1.. _free_page_reporting: 2 3===================== 4Free Page Reporting 5===================== 6 7Free page reporting is an API by which a device can register to receive 8lists of pages that are currently unused by the system. This is useful in 9the case of virtualization where a guest is then able to use this data to 10notify the hypervisor that it is no longer using certain pages in memory. 11 12For the driver, typically a balloon driver, to use of this functionality 13it will allocate and initialize a page_reporting_dev_info structure. The 14field within the structure it will populate is the "report" function 15pointer used to process the scatterlist. It must also guarantee that it can 16handle at least PAGE_REPORTING_CAPACITY worth of scatterlist entries per 17call to the function. A call to page_reporting_register will register the 18page reporting interface with the reporting framework assuming no other 19page reporting devices are already registered. 20 21Once registered the page reporting API will begin reporting batches of 22pages to the driver. The API will start reporting pages 2 seconds after 23the interface is registered and will continue to do so 2 seconds after any 24page of a sufficiently high order is freed. 25 26Pages reported will be stored in the scatterlist passed to the reporting 27function with the final entry having the end bit set in entry nent - 1. 28While pages are being processed by the report function they will not be 29accessible to the allocator. Once the report function has been completed 30the pages will be returned to the free area from which they were obtained. 31 32Prior to removing a driver that is making use of free page reporting it 33is necessary to call page_reporting_unregister to have the 34page_reporting_dev_info structure that is currently in use by free page 35reporting removed. Doing this will prevent further reports from being 36issued via the interface. If another driver or the same driver is 37registered it is possible for it to resume where the previous driver had 38left off in terms of reporting free pages. 39 40Alexander Duyck, Dec 04, 2019 41