Re: [PATCH bpf-next v1 1/2] xdp: recycle Page Pool backed skbs built from XDP frames

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2023/3/3 21:26, Alexander Lobakin wrote:
> From: Yunsheng Lin <linyunsheng@huawei.com>
> Date: Fri, 3 Mar 2023 20:44:24 +0800
> 
>> On 2023/3/3 19:22, Alexander Lobakin wrote:
>>> From: Yunsheng Lin <linyunsheng@huawei.com>
>>> Date: Thu, 2 Mar 2023 10:30:13 +0800
> 
> [...]
> 
>>> And they are fixed :D
>>> No drivers currently which use Page Pool mix PP pages with non-PP. And
>>
>> The wireless adapter which use Page Pool *does* mix PP pages with
>> non-PP, see below discussion:
>>
>> https://lore.kernel.org/netdev/156f3e120bd0757133cb6bc11b76889637b5e0a6.camel@gmail.com/
> 
> Ah right, I remember that (also was fixed).
> Not that I think it is correct to mix them -- for my PoV, a driver
> shoule either give *all* its Rx buffers as PP-backed or not use PP at all.
> 
> [...]
> 
>>> As Jesper already pointed out, not having a quick way to check whether
>>> we have to check ::pp_magic at all can decrease performance. So it's
>>> rather a shortcut.
>>
>> When we are freeing a page by updating the _refcount, I think
>> we are already touching the cache of ::pp_magic.
> 
> But no page freeing happens before checking for skb->pp_recycle, neither
> in skb_pp_recycle() (skb_free_head() etc.)[0] nor in skb_frag_unref()[1].

If we move to per page marker, we probably do not need checking
skb->pp_recycle.

Note both page_pool_return_skb_page() and skb_free_frag() can
reuse the cache line triggered by per page marker checking if
the per page marker is in the 'struct page'.

> 
>>
>> Anyway, I am not sure checking ::pp_magic is correct when a
>> page will be passing between different subsystem and back to
>> the network stack eventually, checking ::pp_magic may not be
>> correct if this happens.
>>
>> Another way is to use the bottom two bits in bv_page, see:
>> https://www.spinics.net/lists/netdev/msg874099.html
>>
>>>
>>>>
>>>>>  
>>>>>  	/* Allow SKB to reuse area used by xdp_frame */
>>>>>  	xdp_scrub_frame(xdpf);
>>>>>
>>>
>>> Thanks,
>>> Olek
>>> .
>>>
> 
> [0] https://elixir.bootlin.com/linux/latest/source/net/core/skbuff.c#L808
> [1]
> https://elixir.bootlin.com/linux/latest/source/include/linux/skbuff.h#L3385
> 
> Thanks,
> Olek
> .
>