Re: [PATCH] sfc: handle NULL returned by xdp_convert_buff_to_frame()

[Kunwu Chan <kunwu.chan@linux.dev>]

On 2025/7/25 18:11, Edward Cree wrote:
> On 7/24/25 10:57, Paolo Abeni wrote:
>> On 7/23/25 2:32 AM, Chenyuan Yang wrote:
>>> 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.”