netdev - Re: [PATCH net-next] net: preserve sock reference when scrubbing the skb.

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Date:   Wed, 27 Jun 2018 12:55:37 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Flavio Leitner <fbl@...hat.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        NetFilter <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: preserve sock reference when scrubbing the skb.

On Wed, Jun 27, 2018 at 12:33 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 06/27/2018 11:59 AM, Cong Wang wrote:
>
> >
> > IIRC, this skb_orphan() was introduced much earlier than TSQ, probably
> > from the beginning of veth.
>
> Sigh
>
> SO_SNDBUF was invented years ago before veth.

Yeah, probably when there was only one stack on one host.
SO_SNDBUF is aligned to networking stack basis.

>
> You focus on TSQ while it is only one of the many things that are broken.
>

I think it is the opposite: this patchset _potentially_ breaks things, not fixes
anything.


> >
> > Leaving the stack should be effectively equivalent to leaving the host,
> > from the view of network isolation.
> >
>
>
> Having a UDP socket being able to burn a cpu and fill a qdisc is a major bug.
>


Very true, network isolation never isolates CPU or memory.
It is cpuset's job to provide physical CPU isolation, not networking
namespace. I don't want to defend this, but it is the current design.


> Bu default (blocking send() syscalls) the following loop should
> block the thread if socket sk_wmem_alloc hits sk_sndbuf, this is
> the beauty of backpressure.
>
> while (1)
>     send(fd, ...);
>
> With skb_orphan(), sk_wmem_alloc will stay around 0, so the loop will burn a cpu
> and fill a qdisc, eventually breaking "network isolation", since other sockets
> might be unable to send a single packet.

Won't the same happen when congestion on a physical connection
between two hosts? Does 'host isolation' break too?

>
> If you have a concrete case where the skb_orphan() is needed, then you will have
> to add a parameter to let the admin opt-in for this.

Please see the other reply from me, where I list 3 or 4 reasons.