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From:   Eric Dumazet <edumazet@google.com>
Date:   Sat, 28 Jul 2018 09:28:52 -0700
Message-ID: <CANn89i+12HvAsvk9crDcqGh=v-DvLjuGQCOzVTPVDTUhHgeDiw@mail.gmail.com>
Subject: Re: [PATCH net-next 1/2] tcp: call tcp_drop() in tcp collapse
To:     Yafang Shao <laoar.shao@gmail.com>
Cc:     David Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Sat, Jul 28, 2018 at 12:43 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Sat, Jul 28, 2018 at 11:38 AM, Eric Dumazet <edumazet@google.com> wrote:
> > On Fri, Jul 27, 2018 at 8:35 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> >> So what about LINUX_MIB_TCPOFOMERGE ?
> >> Regarding LINUX_MIB_TCPOFOMERGE,  a skb is already covered by another
> >> skb, is that dropping the packet or simply lowering the memory
> >> overhead ?
> >
> > What do you think ?
> >
> > If you receive two times the same payload, don't you have to drop one
> > of the duplicate ?
> >
> > There is a a big difference between the two cases.
>
> If the drop caused some data lost (which may then cause retransmition
> or something), then this is a really DROP.
> While if the drop won't cause any data lost, meaning it is a
> non-harmful behavior, I think it should not be defined as DROP.
> This is my suggestion anyway.

Sigh.

We count drops, not because they are ' bad or something went wrong'.

If TCP stack receives twice the same sequence (same payload), we
_drop_ one of the duplicate, so we account for this event.

When ' collapsing'  we reorganize our own storage, not because we have
to drop a payload,
but for some memory pressure reason.
We have specific SNMP counters to account for these, we do not want to
pretend a packet was ' dropped' since it was not.

If we have to _drop_ some packets, it is called Pruning, and we do
properly account for these drops.