Re: Netlink connector

From: Evgeniy Polyakov
Date: Tue Jul 26 2005 - 01:41:10 EST

On Tue, Jul 26, 2005 at 08:14:47AM +0200, Thomas Graf (tgraf@xxxxxxx) wrote:
> * Evgeniy Polyakov <20050726044547.GA32006@xxxxxxxxxxx> 2005-07-26 08:45
> > On Tue, Jul 26, 2005 at 01:46:04AM +0200, Patrick McHardy (kaber@xxxxxxxxx) wrote:
> > > Usually netlink is easily extendable by using nested TLVs. By hiding
> > > this you basically remove this extensibility.
> >
> > Current netlink is not extensible for _many_ different users.
> Patrick's key point was that by hiding some of the functionality
> you remove a lot of the flexbility.
> > It has only 32 sockets.
> You mean MAX_LINKS? That is the current number of reserved
> netlink protocols. The ethertaps are obsolete and can be
> reused so we're currently using 16 out of 256 possible
> protocols. If that is not enough there are ways to work
> around this. However, I also see a need for a generic protocol
> providing a simplified interface for small applications.
> Nevertheless we should take the time and work things out on
> the netlink level first, netlink has issues and we should not
> work around them in a upper layer.
> > > But my main objection is that it sends everything to userspace even
> > > if noone is listening. This can't be used for things that generate
> > > lots of events, and also will get problematic is the number of users
> > > increases.
> >
> > It is a problem for existing netlink - either check in bind time,
> > what could be done for connector, or in socket creation time.
> No, I think you are misunderstanding something. As I said, we can
> easly add a function netlink_nr_subscribers(sk, groups) so the
> check can be done before starting to build the message. This is
> no problem, it simply didn't make sense so far because netlink
> event messages were mostly used for rare events.


> > Actually it is not even a problem, since checking is being done,
> > but after allocation and message filling, such check can be moved into
> > cn_netlink_send() in connector, but different netlink users,
> > who prefers to use different sockets, must perform it by itself in each
> > place, where skb is allocated...
> Sure, which is the right thing, it makes perfect sense to check
> before starting the process of building and event and sending it.
> > Connector is a solution for current situation,
> > it can be deployed with few casualties.
> The problem is that netlink is likely to change in order
> to cope with some recent needs, e.g. ctnetlink but also other
> current issues which need to be addressed. Therefore I suggest
> to build connector on top of the updated netlink so you we have
> one thing less to worry about when thinking about compatibility.
> > Creating a new netlink2 socket for device, which wants to replace ioctl
> > controlling or broadcast it's state is a wrong way.
> Slowly, we might need netlink2 _in case_ we cannot work things
> out without breaking compatibility. This has nothing to do with
> the connector, there are netlink users which have new needs such
> as more groups, at least some of them need the flexibility of
> netlink itself so we have to work things out for them.
> > Different sockets/flows does not allow easy flow control.
> I'm not sure what you mean.

Concider socket overrun - message will be dropped,
using special flags in connector [it's size field was selected to be 4
bytes, and thus has big reserve] this subsystem can requeue message
later after timeout or something similar...

> > We have one pipe - ethernet, and many protocols inside this pipe
> > with different headers - it is the same here - netlink is such a pipe,
> > and with connector it allows to have different protocols in it.
> At least parts of your connector is just a redudant implementation
> of what netlink is already capable of doing. Sure, some of them
> have issues but there is no reason to just build a new protocol on
> top of another one if the protocol beneath has issues which can be
> resolved.
> > > You still have to take care of mixed 64/32 bit environments, u64 fields
> > > for example are differently alligned.
> >
> > Connector has a size in it's header - ioctl does not.
> You have exactly the same issues as netlink as soon as you transfer
> structs, believe it or not.
> > It does not "fix" the "problem" of skb management knowledge, which I
> > described.
> Yes ok, this is a different issue and as Patrick stated already
> those have been mostly worked out by providing a new set of
> macros. Except for a few leftovers, which will be addressed, there
> is no need to call skb functions anymore. The reason the plain
> skb interface was used is simply that the authors of most of the
> netlink using code are in fact very familiar with the skb interface,
> that's it.

I saw your changes - theay are very usefull, but _only_ for sending
part. Kernel receiver still needs dequeuing, freeing and NLKMSG macros.
In first netlink days it also needed skb_recv_msg() or something

> > > You can still built this stuff on top, but the workarounds for netlink
> > > limitations need to be fixed in netlink.
> >
> > I could not call it workaround, I think it is a management layer,
> > which allows :
> Listen, nobody wants to take away your baby. ;-> There are some

Yeah :)

> objections of things which would rather be fixed in the netlink
> layer first and the remaining part that is missing goes into the
> connector. I see a lot of replicated netlink code in the connector
> which is no necessary. I perfectly agree with you that we require
> some form of simplified addressing and easier message handling
> for simple applications but just building another layer on top
> of netlink without respecting the capabilities of netlink itself
> is not the way to go as I see it. For example, we'll probably add
> a new group subscription mechanism to netlink which might perfectly
> suit the needs of your connector.

That is why I raise this question again and againg to see, what ideas should
be moved from connector into netlink and vice versa... :)

> > 1. easy usage. Just register a callback and that is all. Callback will
> > be invoced each time new message arrives. No need to
> > dequeue/free/anything.
> Good point, also doable in netlink directly. Just get rid of the
> usual family_rcv -> family_rcv_skb -> family_rcv_msg process and
> do a callback registration interface instead. However, often the
> processing of a message and the resulting ack must be done as an
> atomic operation, e.g. rtnetlink.

It also better to move into workqueue - just to be sure users will not
do some wrong things...

> > 2. easy usage. Call one function for message delivering, which can
> > care of nonexistent users, perform flow control, congestion control,
> > guarantee delivery and any other.
> I don't understand what exactly you mean but netlink itself
> is not reliable under memory pressure.

Connector has cn_netlink_send() which is a wrapper on top
of skb allocation, queuing and so on.
by flow/congestion control I mean here, that this function
can check for remote peer existing, requeue message if socket
overrun is caught, guarantee that no OOM condition was caught, and
requeue if it was the case and so on.

Evgeniy Polyakov
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