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

[Alexander Lobakin]

From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
Date: Tue, 7 Mar 2023 10:50:34 +0800

> On 2023/3/6 19:58, Alexander Lobakin wrote:
>> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> Date: Mon, 6 Mar 2023 09:09:31 +0800

[...]

>> 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.

You mean false-positives in both directions? Because if ->pp_recycle is
set, the stack can still free non-PP pages. In the opposite case, I mean
when ->pp_recycle is false and an skb page belongs to a page_pool, yes,
there'll be issues.
But I think the deal is to propagate the flag when you want to attach a
PP-backed page to the skb? I mean, if someone decides to mix pages with
different memory models, it's his responsibility to make sure everything
is fine, because it's not a common/intended way. Isn't it?

> 
>>
>>>
>>>>
>>>>>
>>>>> 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

This one is interesting actually. We'd only need one bit -- which is
100% free and available in case of page pointers.

>>>>>
>>>>>>
>>>>>>>
>>>>>>>>  
>>>>>>>>  	/* Allow SKB to reuse area used by xdp_frame */
>>>>>>>>  	xdp_scrub_frame(xdpf);

[...]

Thanks,
Olek