Re: [PATCH net] xen-netback: Fix handling of skbs requiring too many slots

as the provided buffers are fully
utilized.

Signed-off-by: Zoltan Kiss <redacted>
Cc: Paul Durrant <redacted>
Cc: Wei Liu <redacted>
Cc: Ian Campbell <redacted>
Cc: David Vrabel <redacted>
---

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 64ab1d1..65fbcd1 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c

@@ -163,7 +163,10 @@ bool xenvif_rx_ring_slots_available(struct xenvif *vif, int needed)
   * adding 'size' bytes to a buffer which currently contains 'offset'
   * bytes.
   */
-static bool start_new_rx_buffer(int offset, unsigned long size, int head)
+static bool start_new_rx_buffer(int offset,
+				unsigned long size,
+				int head,
+				bool full_coalesce)
  {
  	/* simple case: we have completely filled the current buffer. */
  	if (offset == MAX_BUFFER_OFFSET)

@@ -175,6 +178,7 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
  	 *     (i)   this frag would fit completely in the next buffer
  	 * and (ii)  there is already some data in the current buffer
  	 * and (iii) this is not the head buffer.
+	 * and (iv)  there is no need to fully utilize the buffers
  	 *
  	 * Where:
  	 * - (i) stops us splitting a frag into two copies

@@ -185,6 +189,8 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
  	 *   by (ii) but is explicitly checked because
  	 *   netfront relies on the first buffer being
  	 *   non-empty and can crash otherwise.
+	 * - (iv) is needed for skbs which can use up more than MAX_SKB_FRAGS
+	 *   slot
  	 *
  	 * This means we will effectively linearise small
  	 * frags but do not needlessly split large buffers

@@ -192,7 +198,10 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
  	 * own buffers as before.
  	 */
  	BUG_ON(size > MAX_BUFFER_OFFSET);
-	if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
+	if ((offset + size > MAX_BUFFER_OFFSET) &&
+	    offset &&
+	    !head &&
+	    !full_coalesce)
  		return true;
  
  	return false;

@@ -227,6 +236,13 @@ static struct xenvif_rx_meta *get_next_rx_buffer(struct xenvif *vif,
  	return meta;
  }
  
+struct xenvif_rx_cb {
+	int meta_slots_used;
+	bool full_coalesce;
+};
+
+#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
+
  /*
   * Set up the grant operations for this fragment. If it's a flipping
   * interface, we also set up the unmap request from here.

@@ -261,7 +277,10 @@ static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
  		if (bytes > size)
  			bytes = size;
  
-		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
+		if (start_new_rx_buffer(npo->copy_off,
+					bytes,
+					*head,
+					XENVIF_RX_CB(skb)->full_coalesce)) {
  			/*
  			 * Netfront requires there to be some data in the head
  			 * buffer.

@@ -541,12 +560,6 @@ static void xenvif_add_frag_responses(struct xenvif *vif, int status,
  	}
  }
  
-struct xenvif_rx_cb {
-	int meta_slots_used;
-};
-
-#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
-
  void xenvif_kick_thread(struct xenvif *vif)
  {
  	wake_up(&vif->wq);

@@ -602,10 +615,14 @@ static void xenvif_rx_action(struct xenvif *vif)
  
  		/* To avoid the estimate becoming too pessimal for some
  		 * frontends that limit posted rx requests, cap the estimate
-		 * at MAX_SKB_FRAGS.
+		 * at MAX_SKB_FRAGS. In this case netback will fully coalesce
+		 * the skb into the provided slots.
  		 */
-		if (max_slots_needed > MAX_SKB_FRAGS)
+		if (max_slots_needed > MAX_SKB_FRAGS) {
  			max_slots_needed = MAX_SKB_FRAGS;
+			XENVIF_RX_CB(skb)->full_coalesce = true;
+		} else
+			XENVIF_RX_CB(skb)->full_coalesce = false;