Re: [PATCH net-next v3 0/5] page_pool: recycle buffers

From: Yunsheng Lin
Date: Thu May 06 2021 - 08:34:56 EST

On 2021/5/1 0:24, Ilias Apalodimas wrote:
> [...]
>>>> 1. skb frag page recycling do not need "struct xdp_rxq_info" or
>>>> "struct xdp_mem_info" to bond the relation between "struct page" and
>>>> "struct page_pool", which seems uncessary at this point if bonding
>>>> a "struct page_pool" pointer directly in "struct page" does not cause
>>>> space increasing.
>>> We can't do that. The reason we need those structs is that we rely on the
>>> existing XDP code, which already recycles it's buffers, to enable
>>> recycling. Since we allocate a page per packet when using page_pool for a
>>> driver , the same ideas apply to an SKB and XDP frame. We just recycle the
>> I am not really familar with XDP here, but a packet from hw is either a
>> "struct xdp_frame/xdp_buff" for XDP or a "struct sk_buff" for TCP/IP stack,
>> a packet can not be both "struct xdp_frame/xdp_buff" and "struct sk_buff" at
>> the same time, right?
> Yes, but the payload is irrelevant in both cases and that's what we use
> page_pool for. You can't use this patchset unless your driver usues
> build_skb(). So in both cases you just allocate memory for the payload and

I am not sure I understood why build_skb() matters here. If the head data of
a skb is a page frag and is from page pool, then it's page->signature should be
PP_SIGNATURE, otherwise it's page->signature is zero, so a recyclable skb does
not require it's head data being from a page pool, right?

> decide what the wrap the buffer with (XDP or SKB) later.


>> I am not sure I understand what you meant by "free the skb", does it mean
>> that kfree_skb() is called to free the skb.
> Yes
>> As my understanding, if the skb completely own the page(which means page_count()
>> == 1) when kfree_skb() is called, __page_pool_put_page() is called, otherwise
>> page_ref_dec() is called, which is exactly what page_pool_atomic_sub_if_positive()
>> try to handle it atomically.
> 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?

> [1]
> Cheers
> /Ilias
> .