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

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Date: Thu, 3 Aug 2023 16:26:16 +0800
From: Liang Chen <liangchen.linux@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, edumazet@...gle.com, 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

On Thu, Aug 3, 2023 at 2:37 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed,  2 Aug 2023 15:04:54 +0800 Liang Chen wrote:
> > 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
> > moment(a little nagetive impact on skb_try_coalesce), to avoid potential
> > issues associated with using incorrect skb->pp_recycle information,
> > setting skb->pp_recycle to 0 to reflect the pp state of the skb.
>
> pp_recycle just means that the skb is "page pool aware", there's
> absolutely no harm in having an skb with pp_recycle = 1 and no
> page pool pages attached.

I don't see it causing an error right now either. But it affects
skb_try_coalesce in a negative way from a performance perspective -
from->pp_recycle can be falsely true leading to a coalescing failure
in "(from->pp_recycle && skb_cloned(from))" test, which otherwise
would let the coalesce continue if from->pp_recycle was false. I
wonder if that justifies the need for a fix.

Thanks,
Liang


>
> I vote not to apply this.