Re: [net-next 2/2] tcp: correct handling of extreme menory squeeze

[Eric Dumazet <edumazet@google.com>]

On Sat, Apr 6, 2024 at 8:37 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote:
> >
> > From: Jon Maloy <jmaloy@redhat.com>
> >
> > Testing of the previous commit ("tcp: add support for SO_PEEK_OFF")
> > in this series along with the pasta protocol splicer revealed a bug in
> > the way tcp handles window advertising during extreme memory squeeze
> > situations.
> >
> > The excerpt of the below logging session shows what is happeing:
> >
> > [5201<->54494]:     ==== Activating log @ tcp_select_window()/268 ====
> > [5201<->54494]:     (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE
> > [5201<->54494]:   tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0
> > [5201<->54494]:   ADVERTISING WINDOW SIZE 0
> > [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> >
> > [5201<->54494]: tcp_recvmsg_locked(->)
> > [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> > [5201<->54494]:     NOT calling tcp_send_ack()
> > [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83
> >
> > [...]
>
> I would prefer a packetdrill test, it is not clear what is happening...
>
> In particular, have you used SO_RCVBUF ?
>
> >
> > [5201<->54494]: tcp_recvmsg_locked(->)
> > [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> > [5201<->54494]:     NOT calling tcp_send_ack()
> > [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1
> >
> > [5201<->54494]: tcp_recvmsg_locked(->)
> > [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> > [5201<->54494]:     NOT calling tcp_send_ack()
> > [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0
> >
> > [5201<->54494]: tcp_recvmsg_locked(->)
> > [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]:     NOT calling tcp_send_ack()
> > [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> > [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0
> >
> > We can see that although we are adverising a window size of zero,
> > tp->rcv_wnd is not updated accordingly. This leads to a discrepancy
> > between this side's and the peer's view of the current window size.
> > - The peer thinks the window is zero, and stops sending.
> > - This side ends up in a cycle where it repeatedly caclulates a new
> >   window size it finds too small to advertise.
> >
> > Hence no messages are received, and no acknowledges are sent, and
> > the situation remains locked even after the last queued receive buffer
> > has been consumed.
> >
> > We fix this by setting tp->rcv_wnd to 0 before we return from the
> > function tcp_select_window() in this particular case.
> > Further testing shows that the connection recovers neatly from the
> > squeeze situation, and traffic can continue indefinitely.
> >
> > Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
> > Signed-off-by: Jon Maloy <jmaloy@redhat.com>

I do not think this patch is good. If we reach zero window, it is a
sign something is wrong.

TCP has heuristics to slow down the sender if the receiver does not
drain the receive queue fast enough.

MSG_PEEK is an obvious reason, and SO_RCVLOWAT too.

I suggest you take a look at tcp_set_rcvlowat(), see what is needed
for SO_PEEK_OFF (ab)use ?

In short, when SO_PEEK_OFF is in action :
- TCP needs to not delay ACK when receive queue starts to fill
- TCP needs to make sure sk_rcvbuf and tp->window_clamp grow (if
autotuning is enabled)