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Date:	Tue, 28 Apr 2015 17:28:25 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Anuradha Karuppiah <anuradhak@...ulusnetworks.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Roopa Prabhu <roopa@...ulusnetworks.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	Wilson Kok <wkok@...ulusnetworks.com>
Subject: Re: [RFC PATCH net-next v3 0/4] net: Introduce IFF_PROTO_DOWN flag.

On Tue, Apr 28, 2015 at 1:04 PM, Anuradha Karuppiah
<anuradhak@...ulusnetworks.com> wrote:
> On Tue, Apr 28, 2015 at 12:37 PM, Scott Feldman <sfeldma@...il.com> wrote:
>> On Tue, Apr 28, 2015 at 8:39 AM, Anuradha Karuppiah
>> <anuradhak@...ulusnetworks.com> wrote:
>>>
>>>
>>> On Mon, Apr 27, 2015 at 10:45 PM, Scott Feldman <sfeldma@...il.com> wrote:
>>>>
>>>> On Mon, Apr 27, 2015 at 10:38 AM,  <anuradhak@...ulusnetworks.com> wrote:
>>>> > From: Anuradha Karuppiah <anuradhak@...ulusnetworks.com>
>>>> >
>>>> > User space daemons can detect errors in the network that need to be
>>>> > notified to the switch device drivers.
>>>> >
>>>> > Drivers can react to this error state by doing a phy-down on the
>>>> > switch-port which would result in a carrier-off locally and on the
>>>> > directly connected switch. Doing that would prevent loops and
>>>> > black-holes in the network.
>>>>
>>>> (Sorry if this was asked earlier)
>>>>
>>>> Can the application simply send a SETLINK with IFF_UP clear and the
>>>> port driver's ndo_stop would bring the PHY link down?
>>>
>>>
>>> Yes, Clearing IFF_UP on detecting errors (PROTO_DOWN) is possible and we
>>> tried
>>> that implementation as well. Unfortunately it failed because of the
>>> following
>>> reasons -
>>>
>>> 1. There is no way to disambiguate between admin_down (!IFF_UP) and an
>>> APP/driver enforced error_down (IFF_PROTO_DOWN). Administrator or
>>> automation-scripts that monitor the config assumed that switch-port
>>> configuration had somehow fallen out of sync (and attempted to reinstate the
>>> admin_up repeatedly).
>>>
>>> 2. Automatic error recovery was not possible; consider the following
>>> scenario
>>> for e.g.
>>>    a. The MLAG peer-link is down so the MLAG app on the secondary switch has
>>>       proto_down’ed all the MLAG ports (including switch-port swp1) by
>>> clearing
>>>       IFF_UP.
>>>    b. At the same time the administrator is in the process of making some
>>>       changes on the network connected to swp1. To avoid doing it live he
>>> would
>>>       admin_disable swp1 (!IFF_UP) by doing an "ip link set swp1 down" (this
>>>       is a no-op as event #a has already cleared IFF_UP on swp1).
>>>    c. If the MLAG peer-link recovers at this point the MLAG app on the
>>>       secondary switch would try to automatically recover the MLAG ports
>>>       by clearing proto_down (i.e. setting IFF_UP); including on swp1. Doing
>>>       that overrides the administrator’s directive to keep swp1 admin_down.
>>>       Overriding an admin-down in a live network can be very dangerous so it
>>>       is not possible to do auto-error-recovery unless we have a way to
>>>       disambiguate between the admin and error states
>>
>> That makes sense.
>>
>> Dang, this is so close to IFF_DORMANT.  The interface can be IFF_UP
>> and link mode can be DORMANT.  Can the port driver kill PHY link if
>> dev->flags&IFF_DORMANT in ndo_set_rx_mode()?  Would require
>> IFF_DORMANT is included in dev->flags in __dev_change_flags().
>
> Yes, IFF_DORMANT does seem close to what is needed; in the current/standard
> interpretation IFF_DORMANT keeps the switch port phy-up and running (and most
> PDUs are also exchanged in the dormant state). Like you said we could
> re-interpret IFF_DORMANT in this context to phy-down the switch-port;
> unfortunately we are already using IFF_DORMANT as well (in its standard
> interpretation)...

That makes sense; best to not confuse IFF_DORMANT with this new need.

> We are using the dormant mode (for the MLAG app itself) to hold the MLAG port
> in a brief/transition-ary suspended state when the switch-port link/carrier up
> happens. This has been done to co-ordinate states across the MLAG peer switches
> and to ensure that egress port block masks are programmed on the peer switch
> before transitioning the local switch port to an OPER_UP state. If we didn't do
> that the dual-connected server would see duplicate packets every time a
> link-down to link-up happened on a MLAG port.

How can we see this in action?  I didn't find where the kernel egress
blocks the port when dormant.  What are the requirements for a kernel
port driver to support your MLAG app?  Is this MLAG app available
somewhere?
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