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

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Message-ID: <20201118121422.512ced1d@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net>
Date:   Wed, 18 Nov 2020 12:14:22 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Saeed Mahameed <saeedm@...dia.com>,
        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, 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).