Re: [PATCH net-next] mptcp: adjust tcp rcvspace after moving skbs from ssk to sk queue

[Paolo Abeni <pabeni@redhat.com>]

Hi,

On Fri, 2020-05-22 at 14:43 +0200, Florian Westphal wrote:
> TCP does tcp rcvbuf tuning when copying packets to userspace, e.g. in
> tcp_read_sock().  In case of mptcp, that function is only rarely used
> (when discarding retransmitted duplicate data).
> 
> Instead, skbs are moved from the tcp rx queue to the mptcp socket rx
> queue.
> Adjust subflow rcvbuf when we do so, its the last spot where we can
> adjust the ssk rcvbuf -- later we only have the mptcp-level socket.
> 
> This greatly improves performance on mptcp bulk transfers.
> 
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  net/mptcp/protocol.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index ba9d3d5c625f..dbb86cbb9e77 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -248,6 +248,9 @@ static bool __mptcp_move_skbs_from_subflow(struct mptcp_sock *msk,
>  
>  	*bytes = moved;
>  
> +	if (moved)
> +		tcp_rcv_space_adjust(ssk);
> +
>  	return done;
>  }

It looks like this way ssk rcvbuf will grow up to tcp_rmem[2] even if
there is no user-space reader - assuming the link is fast enough.

Don't we need to somehow cap that? e.g. moving mptcp rcvbuf update in
mptcp_revmsg()?

Thanks,

Paolo