On Mon, Nov 22, 2021 at 04:01:02PM -0800, Mina Almasry wrote:

Add PM_THP_MAPPED MAPPING to allow userspace to detect whether a given virt
address is currently mapped by a transparent huge page or not.  Example
use case is a process requesting THPs from the kernel (via a huge tmpfs
mount for example), for a performance critical region of memory.  The
userspace may want to query whether the kernel is actually backing this
memory by hugepages or not.

So you want this bit to be clear if the memory is backed by a hugetlb
page?

 		if (page && page_mapcount(page) == 1)
 			flags |= PM_MMAP_EXCLUSIVE;
+		if (page && is_transparent_hugepage(page))
+			flags |= PM_THP_MAPPED;

because honestly i'd expect it to be more useful to mean "This memory
is mapped by a PMD entry" and then the code would look like:

		if (page)
			flags |= PM_PMD_MAPPED;

(and put a corresponding change in pagemap_hugetlb_range)