Re: [net, v2.1] tcp: correct handling of extreme memory squeeze

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 16 Jan 2025 16:57:11 -0500
Jon Maloy <jmaloy@redhat.com> wrote:

> Testing with iperf3 using the "pasta" protocol splicer has revealed
> a bug in the way tcp handles window advertising in extreme memory
> squeeze situations. The problem occurs on the 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 this socket.
> 
> A brief summary: 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.

That's a looong paragraph now.

> 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]
> [5201<->39222]:     tcp_data_queue(<-) OFO queue: gap: 65480, len: 0
> [5201<->39222]:     __tcp_transmit_skb(->)
> [5201<->39222]:       tcp_select_window(->) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]:         (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE
> [5201<->39222]:       tcp_select_window(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160, returning 0
> [5201<->39222]:       ADVERTISING WIN 0, ACK_SEQ: 265600160
> [5201<->39222]:     __tcp_transmit_skb(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [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()
> [5201<->39222]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]: tcp_recvmsg_locked(<-) returning 6104 bytes.
>                 [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                 [copied_seq 260040464->260040464 (0), unread 5559696, qlen 85, ofoq 0]
> 
> // 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()
> [5201<->39222]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]: tcp_recvmsg_locked(<-) returning 131072 bytes.
>                 [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                 [copied_seq 260099840->260171536 (71696), unread 5428624, qlen 83, ofoq 0]
> 
> // 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()
> [5201<->39222]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]: tcp_recvmsg_locked(<-) returning 131072 bytes.
>                 [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                 [copied_seq 265469200->265545488 (76288), unread 54672, qlen 1, ofoq 0]
> 
> [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()
> [5201<->39222]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 265469200, tp->rcv_wnd: 262144, tp->rcv_nxt 265600160
> [5201<->39222]: tcp_recvmsg_locked(<-) returning 54672 bytes.
>                 [rcv_nxt 265600160, rcv_wnd 262144, snt_ack 265469200, win_now 131184]
>                 [copied_seq 265600160->265600160 (0), unread 0, qlen 0, ofoq 0]
> 
> // The receive queue is empty, but no new advertisement is 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 setting tp->rcv_wnd and tp->rcv_wup even 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")
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
>
> ---

By the way, for what it's worth:

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

But you should drop the extra newline between Signed-off-by: and ---

-- 
Stefano