On 10.01.25 07:00, Alistair Popple wrote:
Currently ZONE_DEVICE page reference counts are initialised by core memory management code in __init_zone_device_page() as part of the memremap() call which driver modules make to obtain ZONE_DEVICE pages. This initialises page refcounts to 1 before returning them to the driver. This was presumably done because it drivers had a reference of sorts on the page. It also ensured the page could always be mapped with vm_insert_page() for example and would never get freed (ie. have a zero refcount), freeing drivers of manipulating page reference counts. However it complicates figuring out whether or not a page is free from the mm perspective because it is no longer possible to just look at the refcount. Instead the page type must be known and if GUP is used a secondary pgmap reference is also sometimes needed. To simplify this it is desirable to remove the page reference count for the driver, so core mm can just use the refcount without having to account for page type or do other types of tracking. This is possible because drivers can always assume the page is valid as core kernel will never offline or remove the struct page. This means it is now up to drivers to initialise the page refcount as required. P2PDMA uses vm_insert_page() to map the page, and that requires a non-zero reference count when initialising the page so set that when the page is first mapped. Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
LGTM Acked-by: David Hildenbrand <david@xxxxxxxxxx> -- Cheers, David / dhildenb
