netdev - Re: [PATCH net-next] xdp: Fixing skb->pp_recycle flag in generic XDP handling

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Message-ID: <222b7508-ee46-1e7d-d024-436b0373aaea@intel.com>
Date: Wed, 2 Aug 2023 19:09:43 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Liang Chen <liangchen.linux@...il.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <ast@...nel.org>, <daniel@...earbox.net>,
	<john.fastabend@...il.com>, <ilias.apalodimas@...aro.org>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] xdp: Fixing skb->pp_recycle flag in generic XDP
 handling

From: Liang Chen <liangchen.linux@...il.com>
Date: Wed,  2 Aug 2023 15:04:54 +0800

> In the generic XDP processing flow, if an skb with a page pool page
> (skb->pp_recycle == 1) fails to meet XDP packet requirements, it will
> undergo head expansion and linearization of fragment data. As a result,
> skb->head points to a reallocated buffer without any fragments. At this
> point, the skb will not contain any page pool pages. However, the
> skb->pp_recycle flag is still set to 1, which is inconsistent with the
> actual situation. Although it doesn't seem to cause much real harm at the

This means it must be handled in the function which replaces the head,
i.e. pskb_expand_head(). Your change only suppresses one symptom of the
issue.

> moment(a little nagetive impact on skb_try_coalesce), to avoid potential

                  ^^^^^^^^
                  negative

> issues associated with using incorrect skb->pp_recycle information,
> setting skb->pp_recycle to 0 to reflect the pp state of the skb.
> 
> Signed-off-by: Liang Chen <liangchen.linux@...il.com>

I won't say for sure, but may be a candidate for the fixes tree, not
next. This way it would need a "Fixes:" tag here (above the SoB).

> ---
>  net/core/dev.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 10e5a036c706..07baf72be7d7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4934,6 +4934,8 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>  			goto do_drop;
>  		if (skb_linearize(skb))
>  			goto do_drop;
> +		if (skb->pp_recycle)
> +			skb->pp_recycle = 0;
>  	}
>  
>  	act = bpf_prog_run_generic_xdp(skb, xdp, xdp_prog);

Thanks,
Olek