On 2025/7/25 18:11, Edward Cree wrote:

On 7/24/25 10:57, Paolo Abeni wrote:

quoted

On 7/23/25 2:32 AM, Chenyuan Yang wrote:

quoted

The xdp_convert_buff_to_frame() function can return NULL when there is
insufficient headroom in the buffer to store the xdp_frame structure
or when the driver didn't reserve enough tailroom for skb_shared_info.

AFAIC the sfc driver reserves both enough headroom and tailroom, but
this is after ebpf run, which in turn could consume enough headroom to
cause a failure, so I think this makes sense.

Your reasoning seems plausible to me.
However, I think the error path ought to more closely follow the existing
  error cases in logging a ratelimited message and calling the tracepoint.
I think the cleanest way to do this would be:
	if (unlikely(!xdpf))
		err = -ENOBUFS;
	else
		err = efx_xdp_tx_buffers(efx, 1, &xdpf, true);
  so that it can make use of the existing failure path.
Adding the check to efx_xdp_tx_buffers() is also an option.

-ed

Hi Chenyuan,

THX for addressing this edge case. I concur with Edward's suggestion to 
integrate this with the existing error handling flow. This will ensure:
Consistent observability (ratelimited logs + tracepoints)
Centralized resource cleanup
Clear error type differentiation via -ENOBUFS

Proposed refinement:

diff
  case XDP_TX:
      /* Buffer ownership passes to tx on success. */
      xdpf = xdp_convert_buff_to_frame(&xdp);
+    if (unlikely(!xdpf)) {
+        err = -ENOBUFS;
+    } else {
+        err = efx_siena_xdp_tx_buffers(efx, 1, &xdpf, true);
+    }

-    err = efx_siena_xdp_tx_buffers(efx, 1, &xdpf, true);
      if (unlikely(err != 1)) {
          efx_siena_free_rx_buffers(rx_queue, rx_buf, 1);
          if (net_ratelimit())
              netif_err(efx, rx_err, efx->net_dev,
-                  "XDP TX failed (%d)\n", err);
+                  "XDP TX failed (%d)%s\n", err,
+                  err == -ENOBUFS ? " [frame conversion]" : "");
          channel->n_rx_xdp_bad_drops++;
-        trace_xdp_exception(efx->net_dev, xdp_prog, xdp_act);
+        if (err != -ENOBUFS)
+            trace_xdp_exception(efx->net_dev, xdp_prog, xdp_act);
      } else {
          channel->n_rx_xdp_tx++;
      }
      break;


-- Thanks, TAO. --- “Life finds a way.”