Re: [Xen-devel] [RFC] design/API for plugging tmem into existing xen physical memory management code

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 14/02/2009 15:58, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Are all of these allocated at domain startup only?  Or
> are any (shadow pages perhaps?) allocated at relatively
> random times?  If random, what are the consequences
> if the allocation fails?   Isn't it quite possible
> for a random order>0 allocation to fail today due
> to "natural causes"?  E.g. because the currently running
> domains by coincidence (or by ballooning) have used
> up all available memory?  Have we just been "lucky"
> to date, because fragmentation is so bad and ballooning
> is so rarely used, that we haven't seen failures
> of order>0 allocations? (Or maybe have seen them but
> didn't know it because the observable symptoms are
> a failed domain creation or a failed migration?)

I think the per-domain shadow pool is pre-reserved, so it should be okay.
Lack of memory simply causes domain creation failure. Any extra memory that
shadow code would try to allocate would just be gravy I'm pretty sure.

> Perhaps Jan's idea of using xenheap as an "emergency
> fund" for free pages is really a good idea?

It's a can of worms. How big to make the pool? Who should be allowed to
allocate from it and when? What if the emergency pool becomes exhausted?

> That's a reasonable idea... maybe with a "scrub_me"
> flag set in the struct page_info by tmem and checked by the
> existing alloc_heap_pages (and ignored if a memflags flag
> is passed to alloc_xxxheap_pages() set to "ignore_scrub_me")?
> There'd also need to be a free_and_scrub_domheap_pages().
>
> If you prefer that approach, I'll give it a go.  But still
> some (most?) of the time, there will be no free pages so
> alloc_heap_pages will still need to have a hook to tmem
> for that case.

I'm not super fussed, it's just an idea to consider. Could doing it this new
way make it less possible to scrub pages asynchronously before they're
needed?

> I *think* these calls are just in python code (domain creation
> and ballooning) and, if so, will just go through the existing
> tmem hypercall.

Well, probably okay.

 -- Keir



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