Re: [Qemu-devel] [PATCH] tun: orphan an skb on tx

[David Woodhouse]

On Wed, 2010-04-14 at 08:58 +0800, Herbert Xu wrote:
> On Tue, Apr 13, 2010 at 08:31:03PM +0200, Eric Dumazet wrote:
> >
> > Herbert Acked your patch, so I guess its OK, but I think it can be
> > dangerous.
> 
> The tun socket accounting was never designed to stop it from
> flooding another tun interface.  It's there to stop it from
> transmitting above a destination interface TX bandwidth and
> cause unnecessary packet drops.  It also limits the total amount
> of kernel memory that can be pinned down by a single tun interface.
> 
> In this case, all we're doing is shifting the accounting from the
> "hardware" queue to the qdisc queue.
> 
> So your ability to flood a tun interface is essentially unchanged.

I've just been looking at VPN performance, using netperf to flood an
openconnect/ocserv connection over GigE and profiling my VPN client.

If I run netperf over the *unencrypted* link, it only sends 1Gb/s of
packets — because the packets are correctly accounted to netperf's UDP
socket until the moment they're actually transmitted on the wire, and
the backpressure works correctly.

When I run over the VPN, netperf thinks it sent 2½ times the amount of
TX traffic. Packets are being dropped by the tun device before even
feeding them up to the VPN client to be sent — presumably because of
this skb_orphan() call. (The client itself should do the right thing,
and only suck packets out of the tun at the rate it can shove them out
*its* UDP socket.)

Did we ever look at the alternative solution of taking ownership only
after a timeout, or on demand when we need to shut down the device?

-- 
dwmw2

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