netdev - Re: [PATCH net-next 3/3] tcp: derive delack_max from rto

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Date: Thu, 21 Sep 2023 06:37:37 -0600
From: David Ahern <dsahern@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski
 <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Soheil Hassas Yeganeh <soheil@...gle.com>,
 Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>,
 netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 3/3] tcp: derive delack_max from rto_min

On 9/20/23 8:16 PM, Eric Dumazet wrote:
> On Wed, Sep 20, 2023 at 11:57 PM David Ahern <dsahern@...nel.org> wrote:
>>
>> On 9/20/23 11:29 AM, Eric Dumazet wrote:
>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>> index 1fc1f879cfd6c28cd655bb8f02eff6624eec2ffc..2d1e4b5ac1ca41ff3db8dc58458d4e922a2c4999 100644
>>> --- a/net/ipv4/tcp_output.c
>>> +++ b/net/ipv4/tcp_output.c
>>> @@ -3977,6 +3977,20 @@ int tcp_connect(struct sock *sk)
>>>  }
>>>  EXPORT_SYMBOL(tcp_connect);
>>>
>>> +u32 tcp_delack_max(const struct sock *sk)
>>> +{
>>> +     const struct dst_entry *dst = __sk_dst_get(sk);
>>> +     u32 delack_max = inet_csk(sk)->icsk_delack_max;
>>> +
>>> +     if (dst && dst_metric_locked(dst, RTAX_RTO_MIN)) {
>>> +             u32 rto_min = dst_metric_rtt(dst, RTAX_RTO_MIN);
>>> +             u32 delack_from_rto_min = max_t(int, 1, rto_min - 1);
>>
>> `u32` type with max_t type set as `int`
> 
> That is because we allow "rto_min 0" in ip route ...
> 
> rto_min - 1 is then 0xFFFFFFFF
> 
> We could argue that "rto_min 0" would be illegal, but this is orthogonal.

My comment is solely about mismatch on data types. I am surprised use of
max_t with mixed data types does not throw a compiler warning.