class PhysicalPageProvider

Page provider implementation that provides requested loaned physical pages.

This is used by contiguous VMOs which have had pages decommitted, when the pages are again

committed. The reason we use a PageProvider for this is it lines up well with the pager model in

the sense that a page request can be processed while not holding the contiguous VMO's lock.

PhysicalPageProvider always operates async (similar to PagerProxy), because we'd like to (in

typical non-overlapping commit/decommit usage) have one batch that covers the entire commit,

regardless of the fact that some of the pages may already be free and therefore could be

immediately obtained. Quite often at least one page will be presently owned by a different

VMO, so we may as well always do one big async batch that deals with all the presently

non-FREE pages.