On Tue, 2024-02-13 at 14:34 +0100, Eric Dumazet wrote:On Tue, Feb 13, 2024 at 2:02 PM Paolo Abeni <pabeni(a)redhat.com> wrote:Storing in sk_peek_seq the tcp next sequence number to be peeked should avoid changes in the non MSG_PEEK cases. AFAICS that would need a new get_peek_off() sock_op and a bit somewhere (in sk_flags?) to discriminate when sk_peek_seq is actually set. Would that be acceptable? Thanks! PaoloOn Tue, 2024-02-13 at 13:24 +0100, Eric Dumazet wrote:This sk_peek_offset protocol, needing sk_peek_offset_bwd() in the non MSG_PEEK case is very strange IMO. Ideally, we should read/write over sk_peek_offset only when MSG_PEEK is used by the caller. That would only touch non fast paths. Since the API is mono-threaded anyway, the caller should not rely on the fact that normal recvmsg() call would 'consume' sk_peek_offset.On Tue, Feb 13, 2024 at 11:49 AM Paolo Abeni <pabeni(a)redhat.com> wrote: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?> @@ -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().