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

[Yunsheng Lin]

On 2023/11/10 10:59, Mina Almasry wrote:
> On Thu, Nov 9, 2023 at 12:30 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>>
>> I'm trying to wrap my head around the whole infra... the above line is
>> confusing. Why do you increment dma_addr? it will be re-initialized in
>> the next iteration.
>>
> 
> That is just a mistake, sorry. Will remove this increment.

You seems to be combining comments in different thread and replying in
one thread, I am not sure that is a good practice and I almost missed the
reply below as I don't seem to be cc'ed.

> 
> On Thu, Nov 9, 2023 at 1:29 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:> >>>
>>>>> 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?
>>
> 
> I think it already does. page_pool_page_put_many() is only called if
> !recycle or !napi_pp_put_page(). In that case
> page_pool_page_put_many() is just a replacement for put_page(),
> because this 'page' may be an iov.

Is there a reason why not calling napi_pp_put_page() for devmem too
instead of calling page_pool_page_put_many()? mem provider has a
'release_page' ops, calling page_pool_page_put_many() directly here
seems to be bypassing the 'release_page' ops, which means devmem is
bypassing most of the main features of page pool.

As far as I can tell, the main features of page pool:
1. Allow lockless allocation and freeing in pool->alloc cache by
   utilizing NAPI non-concurrent context.
2. Allow concurrent allocation and freeing in pool->ring cache by
   utilizing ptr_ring.
3. Allow dma map/unmap and cache sync optimization.
4. Allow detailed stats logging and tracing.
5. Allow some bulk allocation and freeing.
6. support both skb packet and xdp frame.

I am wondering what is the main features that devmem is utilizing
by intergrating into page pool?

It seems the driver can just call netdev_alloc_devmem() and
napi_frag_unref() can call netdev_free_devmem() directly without
intergrating into page pool and it should just works too?

Maybe we should consider creating a new thin layer, in order to
demux to page pool, devmem or other mem type if my suggestion does
not work out too?

>