Re: [Xen-devel] [PATCH v3 3/3] tools/libxc: use superpages during restore of HVM guest

[Olaf Hering <olaf@xxxxxxxxx>]

On Fri, Aug 25, Olaf Hering wrote:

> I think with the new check of max_pages an overallocation can not happen
> anymore. If at some point the domU still has room for a superpage, it
> will be allocated. In case the batch does not fully fill the superpage,
> the holes will be freed. In the next batch no superpage can be allocated
> anymore, but single pages will be used.

There is one case where Over-allocation will happen: assume
x86_hvm_populate_pfns gets a batch of pfns that fit trigger the
allocation of a 1G page. All pfns will fit into that partly populated
superpage. Then the guest has a hole right after the max_pfn of that
batch. The next batch will start in a new superpage. As a result the
freeing part of x86_hvm_populate_pfns will not consider the previous
superpage anymore. Now 512MB are allocated, but unpopulated.

To handle this case the min_pfn/max_pfn have to be global so that the
current batch can free allocated pfns from previous batches.

Olaf

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