linux-kernel - Re: [RFC PATCH net-next v6 09/15] memory-provider: dmabuf devmem memory provider

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Date: Tue, 5 Mar 2024 18:46:55 -0800
From: David Wei <dw@...idwei.uk>
To: Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [RFC PATCH net-next v6 09/15] memory-provider: dmabuf devmem
 memory provider

On 2024-03-05 18:42, Mina Almasry wrote:
> On Tue, Mar 5, 2024 at 6:28 PM David Wei <dw@...idwei.uk> wrote:
>>
>> On 2024-03-04 18:01, Mina Almasry wrote:
>>> +     if (pool->p.queue)
>>> +             binding = READ_ONCE(pool->p.queue->binding);
>>> +
>>> +     if (binding) {
>>> +             pool->mp_ops = &dmabuf_devmem_ops;
>>> +             pool->mp_priv = binding;
>>> +     }
>>
>> This is specific to TCP devmem. For ZC Rx we will need something more
>> generic to let us pass our own memory provider backend down to the page
>> pool.
>>
>> What about storing ops and priv void ptr in struct netdev_rx_queue
>> instead? Then we can both use it.
> 
> Yes, this is dmabuf specific, I was thinking you'd define your own
> member of netdev_rx_queue, and then add something like this to
> page_pool_init:
> 
> +       if (pool->p.queue)
> +               io_uring_metadata = READ_ONCE(pool->p.queue->io_uring_metadata);
> +
> +       /* We don't support rx-queues that are configured for both
> io_uring & dmabuf binding */
> +       BUG_ON(io_uring_metadata && binding);
> +
> +       if (io_uring_metadata) {
> +               pool->mp_ops = &io_uring_ops;
> +               pool->mp_priv = io_uring_metadata;
> +       }
> 
> I.e., we share the pool->mp_ops and the pool->mp_priv but we don't
> really need to share the same netdev_rx_queue member. For me it's a
> dma-buf specific data structure (netdev_dmabuf_binding) and for you
> it's something else.

This adds size to struct netdev_rx_queue and requires checks on whether
both are set. There can be thousands of these structs at any one time so
if we don't need to add size unnecessarily then that would be best.

We can disambiguate by comparing &mp_ops and then cast the void ptr to
our impl specific objects.

What do you not like about this approach?

> 
> page_pool_init() probably needs to validate that the queue is
> configured for dma-buf or io_uring but not both. If it's configured
> for both then the user is doing something funky we shouldn't support.
> 
> Perhaps I can make the intention clearer by renaming 'binding' to
> something more specific to dma-buf like queue->dmabuf_binding, to make
> it clear that this is the dma-buf binding and not some other binding
> like io_uring?
>