Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2024-02-13 at 13:24 +0100, Eric Dumazet wrote:
> On Tue, Feb 13, 2024 at 11:49 AM Paolo Abeni <pabeni@redhat.com> wrote:
> 
> > > @@ -2508,7 +2508,10 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
> > >               WRITE_ONCE(*seq, *seq + used);
> > >               copied += used;
> > >               len -= used;
> > > -
> > > +             if (flags & MSG_PEEK)
> > > +                     sk_peek_offset_fwd(sk, used);
> > > +             else
> > > +                     sk_peek_offset_bwd(sk, used);
> 
> Yet another cache miss in TCP fast path...
> 
> We need to move sk_peek_off in a better location before we accept this patch.
>
> I always thought MSK_PEEK was very inefficient, I am surprised we
> allow arbitrary loops in recvmsg().

Let me double check I read the above correctly: are you concerned by
the 'skb_queue_walk(&sk->sk_receive_queue, skb) {' loop that could
touch a lot of skbs/cachelines before reaching the relevant skb?

The end goal here is allowing an user-space application to read 
incrementally/sequentially the received data while leaving them in
receive buffer.

I don't see a better option than MSG_PEEK, am I missing something?

Thanks,

Paolo