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Add documentation for free page reporting. Currently the only consumer is virtio-balloon, however it is possible that other drivers might make use of this so it is best to add a bit of documetation explaining at a high level how to use the API. Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: David Hildenbrand <david@redhat.com> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> Cc: Luiz Capitulino <lcapitulino@redhat.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Nitesh Narayan Lal <nitesh@redhat.com> Cc: Oscar Salvador <osalvador@suse.de> Cc: Pankaj Gupta <pagupta@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Rik van Riel <riel@surriel.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Wei Wang <wei.w.wang@intel.com> Cc: Yang Zhang <yang.zhang.wz@gmail.com> Cc: wei qi <weiqi4@huawei.com> Link: http://lkml.kernel.org/r/20200211224730.29318.43815.stgit@localhost.localdomain Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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40 lines
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.. _free_page_reporting:
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=====================
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Free Page Reporting
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=====================
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Free page reporting is an API by which a device can register to receive
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lists of pages that are currently unused by the system. This is useful in
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the case of virtualization where a guest is then able to use this data to
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notify the hypervisor that it is no longer using certain pages in memory.
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For the driver, typically a balloon driver, to use of this functionality
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it will allocate and initialize a page_reporting_dev_info structure. The
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field within the structure it will populate is the "report" function
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pointer used to process the scatterlist. It must also guarantee that it can
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handle at least PAGE_REPORTING_CAPACITY worth of scatterlist entries per
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call to the function. A call to page_reporting_register will register the
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page reporting interface with the reporting framework assuming no other
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page reporting devices are already registered.
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Once registered the page reporting API will begin reporting batches of
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pages to the driver. The API will start reporting pages 2 seconds after
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the interface is registered and will continue to do so 2 seconds after any
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page of a sufficiently high order is freed.
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Pages reported will be stored in the scatterlist passed to the reporting
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function with the final entry having the end bit set in entry nent - 1.
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While pages are being processed by the report function they will not be
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accessible to the allocator. Once the report function has been completed
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the pages will be returned to the free area from which they were obtained.
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Prior to removing a driver that is making use of free page reporting it
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is necessary to call page_reporting_unregister to have the
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page_reporting_dev_info structure that is currently in use by free page
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reporting removed. Doing this will prevent further reports from being
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issued via the interface. If another driver or the same driver is
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registered it is possible for it to resume where the previous driver had
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left off in terms of reporting free pages.
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Alexander Duyck, Dec 04, 2019
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