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Re: [Xen-devel] [PATCH V5 2/3] x86/mm: allocate logdirty_ranges for altp2ms

>>> On 13.11.18 at 19:57, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 11/13/18 8:43 PM, Razvan Cojocaru wrote:
>> On 11/13/18 7:57 PM, George Dunlap wrote:
>>> Is there any particular reason we allocate the p2m structures on domain
>>> creation, but not logdirty range structures?  It seems like allocating
>>> altp2m structures on-demand, rather than at domain creation time, might
>>> make a lot of the reasoning here simpler.
>> I assume that this question is not addressed to me, since I'm not able
>> to answer it - I can only assume that having less heap used has been
>> preferred.
> Actually I now realize that you're asking why the hostp2m rangeset is
> created via paging_domain_init() in arch_domain_create() (so immediately
> on domain creation) while I'm allocating the altp2m rangesets on altp2m
> init.
> I'm doing that to save memory, since we can have MAX_ALTP2M altp2ms
> (which is currently 10), and only two active altp2ms - that means that I
> would allocate 10 rangesets and only use two. In fact we're currently
> only using 2 altp2ms and the hostp2m for our #VE work. That saves the
> space required for 8 rangesets. If that's not much, or if you think that
> the benefits of allocating them early outweigh the costs we can switch
> to allocating them on domain creation, like the hostp2m, and perhaps
> always keeping them in sync.

I think George's question had the opposite goal: Rather than just
allocating the rangesets on demand, why don't the entire altp2m
structures get allocated on demand?


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