From:   Jesper Dangaard Brouer <jbrouer@redhat.com>
Message-ID: <11e480dd-7969-7b58-440e-3207b98d0ac5@redhat.com>
Date:   Wed, 15 Mar 2023 15:55:44 +0100
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Cc:     brouer@redhat.com,
        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>,
        John Fastabend <john.fastabend@gmail.com>,
        Menglong Dong <imagedong@tencent.com>,
        Mykola Lysenko <mykolal@fb.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Freysteinn Alfredsson <Freysteinn.Alfredsson@kau.se>
Subject: Re: [PATCH bpf-next v3 3/4] xdp: recycle Page Pool backed skbs built
 from XDP frames
Content-Language: en-US
To:     Alexander Lobakin <aleksander.lobakin@intel.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>
References: <20230313215553.1045175-1-aleksander.lobakin@intel.com>
 <20230313215553.1045175-4-aleksander.lobakin@intel.com>
In-Reply-To: <20230313215553.1045175-4-aleksander.lobakin@intel.com>
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On 13/03/2023 22.55, Alexander Lobakin wrote:
> __xdp_build_skb_from_frame() state(d):
> 
> /* Until page_pool get SKB return path, release DMA here */
> 
> Page Pool got skb pages recycling in April 2021, but missed this
> function.
> 
> xdp_release_frame() is relevant only for Page Pool backed frames and it
> detaches the page from the corresponding page_pool in order to make it
> freeable via page_frag_free(). It can instead just mark the output skb
> as eligible for recycling if the frame is backed by a pp. No change for
> other memory model types (the same condition check as before).
> cpumap redirect and veth on Page Pool drivers now become zero-alloc (or
> almost).
> 
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
>   net/core/xdp.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 8c92fc553317..a2237cfca8e9 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -658,8 +658,8 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>   	 * - RX ring dev queue index	(skb_record_rx_queue)
>   	 */
>   
> -	/* Until page_pool get SKB return path, release DMA here */
> -	xdp_release_frame(xdpf);
> +	if (xdpf->mem.type == MEM_TYPE_PAGE_POOL)
> +		skb_mark_for_recycle(skb);

I hope this is safe ;-) ... Meaning hopefully drivers does the correct
thing when XDP_REDIRECT'ing page_pool pages.

Looking for drivers doing weird refcnt tricks and XDP_REDIRECT'ing, I
noticed the driver aquantia/atlantic (in aq_get_rxpages_xdp), but I now
see this is not using page_pool, so it should be affected by this (but I
worry if atlantic driver have a potential race condition for its refcnt
scheme).

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