-ranged-confusing-4505@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d2121faf133ac3bf9531b53a7e21273649a08517 Mon Sep 17 00:00:00 2001
From: Koichiro Den <[email protected]>
Date: Thu, 6 Aug 2026 12:25:37 +0900
Subject: [PATCH] NTB: ntb_netdev: Preserve RX queue depth on allocation
failure

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.

index c3a6ba96fc8a..029a4a532a10 100644
@@ -127,8 +127,8 @@ static void ntb_netdev_rx_handler(struct ntb_transport_qp *qp, void *qp_data,
 {
 	struct ntb_netdev_queue *q = qp_data;
 	struct ntb_netdev *dev = q->ntdev;
+	struct sk_buff *skb, *new_skb;
 	struct net_device *ndev;
-	struct sk_buff *skb;
 	int rc;
 
 	ndev = dev->ndev;

@@ -144,6 +144,12 @@ 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;

@@ -157,12 +163,7 @@ 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);

From: Koichiro Den <[email protected]>

[ 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.

---
 drivers/net/ntb_netdev.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

index b701ee83e64a8..7e562751d6ddc 100644
@@ -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