Re: [Xen-devel] long latency of domain shutdown

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 8/5/08 13:33, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> In actuality, since we know that we never have 'cross-domain' pagetable type
>> references, we should actually be able to zap pagetable reference counts to
>> zero. The only reason we don't do that right now is really because it
>> provides good debugging info to see whether a domain's refcounts have got
>> screwed up. But that would not prevent us doing something faster for NDEBUG
>> builds, at least.
>> 
>> Does that make sense?
> 
> Yes, except for me not immediately seeing why this is then not also a
> problem for the current circular reference handling.

Because ultimately the reference(s) that are still being held on the page we
are unvalidating and calling free_lX_table() on will get dropped, due to the
fact we are breaking the circular chain and calling
free_lX_table()->put_page_and_type()->...

> But really, rather than introducing (and fixing) the hack here I'd much
> prefer a generic solution to the problem, and you didn't say a word on
> the thoughts I had on that (but in a mail a couple of days ago you
> indicated you might get around doing something in that area yourself,
> so I half way implied you may have a mechanism in mind already).

I don't have a very clear plan, except that some kind of continuation
(basically encoding of how far we got) must be encoded in the page_info
structure. We should be able to find spare bits for a page which is in this
in-between state.

 -- Keir



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