Re: [net,v3] tcp: correct handling of extreme memory squeeze

[Jason Xing <kerneljasonxing@gmail.com>]

On Tue, Jan 28, 2025 at 7:13 AM <jmaloy@redhat.com> wrote:
>
> From: Jon Maloy <jmaloy@redhat.com>
>
> Testing with iperf3 using the "pasta" protocol splicer has revealed
> a bug in the way tcp handles window advertising in extreme memory
> squeeze situations.
>
> Under memory pressure, a socket endpoint may temporarily advertise
> a zero-sized window, but this is not stored as part of the socket data.
> The reasoning behind this is that it is considered a temporary setting
> which shouldn't influence any further calculations.
>
> However, if we happen to stall at an unfortunate value of the current
> window size, the algorithm selecting a new value will consistently fail
> to advertise a non-zero window once we have freed up enough memory.
> This means that this side's notion of the current window size is
> different from the one last advertised to the peer, causing the latter
> to not send any data to resolve the sitution.
>
> The problem occurs on the iperf3 server side, and the socket in question
> is a completely regular socket with the default settings for the
> fedora40 kernel. We do not use SO_PEEK or SO_RCVBUF on the socket.
>
> The following excerpt of a logging session, with own comments added,
> shows more in detail what is happening:
>
> //              tcp_v4_rcv(->)
> //                tcp_rcv_established(->)
> [5201<->39222]:     ==== Activating log @ net/ipv4/tcp_input.c/tcp_data_queue()/5257 ====
> [5201<->39222]:     tcp_data_queue(->)
> [5201<->39222]:        DROPPING skb [265600160..265665640], reason: SKB_DROP_REASON_PROTO_MEM
>                        [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                        [copied_seq 259909392->260034360 (124968), unread 5565800, qlen 85, ofoq 0]
>                        [OFO queue: gap: 65480, len: 0]
> [5201<->39222]:     tcp_data_queue(<-)
> [5201<->39222]:     __tcp_transmit_skb(->)
>                         [tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
> [5201<->39222]:       tcp_select_window(->)
> [5201<->39222]:         (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) ? --> TRUE
>                         [tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
>                         returning 0
> [5201<->39222]:       tcp_select_window(<-)
> [5201<->39222]:       ADVERTISING WIN 0, ACK_SEQ: 265600160
> [5201<->39222]:     [__tcp_transmit_skb(<-)
> [5201<->39222]:   tcp_rcv_established(<-)
> [5201<->39222]: tcp_v4_rcv(<-)
>
> // Receive queue is at 85 buffers and we are out of memory.
> // We drop the incoming buffer, although it is in sequence, and decide
> // to send an advertisement with a window of zero.
> // We don't update tp->rcv_wnd and tp->rcv_wup accordingly, which means
> // we unconditionally shrink the window.
>
> [5201<->39222]: tcp_recvmsg_locked(->)
> [5201<->39222]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]:     [new_win = 0, win_now = 131184, 2 * win_now = 262368]
> [5201<->39222]:     [new_win >= (2 * win_now) ? --> time_to_ack = 0]
> [5201<->39222]:     NOT calling tcp_send_ack()
>                     [tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
> [5201<->39222]:   __tcp_cleanup_rbuf(<-)
>                   [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                   [copied_seq 260040464->260040464 (0), unread 5559696, qlen 85, ofoq 0]
>                   returning 6104 bytes
> [5201<->39222]: tcp_recvmsg_locked(<-)
>
> // After each read, the algorithm for calculating the new receive
> // window in __tcp_cleanup_rbuf() finds it is too small to advertise
> // or to update tp->rcv_wnd.
> // Meanwhile, the peer thinks the window is zero, and will not send
> // any more data to trigger an update from the interrupt mode side.
>
> [5201<->39222]: tcp_recvmsg_locked(->)
> [5201<->39222]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]:     [new_win = 262144, win_now = 131184, 2 * win_now = 262368]
> [5201<->39222]:     [new_win >= (2 * win_now) ? --> time_to_ack = 0]
> [5201<->39222]:     NOT calling tcp_send_ack()
>                     [tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
> [5201<->39222]:   __tcp_cleanup_rbuf(<-)
>                   [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                   [copied_seq 260099840->260171536 (71696), unread 5428624, qlen 83, ofoq 0]
>                   returning 131072 bytes
> [5201<->39222]: tcp_recvmsg_locked(<-)
>
> // The above pattern repeats again and again, since nothing changes
> // between the reads.
>
> [...]
>
> [5201<->39222]: tcp_recvmsg_locked(->)
> [5201<->39222]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]:     [new_win = 262144, win_now = 131184, 2 * win_now = 262368]
> [5201<->39222]:     [new_win >= (2 * win_now) ? --> time_to_ack = 0]
> [5201<->39222]:     NOT calling tcp_send_ack()
>                     [tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160]
> [5201<->39222]:   __tcp_cleanup_rbuf(<-)
>                   [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                   [copied_seq 265600160->265600160 (0), unread 0, qlen 0, ofoq 0]
>                   returning 54672 bytes
> [5201<->39222]: tcp_recvmsg_locked(<-)
>
> // The receive queue is empty, but no new advertisement has been sent.
> // The peer still thinks the receive window is zero, and sends nothing.
> // We have ended up in a deadlock situation.
>
> Furthermore, we have observed that in these situations this side may
> send out an updated 'th->ack_seq´ which is not stored in tp->rcv_wup
> as it should be. Backing ack_seq seems to be harmless, but is of
> course still wrong from a protocol viewpoint.
>
> We fix this by updating the socket state correctly when a packet has
> been dropped because of memory exhaustion and we have to advertize
> a zero window.
>
> Further testing shows that the connection recovers neatly from the
> squeeze situation, and traffic can continue indefinitely.
>
> Fixes: e2142825c120 ("net: tcp: send zero-window ACK when no memory")
> Cc: Menglong Dong <menglong8.dong@gmail.com>
> Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

I reckon that adding more description about why this case can be
triggered (the reason behind this case is the other side using other
kernels doesn't periodically send a window probe) is really necessary.

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>

Thanks,
Jason