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Date: Mon, 11 Jul 2022 11:06:53 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Yonglong Li <liyonglong@...natelecom.cn>
Cc: netdev <netdev@...r.kernel.org>,
David Miller <davem@...emloft.net>,
David Ahern <dsahern@...nel.org>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v3] tcp: make retransmitted SKB fit into the send window
On Mon, Jul 11, 2022 at 9:56 AM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>
> current code of __tcp_retransmit_skb only check TCP_SKB_CB(skb)->seq
> in send window, and TCP_SKB_CB(skb)->seq_end maybe out of send window.
> If receiver has shrunk his window, and skb is out of new window, it
> should retransmit a smaller portion of the payload.
>
> test packetdrill script:
> 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
> +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
>
> +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
> +0 > S 0:0(0) win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8>
> +.05 < S. 0:0(0) ack 1 win 6000 <mss 1000,nop,nop,sackOK>
> +0 > . 1:1(0) ack 1
>
> +0 write(3, ..., 10000) = 10000
>
> +0 > . 1:2001(2000) ack 1 win 65535
> +0 > . 2001:4001(2000) ack 1 win 65535
> +0 > . 4001:6001(2000) ack 1 win 65535
>
> +.05 < . 1:1(0) ack 4001 win 1001
>
> and tcpdump show:
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 1:2001, ack 1, win 65535, length 2000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 2001:4001, ack 1, win 65535, length 2000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
> 192.0.2.1.8080 > 192.168.226.67.55: Flags [.], ack 4001, win 1001, length 0
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
>
> when cient retract window to 1001, send window is [4001,5002],
> but TLP send 5001-6001 packet which is out of send window.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Yonglong Li <liyonglong@...natelecom.cn>
> ---
> net/ipv4/tcp_output.c | 36 ++++++++++++++++++++++++------------
> 1 file changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 18c913a..efd0f05 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -3100,7 +3100,6 @@ static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
> static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
> int space)
> {
> - struct tcp_sock *tp = tcp_sk(sk);
> struct sk_buff *skb = to, *tmp;
> bool first = true;
>
> @@ -3123,14 +3122,18 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
> continue;
> }
>
> - if (space < 0)
> - break;
> -
> - if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
> + if (space < 0) {
> + if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE,
> + skb, space + skb->len,
> + tcp_current_mss(sk), GFP_ATOMIC)))
What are you doing here ?
This seems wrong.
Can we please stick to the patch I sent earlier.
If you want to amend it later, you can do this in a separate patch,
with a clear explanation.
> + break;
> + tcp_collapse_retrans(sk, to);
> break;
> + }
>
> if (!tcp_collapse_retrans(sk, to))
> break;
> +
> }
> }
>
> @@ -3144,7 +3147,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
> struct tcp_sock *tp = tcp_sk(sk);
> unsigned int cur_mss;
> int diff, len, err;
> -
> + int avail_wnd;
>
> /* Inconclusive MTU probe */
> if (icsk->icsk_mtup.probe_size)
> @@ -3166,17 +3169,25 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
> return -EHOSTUNREACH; /* Routing failure or similar. */
>
> cur_mss = tcp_current_mss(sk);
> + avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
>
> /* If receiver has shrunk his window, and skb is out of
> * new window, do not retransmit it. The exception is the
> * case, when window is shrunk to zero. In this case
> - * our retransmit serves as a zero window probe.
> + * our retransmit of one segment serves as a zero window probe.
> */
> - if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
> - TCP_SKB_CB(skb)->seq != tp->snd_una)
> - return -EAGAIN;
> + if (avail_wnd <= 0) {
> + if (TCP_SKB_CB(skb)->seq != tp->snd_una)
> + return -EAGAIN;
> + avail_wnd = cur_mss;
> + }
>
> len = cur_mss * segs;
> + if (len > avail_wnd) {
> + len = rounddown(avail_wnd, cur_mss);
> + if (!len)
> + len = avail_wnd;
> + }
> if (skb->len > len) {
> if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
> cur_mss, GFP_ATOMIC))
> @@ -3190,8 +3201,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
> diff -= tcp_skb_pcount(skb);
> if (diff)
> tcp_adjust_pcount(sk, skb, diff);
> - if (skb->len < cur_mss)
> - tcp_retrans_try_collapse(sk, skb, cur_mss);
> + avail_wnd = min_t(int, avail_wnd, cur_mss);
> + if (skb->len < avail_wnd)
> + tcp_retrans_try_collapse(sk, skb, avail_wnd);
> }
>
> /* RFC3168, section 6.1.1.1. ECN fallback */
> --
> 1.8.3.1
>
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