Re: [RFC PATCH v3 05/12] netdev: netdevice devmem allocator

[Yunsheng Lin]

On 2023/11/8 6:10, Mina Almasry wrote:
> On Mon, Nov 6, 2023 at 3:44 PM David Ahern <dsahern@xxxxxxxxxx> wrote:
>>
>> On 11/5/23 7:44 PM, Mina Almasry wrote:
>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>> index eeeda849115c..1c351c138a5b 100644
>>> --- a/include/linux/netdevice.h
>>> +++ b/include/linux/netdevice.h
>>> @@ -843,6 +843,9 @@ struct netdev_dmabuf_binding {
>>>  };
>>>
>>>  #ifdef CONFIG_DMA_SHARED_BUFFER
>>> +struct page_pool_iov *
>>> +netdev_alloc_devmem(struct netdev_dmabuf_binding *binding);
>>> +void netdev_free_devmem(struct page_pool_iov *ppiov);
>>
>> netdev_{alloc,free}_dmabuf?
>>
> 
> Can do.
> 
>> I say that because a dmabuf can be host memory, at least I am not aware
>> of a restriction that a dmabuf is device memory.
>>
> 
> In my limited experience dma-buf is generally device memory, and
> that's really its use case. CONFIG_UDMABUF is a driver that mocks
> dma-buf with a memfd which I think is used for testing. But I can do
> the rename, it's more clear anyway, I think.
> 
> On Mon, Nov 6, 2023 at 11:45 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>
>> On 2023/11/6 10:44, Mina Almasry wrote:
>>> +
>>> +void netdev_free_devmem(struct page_pool_iov *ppiov)
>>> +{
>>> +     struct netdev_dmabuf_binding *binding = page_pool_iov_binding(ppiov);
>>> +
>>> +     refcount_set(&ppiov->refcount, 1);
>>> +
>>> +     if (gen_pool_has_addr(binding->chunk_pool,
>>> +                           page_pool_iov_dma_addr(ppiov), PAGE_SIZE))
>>
>> When gen_pool_has_addr() returns false, does it mean something has gone
>> really wrong here?
>>
> 
> Yes, good eye. gen_pool_has_addr() should never return false, but then
> again, gen_pool_free()  BUG_ON()s if it doesn't find the address,
> which is an extremely severe reaction to what can be a minor bug in
> the accounting. I prefer to leak rather than crash the machine. It's a
> bit of defensive programming that is normally frowned upon, but I feel
> like in this case it's maybe warranted due to the very severe reaction
> (BUG_ON).

I would argue that why is the above defensive programming not done in the
gen_pool core:)

>