Message-ID: <fba8f996-003f-1fb1-d5f0-09ef6fc24f51@intel.com>
Date:   Thu, 9 Mar 2023 17:27:52 +0100
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Subject: Re: [PATCH bpf-next v1 1/2] xdp: recycle Page Pool backed skbs built
 from XDP frames
Content-Language: en-US
To:     Yunsheng Lin <linyunsheng@huawei.com>
CC:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Larysa Zaremba <larysa.zaremba@intel.com>,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        Song Liu <song@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>, <bpf@vger.kernel.org>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <20230301160315.1022488-1-aleksander.lobakin@intel.com>
 <20230301160315.1022488-2-aleksander.lobakin@intel.com>
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 <7ffbcac4-f4f2-5579-fd55-35813fbd792c@huawei.com>
 <9b5b88da-0d2d-d3f3-6ee1-7e4afc2e329a@intel.com>
 <98aa093a-e772-8882-b0e3-5895fd747e59@huawei.com>
 <0bc28bea-78f5-bcce-2d45-e6f6d1a7ed40@intel.com>
 <605cad27-2bf3-7913-877e-d2870892ecd5@huawei.com>
 <9e8a9346-37f4-7c5d-f1d0-cbba3de805db@intel.com>
 <1cea3621-8f5d-ba95-1b0b-e245ce770abf@huawei.com>
From:   Alexander Lobakin <aleksander.lobakin@intel.com>
In-Reply-To: <1cea3621-8f5d-ba95-1b0b-e245ce770abf@huawei.com>
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From: Yunsheng Lin <linyunsheng@huawei.com>
Date: Wed, 8 Mar 2023 14:27:13 +0800

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