Date:   Fri, 7 May 2021 16:28:30 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
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Subject: Re: [PATCH net-next v3 0/5] page_pool: recycle buffers

On 2021/5/7 15:06, Ilias Apalodimas wrote:
> On Fri, May 07, 2021 at 11:23:28AM +0800, Yunsheng Lin wrote:
>> On 2021/5/6 20:58, Ilias Apalodimas wrote:
>>>>>>
>>>>>
>>>>> Not really, the opposite is happening here. If the pp_recycle bit is set we
>>>>> will always call page_pool_return_skb_page().  If the page signature matches
>>>>> the 'magic' set by page pool we will always call xdp_return_skb_frame() will
>>>>> end up calling __page_pool_put_page(). If the refcnt is 1 we'll try
>>>>> to recycle the page.  If it's not we'll release it from page_pool (releasing
>>>>> some internal references we keep) unmap the buffer and decrement the refcnt.
>>>>
>>>> Yes, I understood the above is what the page pool do now.
>>>>
>>>> But the question is who is still holding an extral reference to the page when
>>>> kfree_skb()? Perhaps a cloned and pskb_expand_head()'ed skb is holding an extral
>>>> reference to the same page? So why not just do a page_ref_dec() if the orginal skb
>>>> is freed first, and call __page_pool_put_page() when the cloned skb is freed later?
>>>> So that we can always reuse the recyclable page from a recyclable skb. This may
>>>> make the page_pool_destroy() process delays longer than before, I am supposed the
>>>> page_pool_destroy() delaying for cloned skb case does not really matters here.
>>>>
>>>> If the above works, I think the samiliar handling can be added to RX zerocopy if
>>>> the RX zerocopy also hold extral references to the recyclable page from a recyclable
>>>> skb too?
>>>>
>>>
>>> Right, this sounds doable, but I'll have to go back code it and see if it
>>> really makes sense.  However I'd still prefer the support to go in as-is
>>> (including the struct xdp_mem_info in struct page, instead of a page_pool
>>> pointer).
>>>
>>> There's a couple of reasons for that.  If we keep the struct xdp_mem_info we
>>> can in the future recycle different kind of buffers using __xdp_return().
>>> And this is a non intrusive change if we choose to store the page pool address
>>> directly in the future.  It just affects the internal contract between the
>>> page_pool code and struct page.  So it won't affect any drivers that already
>>> use the feature.
>>
>> This patchset has embeded a signature field in "struct page", and xdp_mem_info
>> is stored in page_private(), which seems not considering the case for associating
>> the page pool with "struct page" directly yet? 
> 
> Correct
> 
>> Is the page pool also stored in
>> page_private() and a different signature is used to indicate that?
> 
> No only struct xdp_mem_info as you mentioned before
> 
>>
>> I am not saying we have to do it in this patchset, but we have to consider it
>> while we are adding new signature field to "struct page", right?
> 
> We won't need a new signature.  The signature in both cases is there to 
> guarantee the page you are trying to recycle was indeed allocated by page_pool.
> 
> Basically we got two design choices here: 
> - We store the page_pool ptr address directly in page->private and then,
>   we call into page_pool APIs directly to do the recycling.
>   That would eliminate the lookup through xdp_mem_info and the
>   XDP helpers to locate page pool pointer (through __xdp_return).
> - You store the xdp_mem_info on page_private.  In that case you need to go
>   through __xdp_return()  to locate the page_pool pointer. Although we might
>   loose some performance that would allow us to recycle additional memory types
>   and not only MEM_TYPE_PAGE_POOL (in case we ever need it).

So the signature field  in "struct page" is used to only indicate a page is
from a page pool, then how do we tell the content of page_private() if both of
the above choices are needed, we might still need an extra indicator to tell
page_private() is page_pool ptr or xdp_mem_info.

It seems storing the page pool ptr in page_private() is clear for recyclable
page from a recyclable skb use case, and the use case for storing xdp_mem_info
in page_private() is unclear yet? As XDP seems to have the xdp_mem_info in the
"struct xdp_frame", so it does not need the xdp_mem_info from page_private().

If the above is true, what does not really makes sense to me here is that:
why do we first implement a unclear use case for storing xdp_mem_info in
page_private(), why not implement the clear use case for storing page pool ptr
in page_private() first?

> 
> 
> I think both choices are sane.  What I am trying to explain here, is
> regardless of what we choose now, we can change it in the future without
> affecting the API consumers at all.  What will change internally is the way we
> lookup the page pool pointer we are trying to recycle.

It seems the below API need changing?
+static inline void skb_mark_for_recycle(struct sk_buff *skb, struct page *page,
+					struct xdp_mem_info *mem)


> 
> [...]
> 
> 
> Cheers
> /Ilias
> 
> .
> 

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