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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Eric Dumazet <edumazet@google.com>
Date: Mon, 13 Jan 2025 13:55:18 +0100

> On Mon, Jan 13, 2025 at 12:28 PM Thomas Bogendoerfer
> <tbogendoerfer@suse.de> wrote:
>>
>> On Thu, 9 Jan 2025 15:56:24 +0100
>> Eric Dumazet <edumazet@google.com> wrote:
>>
>>> On Thu, Jan 9, 2025 at 3:27 PM Thomas Bogendoerfer
>>> <tbogendoerfer@suse.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@suse.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.
>>
>> ok
>>
>>> Can you remind us why we have this skb_cloned() check here ?
>>
>> some fields of the ip/tcp header are going to be changed in the first gro
>> segment
> 
> Presumably we should test skb_header_cloned()
> 
> This means something like skb_cow_head(skb, 0) could be much more
> reasonable than skb_copy().

Maybe skb_try_make_writable() would fit?

> 
> diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
> index ff8e5b64bf6b76451a69e3eae132b593c60ee204..bd8966484da3fe85d1d87bf847d3730d7ad094e5
> 100644
> --- a/net/core/gro_cells.c
> +++ b/net/core/gro_cells.c
> @@ -20,7 +20,7 @@ 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) || skb_cow_head(skb, 0)) {
>                 res = netif_rx(skb);
>                 goto unlock;
>         }

Thanks,
Olek