Re: [PATCH net-next] net: devmem: add netdev_has_dmabuf_binding() helper

[Dragos Tatulea <dtatulea@nvidia.com>]

On 11.08.26 09:24, Dragos Tatulea wrote:
> 
> 
> On 11.08.26 00:17, bobbyeshleman wrote:
>> On Mon, Aug 10, 2026 at 08:54:45PM +0300, Dragos Tatulea wrote:> [...]
>>> +bool netdev_has_dmabuf_binding(struct net_device *dev, struct device *dma_dev)
>>> +{
>>> +	struct net_devmem_dmabuf_binding *binding;
>>> +	unsigned long id;
>>> +	bool found = false;
>>> +
>>> +	rcu_read_lock();
>>> +	xa_for_each(&net_devmem_dmabuf_bindings, id, binding) {
>>> +		if (READ_ONCE(binding->dev) == dev &&
>>> +		    (!dma_dev || binding->attachment->dev == dma_dev)) {
>>> +			found = true;
>>> +			break;
>>> +		}
>>> +	}
>>> +	rcu_read_unlock();
>>> +
>>> +	return found;
>>> +}
>>> +EXPORT_SYMBOL_GPL(netdev_has_dmabuf_binding);
>>> +
>>>  void net_devmem_get_net_iov(struct net_iov *niov)
>>>  {
>>>  	net_devmem_dmabuf_binding_get(net_devmem_iov_binding(niov));
>>> -- 
>>> 2.54.0
>>>
>>
>> I think netdev_has_dmabuf_binding() can return false even when there
>> exists one or more in-flight skbs (e.g., in sk_write_queue) backed by
>> the txq binding (won't technically be inactive until the final reference
>> is dropped and the binding is freed)? If so, can this still detect when
>> it is safe to swap the underlying dma dev?
>>
> Oh, good point! I didn't realize that the binding is refcounted and can
> outlive its xarray slot.
>
I can only think of adding another list that tracks bindings for their
lifetime or between xarray erasure and deletion. Not too happy about it
though.

There's also the possibility of iterating over dmabufs and their attachment.
But this requires a function in dmabuf to scan the dmabufs and attachments
under the internal lock similar to dma_buf_debug_show(). But I don't think
that will be accepted. So this is even worse.

Thanks,
Dragos