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