xen: netback: handle compound page fragments on transmit.

An SKB paged fragment can consist of a compound page with order > 0.
However the netchannel protocol deals only in PAGE_SIZE frames.

Handle this in netbk_gop_frag_copy and xen_netbk_count_skb_slots by
iterating over the frames which make up the page.

Signed-off-by: Ian Campbell <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Sander Eikelenboom <[e-mail]>
Tested-by: Konrad Rzeszutek Wilk <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/xen-netback/netback.c

···
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 		unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
+		unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
 		unsigned long bytes;
+
+		offset &= ~PAGE_MASK;
+
 		while (size > 0) {
+			BUG_ON(offset >= PAGE_SIZE);
 			BUG_ON(copy_off > MAX_BUFFER_OFFSET);
 
-			if (start_new_rx_buffer(copy_off, size, 0)) {
+			bytes = PAGE_SIZE - offset;
+
+			if (bytes > size)
+				bytes = size;
+
+			if (start_new_rx_buffer(copy_off, bytes, 0)) {
 				count++;
 				copy_off = 0;
 			}
 
-			bytes = size;
 			if (copy_off + bytes > MAX_BUFFER_OFFSET)
 				bytes = MAX_BUFFER_OFFSET - copy_off;
 
 			copy_off += bytes;
+
+			offset += bytes;
 			size -= bytes;
+
+			if (offset == PAGE_SIZE)
+				offset = 0;
 		}
 	}
 	return count;
···
 	unsigned long bytes;
 
 	/* Data must not cross a page boundary. */
-	BUG_ON(size + offset > PAGE_SIZE);
+	BUG_ON(size + offset > PAGE_SIZE<<compound_order(page));
 
 	meta = npo->meta + npo->meta_prod - 1;
 
+	/* Skip unused frames from start of page */
+	page += offset >> PAGE_SHIFT;
+	offset &= ~PAGE_MASK;
+
 	while (size > 0) {
+		BUG_ON(offset >= PAGE_SIZE);
 		BUG_ON(npo->copy_off > MAX_BUFFER_OFFSET);
 
-		if (start_new_rx_buffer(npo->copy_off, size, *head)) {
+		bytes = PAGE_SIZE - offset;
+
+		if (bytes > size)
+			bytes = size;
+
+		if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
 			/*
 			 * Netfront requires there to be some data in the head
 			 * buffer.
···
 			meta = get_next_rx_buffer(vif, npo);
 		}
 
-		bytes = size;
 		if (npo->copy_off + bytes > MAX_BUFFER_OFFSET)
 			bytes = MAX_BUFFER_OFFSET - npo->copy_off;
 
···
 
 		offset += bytes;
 		size -= bytes;
+
+		/* Next frame */
+		if (offset == PAGE_SIZE && size) {
+			BUG_ON(!PageCompound(page));
+			page++;
+			offset = 0;
+		}
 
 		/* Leave a gap for the GSO descriptor. */
 		if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)