VIMAGE UDP memory leak fix

[Robert N. M. Watson]

On 20 Nov 2014, at 23:29, Marko Zec <zec at fer.hr> wrote:

>> Can folks take a look at this?
>> 
>> https://reviews.freebsd.org/D1201
> 
> All UMA zones used in the network stack have been marked as
> UMA_ZONE_NOFREE for ages, probably for a reason, so perhaps it might
> not hurt to provide more insight why and how it suddenly became safe to
> remove that flag?

Historically, this was (if I recall) a property of the way data was exported for netstat, which depended on memory stability of various data types. We have worked quite hard to remove the causes of those sorts of dependencies by introducing stronger reference and memory ownership models throughout the stack -- in the case of inpcbs, for example, we introduced a true reference model during the multiprocessing scalability work (which, it should be pointed out, was enormously painful and took years to shake the bugs out of due to complexity/subtlety). It may be that it is now safe to remove UMA_ZONE_NOFREE for some of the types where it was previously required -- but it's definitely something you want to do intentionally and in the context of a careful analysis to convince yourself that all the causes have been addressed. A fair amount of stress testing in low-memory conditions wouldn't hurt either...

Robert