From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH] tun: only queue packets on device Date: Mon, 03 Dec 2012 13:41:32 -0500 (EST) Message-ID: <20121203.134132.558381056546064453.davem@davemloft.net> References: <20121203131942.GA27953@redhat.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, jasowang@redhat.com, nhorman@tuxdriver.com, ramirose@gmail.com, linux-kernel@vger.kernel.org To: mst@redhat.com Return-path: In-Reply-To: <20121203131942.GA27953@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org From: "Michael S. Tsirkin" Date: Mon, 3 Dec 2012 15:19:43 +0200 > Historically tun supported two modes of operation: > - in default mode, a small number of packets would get queued > at the device, the rest would be queued in qdisc > - in one queue mode, all packets would get queued at the device > > This might have made sense up to a point where we made the > queue depth for both modes the same and set it to > a huge value (500) so unless the consumer > is stuck the chance of losing packets is small. > > Thus in practice both modes behave the same, but the > default mode has some problems: > - if packets are never consumed, fragments are never orphaned > which cases a DOS for sender using zero copy transmit > - overrun errors are hard to diagnose: fifo error is incremented > only once so you can not distinguish between > userspace that is stuck and a transient failure, > tcpdump on the device does not show any traffic > > Userspace solves this simply by enabling IFF_ONE_QUEUE > but there seems to be little point in not doing the > right thing for everyone, by default. > > Signed-off-by: Michael S. Tsirkin Now that TUN_NO_QUEUE has no real effect and is a NOP, please document it as such both in if_tun.h and the places in the driver that flip the bit based upon userspace requests. Thanks.