netdev - Re: [PATCH net] gro_cells: Avoid packet re-ordering for cloned skbs

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Message-ID: <CANn89iKY1x11hHgQDsVtTYe6L_FtN4SKpzFhPk-8fYPp5Wp4ng@mail.gmail.com>
Date: Thu, 9 Jan 2025 15:56:24 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Thomas Bogendoerfer <tbogendoerfer@...e.de>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] gro_cells: Avoid packet re-ordering for cloned skbs

On Thu, Jan 9, 2025 at 3:27 PM Thomas Bogendoerfer
<tbogendoerfer@...e.de> wrote:
>
> gro_cells_receive() passes a cloned skb directly up the stack and
> could cause re-ordering against segments still in GRO. To avoid
> this copy the skb and let GRO do it's work.
>
> Fixes: c9e6bc644e55 ("net: add gro_cells infrastructure")
> Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@...e.de>
> ---
>  net/core/gro_cells.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
> index ff8e5b64bf6b..2f8d688f9d82 100644
> --- a/net/core/gro_cells.c
> +++ b/net/core/gro_cells.c
> @@ -20,11 +20,20 @@ int gro_cells_receive(struct gro_cells *gcells, struct sk_buff *skb)
>         if (unlikely(!(dev->flags & IFF_UP)))
>                 goto drop;
>
> -       if (!gcells->cells || skb_cloned(skb) || netif_elide_gro(dev)) {
> +       if (!gcells->cells || netif_elide_gro(dev)) {
> +netif_rx:
>                 res = netif_rx(skb);
>                 goto unlock;
>         }
> +       if (skb_cloned(skb)) {
> +               struct sk_buff *n;
>
> +               n = skb_copy(skb, GFP_KERNEL);

I do not think we want this skb_copy(). This is going to fail too often.

Can you remind us why we have this skb_cloned() check here ?