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

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Message-ID: <CANn89iL-CcBxQUvJDn7o2ETSBnwf047hXJEf=q=O3m+qAenPFw@mail.gmail.com>
Date: Mon, 13 Jan 2025 13:55:18 +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 Mon, Jan 13, 2025 at 12:28 PM Thomas Bogendoerfer
<tbogendoerfer@...e.de> wrote:
>
> On Thu, 9 Jan 2025 15:56:24 +0100
> Eric Dumazet <edumazet@...gle.com> wrote:
>
> > 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.
>
> 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().

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;
        }