| .. _free_page_reporting: |
| |
| ===================== |
| 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 |
