netdev - Re: [PATCH net] net: let skb_orphan

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Message-ID: <CANn89i++Scdr8f7PbOk0ZkX6+NwhaQBTKmqXXiaQAjFXDijAsQ@mail.gmail.com>
Date:   Tue, 30 Mar 2021 17:29:56 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: let skb_orphan_partial wake-up waiters.

On Tue, Mar 30, 2021 at 5:18 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Tue, 2021-03-30 at 16:40 +0200, Eric Dumazet wrote:
> > On Tue, Mar 30, 2021 at 4:39 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > On Tue, Mar 30, 2021 at 4:25 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > > > Currently the mentioned helper can end-up freeing the socket wmem
> > > > without waking-up any processes waiting for more write memory.
> > > >
> > > > If the partially orphaned skb is attached to an UDP (or raw) socket,
> > > > the lack of wake-up can hang the user-space.
> > > >
> > > > Address the issue invoking the write_space callback after
> > > > releasing the memory, if the old skb destructor requires that.
> > > >
> > > > Fixes: f6ba8d33cfbb ("netem: fix skb_orphan_partial()")
> > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > > ---
> > > >  net/core/sock.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/net/core/sock.c b/net/core/sock.c
> > > > index 0ed98f20448a2..7a38332d748e7 100644
> > > > --- a/net/core/sock.c
> > > > +++ b/net/core/sock.c
> > > > @@ -2137,6 +2137,8 @@ void skb_orphan_partial(struct sk_buff *skb)
> > > >
> > > >                 if (refcount_inc_not_zero(&sk->sk_refcnt)) {
> > > >                         WARN_ON(refcount_sub_and_test(skb->truesize, &sk->sk_wmem_alloc));
> > > > +                       if (skb->destructor == sock_wfree)
> > > > +                               sk->sk_write_space(sk);
> > >
> > > Interesting.
> > >
> > > Why TCP is not a problem here ?
>
> AFAICS, tcp_wfree() does not call sk->sk_write_space(). Processes
> waiting for wmem are woken by ack processing.
>
> > > I would rather replace WARN_ON(refcount_sub_and_test(skb->truesize,
> > > &sk->sk_wmem_alloc)) by :
> > >                         skb_orphan(skb);
> >
> > And of course re-add
> >                         skb->sk = sk;
>
> Double checking to be sure. The patched slice of skb_orphan_partial()
> will then look like:
>
>         if (can_skb_orphan_partial(skb)) {
>                 struct sock *sk = skb->sk;
>
>                 if (refcount_inc_not_zero(&sk->sk_refcnt)) {
>                         skb_orphan(skb);
>                         skb->sk = sk;
>                         skb->destructor = sock_efree;
>                 }
>         } // ...
>
> Am I correct?
>

Yes.

We also could add a helper for the whole construct, since many other
paths do almost the same.
(They might use sock_hold(), but it seems safe to use the
refcount_inc_not_zero())
Or they omit the skb_orphan() (see can_skb_set_owner), which seems also risky.

static inline void skb_set_owner_sk_safe(sk, skb)
{
    if (sk && refcount_inc_not_zero(&sk->sk_refcnt)) {
           skb_orphan(skb);
           skb->destructor = sock_efree;
          skb->sk = sk;
    }
}



> Thanks!
>
> Paolo
>