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Subject: Re: [PATCH net-next v2] net: tcp: support to probe tcp receiver OOM
From:   Menglong Dong <menglong8.dong@gmail.com>
In-Reply-To: <CANn89iJMzChaDsB+bPAuCEDUHVApsYs8KtD3oEC+oU_Qvi1KvQ@mail.gmail.com>
Date:   Wed, 19 Jul 2023 20:35:39 +0800
Cc:     ncardwell@google.com, davem@davemloft.net, kuba@kernel.org,
        pabeni@redhat.com, corbet@lwn.net, dsahern@kernel.org,
        kuniyu@amazon.com, morleyd@google.com, imagedong@tencent.com,
        mfreemon@cloudflare.com, mubashirq@google.com,
        netdev@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
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References: <20230713112404.2022373-1-imagedong@tencent.com>
 <CANn89iJMzChaDsB+bPAuCEDUHVApsYs8KtD3oEC+oU_Qvi1KvQ@mail.gmail.com>
To:     Eric Dumazet <edumazet@google.com>
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> On Jul 18, 2023, at 00:46, Eric Dumazet <edumazet@google.com> wrote:
>=20
> On Thu, Jul 13, 2023 at 1:24=E2=80=AFPM <menglong8.dong@gmail.com> =
wrote:
>>=20
>> From: Menglong Dong <imagedong@tencent.com>
>>=20
>> For now, skb will be dropped directly if rmem schedule fails, which =
means
>> tcp_try_rmem_schedule() returns an error. This can happen on =
following
>> cases:
>>=20
>> 1. The total memory allocated for TCP protocol is up to tcp_mem[2], =
and
>>   the receive queue of the tcp socket is not empty.
>> 2. The receive buffer of the tcp socket is full, which can happen on =
small
>>   packet cases.
>>=20
>> If the user hangs and doesn't take away the packet in the receive =
queue
>> with recv() or read() for a long time, the sender will keep
>> retransmitting until timeout, and the tcp connection will break.
>>=20
>> In order to handle such case, we introduce the tcp protocol OOM =
detection
>> in following steps, as Neal Cardwell suggested:
>>=20
>=20
> For the record, I dislike this patch. I am not sure what Neal had in =
mind.
>=20
> I suggested instead to send an ACK RWIN 0, whenever we were under
> extreme memory pressure,
> and we only could queue one skb in the receive queue.
>=20
> For details, look at the points we call sk_forced_mem_schedule().
> This would be a matter of refactoring code around it, in =
tcp_data_queue()
>=20
> The patch would be much simpler. Nothing changed at the sender side :/

I think you are right. I misunderstood the code in =
tcp_retransmit_timer().
It seems that it already handle the window shrink case properly.

Let me do more testing first.

Thanks!
Menglong Dong