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

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Message-ID: <fba8f996-003f-1fb1-d5f0-09ef6fc24f51@intel.com>
Date:   Thu, 9 Mar 2023 17:27:52 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Song Liu <song@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 1/2] xdp: recycle Page Pool backed skbs built
 from XDP frames

From: Yunsheng Lin <linyunsheng@...wei.com>
Date: Wed, 8 Mar 2023 14:27:13 +0800

> On 2023/3/8 2:14, Alexander Lobakin wrote:
>> From: Yunsheng Lin <linyunsheng@...wei.com>
>> 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