Re: [PATCH mptcp-next v2 2/3] mptcp: allow overridden write_space to be invoked

[Matthieu Baerts <matttbe@kernel.org>]

Hi Geliang, Paolo,

On 04/01/2026 06:29, Geliang Tang wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> psock may override its own sk_write_space functions. This patch ensures
> that the overridden sk_write_space can be invoked by MPTCP.
> 
> Note: This patch was initially included in the NVME MPTCP set.
> 
> Suggested-by: Paolo Abeni <pabeni@redhat.com>
> Co-developed-by: Gang Yan <yangang@kylinos.cn>
> Signed-off-by: Gang Yan <yangang@kylinos.cn>
> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> ---
>  net/mptcp/protocol.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index cd5266099993..f5d4d7d030f2 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -1014,7 +1014,7 @@ static inline void mptcp_write_space(struct sock *sk)
>  	/* pairs with memory barrier in mptcp_poll */
>  	smp_mb();
>  	if (mptcp_stream_memory_free(sk, 1))
> -		sk_stream_write_space(sk);
> +		INDIRECT_CALL_1(sk->sk_write_space, sk_stream_write_space, sk);

As I mentioned on the netdev ML when sending this patch upstream, there
is something fishy here: sk->sk_write_space is never set to
sk_stream_write_space here with the MPTCP sockets, but to
sock_def_write_space. (Subflow sockets use sk_stream_write_space.)

In other words, this patch changes the behaviour, which was not supposed
to be the case, now calling sock_def_write_space() instead of
sk_stream_write_space().

So either sk->sk_write_space should be set to sk_stream_write_space for
the MPTCP sockets (I guess, I don't think sk->sk_write_space() is ever
called with an MPTCP socket for the moment), or sock_def_write_space()
should have been called before (then a fix is needed).

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.