netdev - Re: [PATCH net 2/2] net: Call skb destructor on NAPI_GRO_FREE_STOLEN

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Date:   Wed, 25 Nov 2020 14:11:36 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        Tariq Toukan <tariqt@...dia.com>
Subject: Re: [PATCH net 2/2] net: Call skb destructor on
 NAPI_GRO_FREE_STOLEN_HEAD

On Wed, 2020-11-18 at 21:21 +0100, Eric Dumazet wrote:
> On Wed, Nov 18, 2020 at 9:14 PM Jakub Kicinski <kuba@...nel.org>
> wrote:
> > On Wed, 18 Nov 2020 21:02:29 +0100 Eric Dumazet wrote:
> > > On Wed, Nov 18, 2020 at 8:22 PM Jakub Kicinski <kuba@...nel.org>
> > > wrote:
> > > > On Tue, 17 Nov 2020 12:33:55 -0800 Saeed Mahameed wrote:
> > > > > From: Maxim Mikityanskiy <maximmi@...lanox.com>
> > > > > 
> > > > > All GRO flows except one call skb->destructor, however,
> > > > > GRO_MERGED_FREE
> > > > > doesn't do it in case of NAPI_GRO_FREE_STOLEN_HEAD. For
> > > > > better
> > > > > consistency and to add resiliency against the drivers that
> > > > > may pass SKBs
> > > > > with a destructor, this patch changes
> > > > > napi_skb_free_stolen_head to use
> > > > > skb_release_head_state, which should perform all the needed
> > > > > cleanups,
> > > > > including a call to the destructor. This way the code of
> > > > > GRO_MERGED_FREE
> > > > > becomes similar to kfree_skb_partial.
> > > > > 
> > > > > Fixes: e44699d2c280 ("net: handle NAPI_GRO_FREE_STOLEN_HEAD
> > > > > case also in napi_frags_finish()")
> > > > > Fixes: d7e8883cfcf4 ("net: make GRO aware of skb->head_frag")
> > > > > Signed-off-by: Maxim Mikityanskiy <maximmi@...lanox.com>
> > > > > Reviewed-by: Tariq Toukan <tariqt@...dia.com>
> > > > > Signed-off-by: Saeed Mahameed <saeedm@...dia.com>
> > > > 
> > > > CC Eric for GRO expertise.
> > > 
> > > Thanks for CCing me.
> > > 
> > > Since when drivers can pass funny skbs with destructors ???
> > > 
> > > Can we please stop adding more cycles to _already_ expensive GRO
> > > ?
> > 
> > I don't think they do that today much (save for the ktls
> > optimization
> > in mlx5 Maxim is fixing separately). But I believe the idea of
> > early
> > demux in XDP had been floated in the past.
> > 
> > If we don't want that to happen we should document it (stating the
> > obvious).
> 
> This is a patch targeting the net tree, with Fixes: tag pretending
> this is an old bug.
> 
> How can we possibly merge two skbs if they have destructors ?
> 
> We do not make sure it is even possible.
> 
> Many destructors track skb->truesize against a socket wmem_alloc or
> rmem_alloc,
> this stuff can not possibly work, unless stronger checks in GRO,
> since
> GRO changes skb->truesize
> without checking skb->destructor.
> 
> If skb has a destructor, just bypass GRO completely, this is the only
> thing we can do.
> This would be quite unfortunate to add such a check "just because
> someone tries to fool us"
> 

Thanks Eric !!
We don't actually need this patch, as the kTLS SKBs are handled locally
in the drivers, I think we don't need to add any extra check in the
datapath and just enforce the policy somehow with debug macros
maybe WARN_ONE_ONCE()

I will drop this patch, but the XDP folks who are going to implement
XDP early demux should take care of this themselves.

> diff --git a/net/core/dev.c b/net/core/dev.c
> index
> 4bfdcd6b20e8836e2884c51c6ce349ed54130bfa..76f0a627b6a1ee02339a724ecb6
> e4dbade80501b
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5920,7 +5920,7 @@ static enum gro_result dev_gro_receive(struct
> napi_struct *napi, struct sk_buff
>         int same_flow;
>         int grow;
> 
> -       if (netif_elide_gro(skb->dev))
> +       if (netif_elide_gro(skb->dev) || skb->destructor)
>                 goto normal;
> 
>         gro_head = gro_list_prepare(napi, skb);