[RFC PATCH v3 04/12] netdev: support binding dma-buf to netdevice

[Yunsheng Lin]

On 2023/11/9 10:22, Mina Almasry wrote:
> On Tue, Nov 7, 2023 at 7:40 PM Yunsheng Lin <linyunsheng at huawei.com> wrote:
>>
>> On 2023/11/8 5:59, Mina Almasry wrote:
>>> On Mon, Nov 6, 2023 at 11:46 PM Yunsheng Lin <linyunsheng at huawei.com> wrote:
>>>>
>>>> On 2023/11/6 10:44, Mina Almasry wrote:
>>>>> +
>>>>> +void __netdev_devmem_binding_free(struct netdev_dmabuf_binding *binding)
>>>>> +{
>>>>> +     size_t size, avail;
>>>>> +
>>>>> +     gen_pool_for_each_chunk(binding->chunk_pool,
>>>>> +                             netdev_devmem_free_chunk_owner, NULL);
>>>>> +
>>>>> +     size = gen_pool_size(binding->chunk_pool);
>>>>> +     avail = gen_pool_avail(binding->chunk_pool);
>>>>> +
>>>>> +     if (!WARN(size != avail, "can't destroy genpool. size=%lu, avail=%lu",
>>>>> +               size, avail))
>>>>> +             gen_pool_destroy(binding->chunk_pool);
>>>>
>>>>
>>>> Is there any other place calling the gen_pool_destroy() when the above
>>>> warning is triggered? Do we have a leaking for binding->chunk_pool?
>>>>
>>>
>>> gen_pool_destroy BUG_ON() if it's not empty at the time of destroying.
>>> Technically that should never happen, because
>>> __netdev_devmem_binding_free() should only be called when the refcount
>>> hits 0, so all the chunks have been freed back to the gen_pool. But,
>>> just in case, I don't want to crash the server just because I'm
>>> leaking a chunk... this is a bit of defensive programming that is
>>> typically frowned upon, but the behavior of gen_pool is so severe I
>>> think the WARN() + check is warranted here.
>>
>> It seems it is pretty normal for the above to happen nowadays because of
>> retransmits timeouts, NAPI defer schemes mentioned below:
>>
>> https://lkml.kernel.org/netdev/168269854650.2191653.8465259808498269815.stgit@firesoul/
>>
>> And currently page pool core handles that by using a workqueue.
> 
> Forgive me but I'm not understanding the concern here.
> 
> __netdev_devmem_binding_free() is called when binding->ref hits 0.
> 
> binding->ref is incremented when an iov slice of the dma-buf is
> allocated, and decremented when an iov is freed. So,
> __netdev_devmem_binding_free() can't really be called unless all the
> iovs have been freed, and gen_pool_size() == gen_pool_avail(),
> regardless of what's happening on the page_pool side of things, right?

I seems to misunderstand it. In that case, it seems to be about
defensive programming like other checking.

By looking at it more closely, it seems napi_frag_unref() call
page_pool_page_put_many() directly， which means devmem seems to
be bypassing the napi_safe optimization.

Can napi_frag_unref() reuse napi_pp_put_page() in order to reuse
the napi_safe optimization?

>