[PATCH net 4/4] virtio-net: allow more allocated space for mergeable XDP

[Bui Quang Minh <minhquangbui99@gmail.com>]

When the mergeable receive buffer is prefilled before XDP is set, it
does not reserve the space for XDP_PACKET_HEADROOM and skb_shared_info.
So when XDP is set and this buffer is used to receive frame, we need to
create a new buffer with reserved headroom, tailroom and copy the frame
data over. Currently, the new buffer's size is restricted to PAGE_SIZE
only. If the frame data's length + headroom + tailroom exceeds
PAGE_SIZE, the frame is dropped.

However, it seems like there is no restriction on the total size in XDP.
So we can just increase the size of new buffer to 2 * PAGE_SIZE in that
case and continue to process the frame.

In my opinion, the current drop behavior is fine and expected so this
commit is just an improvement not a bug fix.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
---
 drivers/net/virtio_net.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 844cb2a78be0..663cec686045 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2277,13 +2277,26 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
 					      len);
 		if (!xdp_page)
 			return NULL;
+
+		*frame_sz = PAGE_SIZE;
 	} else {
+		unsigned int total_len;
+
 		xdp_room = SKB_DATA_ALIGN(XDP_PACKET_HEADROOM +
 					  sizeof(struct skb_shared_info));
-		if (*len + xdp_room > PAGE_SIZE)
+		total_len = *len + xdp_room;
+
+		/* This must never happen because len cannot exceed PAGE_SIZE */
+		if (unlikely(total_len > 2 * PAGE_SIZE))
 			return NULL;
 
-		xdp_page = alloc_page(GFP_ATOMIC);
+		if (total_len > PAGE_SIZE) {
+			xdp_page = alloc_pages(GFP_ATOMIC, 1);
+			*frame_sz = 2 * PAGE_SIZE;
+		} else {
+			xdp_page = alloc_page(GFP_ATOMIC);
+			*frame_sz = PAGE_SIZE;
+		}
 		if (!xdp_page)
 			return NULL;
 
@@ -2291,8 +2304,6 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
 		       page_address(*page) + offset, *len);
 	}
 
-	*frame_sz = PAGE_SIZE;
-
 	put_page(*page);
 
 	*page = xdp_page;
-- 
2.43.0