netdev - Re: [PATCH] net: orphan queued skbs if device tx can stall

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Date:	Mon, 9 Apr 2012 11:34:02 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Herbert Xu <herbert@...dor.hengli.com.au>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <hadi@...erus.ca>,
	Stephen Hemminger <shemminger@...tta.com>,
	Jason Wang <jasowang@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jiri Pirko <jpirko@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH] net: orphan queued skbs if device tx can stall

On Mon, Apr 09, 2012 at 04:29:20PM +0800, Herbert Xu wrote:
> On Mon, Apr 09, 2012 at 10:39:54AM +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 09, 2012 at 03:33:54PM +0800, Herbert Xu wrote:
> > > On Mon, Apr 09, 2012 at 10:28:49AM +0300, Michael S. Tsirkin wrote:
> > > >
> > > > > 1) Doesn't this break local UDP push-back?
> > > > 
> > > > What is meant by UDP pushback here? Two tap
> > > > devices communicating by UDP packets locally?
> > > > This was always broken, see below.
> > > 
> > > I mean push-back from UDP transmission to the physical NIC.
> > > 
> > > Your patch breaks that
> > 
> > I think there's some misunderstanding. pushback is only disabled
> > for destinations that set IFF_TX_CAN_STALL. I expect that
> > no physical NICs set this flag - only tun and possibly
> > other userspace-controlled devices in the future.
> 
> I'm talking about an app running in the guest transmitting UDP
> to the physical NIC via virtio/vhost.  This will break with your
> patch.
> 
> Cheers,

Puzzled. This patch orphans skbs only if the destination device sets
IFF_TX_CAN_STALL.  Since the physical NIC doesn't stall forever it never
sets this flag.

So it seems that this patch should not affect the configuration
you describe at all.

Could you please clarify? What did I miss?

> -- 
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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