Subject: Re: [RFC mptcp-next] tcp: mptcp: use mptcp receive buffer space to select rcv window

From: Florian Westphal <[email-masked]>
Date: Wed, 18 Mar 2020 19:05:26 +0100

Eric Dumazet <[email-masked]> wrote:
> > @@ -2771,6 +2771,11 @@ u32 __tcp_select_window(struct sock *sk)
> >  	int full_space = min_t(int, tp->window_clamp, allowed_space);
> >  	int window;
> >  
> > +	if (sk_is_mptcp(sk)) {
> > +		mptcp_space(sk, &free_space, &allowed_space);
> > +		full_space = min_t(int, tp->window_clamp, allowed_space);
> > +	}
> 
> You could move the full_space = min_t(int, tp->window_clamp, allowed_space);
> after this block factorize it.

Indeed, will do.

> > diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> > index 40ad7995b13b..aefcbb8bb737 100644
> > --- a/net/mptcp/subflow.c
> > +++ b/net/mptcp/subflow.c
> > @@ -745,6 +745,23 @@ bool mptcp_subflow_data_available(struct sock *sk)
> >  	return subflow->data_avail;
> >  }
> >  
> > +/* If ssk has an mptcp parent socket, use the mptcp rcvbuf occupancy,
> > + * not the ssk one.
> > + *
> > + * In mptcp, rwin is about the mptcp-level connection data.
> > + *
> > + * Data that is still on the ssk rx queue can thus be ignored,
> > + * as far as mptcp peer is concerened that data is still inflight.
> > + */
> > +void mptcp_space(const struct sock *ssk, int *space, int *full_space)
> > +{
> > +	const struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
> > +	const struct sock *sk = READ_ONCE(subflow->conn);
> 
> What are the rules protecting subflow->conn lifetime ?
> 
> Why dereferencing sk after this line is safe ?

Subflow sockets hold a reference on the master/parent mptcp-socket.