M_NOFREE removal (was Re: svn commit: r254520 - in head/sys: kern sys)

[Andre Oppermann]

On 20.08.2013 00:38, Peter Grehan wrote:
> Hi Andre,
>
>   (moving to the more appropriate freebsd-net)
>
>> I'm sorry for ambushing but this stuff has to be done.  I have provided
>> an alternative way of handling it and I'm happy to help you with your
>> use case to make it good for you and to prevent the mbuf system from
>> getting bloated and hackish again.
>
>   Sure. I'm not really upset since my code wasn't too far along, but with any API, you never know
> who consumers might be so it's always worth being proactive about announcing it's removal.

Agreed.  OTOH there wasn't any in-tree user of it and posting before such removals
always yields at least one obscure hand wavy use or potential use of it.  So nothing
ever happens.

>> Can you please describe your intended use of M_NOFREE to better understand
>> the shortcomings of the current mbuf systems and the additional advantages
>> of the M_NOFREE case?
>
>   I was looking at something similar to Linux's vhost-net, where a guest's virtio ring would be
> processed in-kernel. An mbuf chain with external buffers would be used to pass guest tx buffer/len
> segments directly into FreeBSD drivers.

Is that like page flipping?

>   The intent of M_NOFREE was to avoid small mbuf allocations/frees in what is a hot path. This code
> was intended to run at 10/40G.
>
>   Note this code isn't really generic - it would require interfaces to be 'owned' by the guest,
> except that direct PCI-level pass-through wouldn't be needed.

Do you have some example code showing how that is or can be done?

>   If there's an alternative to M_NOFREE, I'd be more than happy to use that.

Set up your own (*ext_free) function and omit freeing of the mbuf itself.  Make
sure to properly track your mbufs to avoid leaking them.

-- 
Andre