Date:	Thu, 17 Apr 2008 22:03:07 +0200
From:	Tomasz Grobelny <tomasz@...belny.oswiecenia.net>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	dccp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [DCCP] [RFC] [Patchv2 1/1]: Queuing policies -- reworked version of Tomasz's patch set

Dnia Wednesday 16 of April 2008, Gerrit Renker napisał:
> | > | | What happens if application is compiled with dccp_packet_info
> | > | | containing only one field (priority) and kernel containing two
> | > | | fields (for example priority and later added expiry time)? Wouldn't
> | > | | that implementation make extensions to dccp_packet_info impossible?
> | > |
> | > | Had not looked at it from this point of view. So you are suggesting
> | > |
> | > | 	if (msg->msg_control != NULL && msg->msg_controllen >= sizeof(*dcp))
> | > | 		dcp = (struct dccp_packet_info *)msg->msg_control;
> | > |
> | > | instead? Agreed, point #1 to change in the patch.
> | >
> | > No, the above is nonsense. If the application uses an earlier version
> | > of the API, then it needs to be recompiled, there is no way it could
> | > pretend to have an adequate size.
> |
> | That would effectively stop development of any policy. And it's not about
> | pretending the structure has different size, it's just about using only
> | the data that is provided by the application (even though it may be
> | incomplete).
> |
> | > So my take is that still the '==' is correct. But maybe this is a known
> | > problem?
> |
> | I can't see a problem with copying smaller struct to bigger one. It would
> | work like that:
> | 1. set default values for in-kernel structure,
> | 2. copy min(msg_control, sizeof(struct dccp_packet_info)) bytes from
> | userspace structure to the kernel one.
> |
> | In that sense the code above is in fact wrong but the idea alone should
> | be ok. --
>
> There is a more serious problem here: apparently no one tried to compile
> the `qpolicy' subtree since the changes on Monday. It fails with the
> BUILD_BUG_ON.
>
> I tried yesterday evening and found that there is not even a possibility
> of adding a single field to struct dccp_skb_cb: with the addition of the
> inet{,6}_skb_parm, the struct has reached its maximum size of 48 bytes
> (try printk("%u", sizeof(struct dccp_skb_cb));).
>
> There is a solution to this, will post a revised patch shortly.
>
What you proposed in the patch should work ok for qpolicies for now. But 
sooner or later the need to add a field to struct dccp_skb_cb will arise. So 
maybe we should think about it now...
Other possibility apart from what you proposed in the patch is creating struct 
dccp_skb_cb_ext, move to it some fields from struct dccp_skb_cb and in struct 
add a pointer to this new struct dccp_skb_cb_ext. Ok, maybe it is not the 
prettiest idea but in case somebody needs yet another additional field in 
struct dccp_skb_cb_ext it would be nice to have an idea how to do it.

> With regard to your point above: I think there is a way of keeping the
> framework open without bending over backwards to ensure the binary
> compatibility.
>
While we are developing in the test tree compatibility is not important at 
all. But real world needs compatibility, especially if it's not that 
expensive. Otherwise people will get weird behaviour without any messages 
indicating what is wrong. And it's certainly not how code should be written. 
It can't be that binary package of VLC (or any other streaming server) cannot 
use newer kernel version because we added a new field.

> My question for you in this regard: which policy can you think of whose
> priorities/preferences can not be mapped into an unsigned 32-bit
> integer? I think that such a number is sufficiently expressive enough
> to cater for a wide range of policies. It can be interpreted as
>  * symbolic value (enum)
>  * ascii string (similar to DCCP service code)
>  * millisecond timeout (with room for up to 7 weeks)
>  * microsecond timeout (with room for up to 1.19 hours)
>  * priority values (the large range 0..2^32-1 can be partitioned)
>  * ... ?
>
Putting any of those values in an integer is pretty straightforward. But when 
you want to put both timeout and priority things get messy. Would it be 
possible at least to use structure like that:
struct dccp_packet_info
{
	s8	priority;
	u16	timeout_mantissa:10;
	u16	timeout_exponent:6;
}
? That would still fit in 32-bit integer (so could be stored in 
skb->priority), but would provide a much cleaner interface.
-- 
Regards,
Tomasz Grobelny
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