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: Wed, 8 Mar 2023 14:27:13 +0800

> On 2023/3/8 2:14, Alexander Lobakin wrote:
>> From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> Date: Tue, 7 Mar 2023 10:50:34 +0800

[...]

>> 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.
> 
> That may depends on what is a PP pages and what is a non-PP pages, it seems
> hard to answer now.
> 
> For a skb with ->pp_recycle being true and its frag page with page->pp_magic
> being PP_SIGNATURE, when calling skb_clone()/pskb_expand_head() or
> skb_try_coalesce(), we may call __skb_frag_ref() for the frag page, which
> mean a page with page->pp_magic being PP_SIGNATURE can be both PP page
> and non-PP page at the same time. So it is important to set the ->pp_recycle
> correctly, and it seems hard to get that right from past experience，that's
> why a per page marker is suggested.

Oh well, I didn't know that :s
Thanks for the expl.

[...]

Olek