selftests/mm: uffd-unit-tests support for hugepages > 2M

uffd-unit-tests uses a memory area with a fixed 32M size.  Then it
calculates the number of pages by dividing by page_size, which itself is
either the base page size or the PMD huge page size depending on the test
config.  For the latter, we end up with nr_pages=1 for arm64 16K base
pages, and nr_pages=0 for 64K base pages.  This doesn't end well.

So let's make the 32M size a floor and also ensure that we have at least 2
pages given the PMD size.  With this change, the tests pass on arm64 64K
base page size configuration.

Link: https://lkml.kernel.org/r/20250318174343.243631-2-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Rafael Aquini <raquini@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/uffd-unit-tests.c

@@ -26,6 +26,8 @@
 #define ALIGN_UP(x, align_to) \
 	((__typeof__(x))((((unsigned long)(x)) + ((align_to)-1)) & ~((align_to)-1)))
 
+#define MAX(a, b) (((a) > (b)) ? (a) : (b))
+
 struct mem_type {
 	const char *name;
 	unsigned int mem_flag;
@@ -196,7 +198,8 @@ uffd_setup_environment(uffd_test_args_t *args, uffd_test_case_t *test,
 	else
 		page_size = psize();
 
-	nr_pages = UFFD_TEST_MEM_SIZE / page_size;
+	/* Ensure we have at least 2 pages */
+	nr_pages = MAX(UFFD_TEST_MEM_SIZE, page_size * 2) / page_size;
 	/* TODO: remove this global var.. it's so ugly */
 	nr_parallel = 1;