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Free Page Reporting
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Free page reporting is an API by which a device can register to receive
lists of pages that are currently unused by the system. This is useful in
the case of virtualization where a guest is then able to use this data to
notify the hypervisor that it is no longer using certain pages in memory.

For the driver, typically a balloon driver, to use of this functionality
it will allocate and initialize a page_reporting_dev_info structure. The
field within the structure it will populate is the "report" function
pointer used to process the scatterlist. It must also guarantee that it can
handle at least PAGE_REPORTING_CAPACITY worth of scatterlist entries per
call to the function. A call to page_reporting_register will register the
page reporting interface with the reporting framework assuming no other
page reporting devices are already registered.

Once registered the page reporting API will begin reporting batches of
pages to the driver. The API will start reporting pages 2 seconds after
the interface is registered and will continue to do so 2 seconds after any
page of a sufficiently high order is freed.

Pages reported will be stored in the scatterlist passed to the reporting
function with the final entry having the end bit set in entry nent - 1.
While pages are being processed by the report function they will not be
accessible to the allocator. Once the report function has been completed
the pages will be returned to the free area from which they were obtained.

Prior to removing a driver that is making use of free page reporting it
is necessary to call page_reporting_unregister to have the
page_reporting_dev_info structure that is currently in use by free page
reporting removed. Doing this will prevent further reports from being
issued via the interface. If another driver or the same driver is
registered it is possible for it to resume where the previous driver had
left off in terms of reporting free pages.

Alexander Duyck, Dec 04, 2019