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1=====================2Free Page Reporting3=====================4 5Free page reporting is an API by which a device can register to receive6lists of pages that are currently unused by the system. This is useful in7the case of virtualization where a guest is then able to use this data to8notify the hypervisor that it is no longer using certain pages in memory.9 10For the driver, typically a balloon driver, to use of this functionality11it will allocate and initialize a page_reporting_dev_info structure. The12field within the structure it will populate is the "report" function13pointer used to process the scatterlist. It must also guarantee that it can14handle at least PAGE_REPORTING_CAPACITY worth of scatterlist entries per15call to the function. A call to page_reporting_register will register the16page reporting interface with the reporting framework assuming no other17page reporting devices are already registered.18 19Once registered the page reporting API will begin reporting batches of20pages to the driver. The API will start reporting pages 2 seconds after21the interface is registered and will continue to do so 2 seconds after any22page of a sufficiently high order is freed.23 24Pages reported will be stored in the scatterlist passed to the reporting25function with the final entry having the end bit set in entry nent - 1.26While pages are being processed by the report function they will not be27accessible to the allocator. Once the report function has been completed28the pages will be returned to the free area from which they were obtained.29 30Prior to removing a driver that is making use of free page reporting it31is necessary to call page_reporting_unregister to have the32page_reporting_dev_info structure that is currently in use by free page33reporting removed. Doing this will prevent further reports from being34issued via the interface. If another driver or the same driver is35registered it is possible for it to resume where the previous driver had36left off in terms of reporting free pages.37 38Alexander Duyck, Dec 04, 201939