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

[Yunsheng Lin]

On 2023/3/6 19:58, Alexander Lobakin wrote:
> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> Date: Mon, 6 Mar 2023 09:09:31 +0800
> 
>> On 2023/3/3 21:26, Alexander Lobakin wrote:
>>> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>>> Date: Fri, 3 Mar 2023 20:44:24 +0800
>>>
>>>> On 2023/3/3 19:22, Alexander Lobakin wrote:
>>>>> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>>>>> 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@xxxxxxxxx/
>>>
>>> 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'.
> 
> Ah, from that perspective. Yes, you're probably right, but would need to
> be tested anyway. I don't see any open problems with the PP recycling
> right now on the lists, but someone may try to change it one day.
> Anyway, this flag is only to do a quick test. We do have
> sk_buff::pfmemalloc, but this flag doesn't mean every page from this skb
> was pfmemalloced.

The point seems to be that sk_buff::pfmemalloc allow false positive, which
means skb->pfmemalloc can be set to true while every page from this skb is
not pfmemalloced as you mentioned.

While skb->pp_recycle can't allow false positive, if that happens, reference
counting of the page will not be handled properly if pp and non-pp skb shares
the page as the wireless adapter does.

> 
>>
>>>
>>>>
>>>> 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
>>> .
>>>
> 
> Thanks,
> Olek
> 
> .
>