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Date: Mon, 11 Jul 2022 17:34:06 +0800
From: Yonglong Li <liyonglong@...natelecom.cn>
To: Eric Dumazet <edumazet@...gle.com>
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 7/11/2022 5:06 PM, Eric Dumazet wrote:
> 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.
I think we can collapse more data if skb->len < avail_wnd. If you think doing this is wrong
I will remove it.
>
> 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
>>
>
--
Li YongLong
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