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

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Date:   Wed, 18 Nov 2020 21:21:06 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
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, 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"

diff --git a/net/core/dev.c b/net/core/dev.c
index 4bfdcd6b20e8836e2884c51c6ce349ed54130bfa..76f0a627b6a1ee02339a724ecb6e4dbade80501b
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);