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- =====================
- Free Page Reporting
- =====================
- 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
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