selftests/mm: fix division-by-zero in uffd-unit-tests

Commit 4dfd4bb ("selftests/mm/uffd: refactor non-composite global
vars into struct") moved some of the operations previously implemented in
uffd_setup_environment() earlier in the main test loop.

The calculation of nr_pages, which involves a division by page_size, now
occurs before checking that default_huge_page_size() returns a non-zero
This leads to a division-by-zero error on systems with !CONFIG_HUGETLB.

Fix this by relocating the non-zero page_size check before the nr_pages
calculation, as it was originally implemented.

Link: https://lkml.kernel.org/r/20251113034623.3127012-1-cmllamas@google.com
Fixes: 4dfd4bb ("selftests/mm/uffd: refactor non-composite global vars into struct")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Ujwal Kundur <ujwal.kundur@gmail.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Carlos Llamas

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

@@ -1758,10 +1758,15 @@ int main(int argc, char *argv[])
 			uffd_test_ops = mem_type->mem_ops;
 			uffd_test_case_ops = test->test_case_ops;
 
-			if (mem_type->mem_flag & (MEM_HUGETLB_PRIVATE | MEM_HUGETLB))
+			if (mem_type->mem_flag & (MEM_HUGETLB_PRIVATE | MEM_HUGETLB)) {
 				gopts.page_size = default_huge_page_size();
-			else
+				if (gopts.page_size == 0) {
+					uffd_test_skip("huge page size is 0, feature missing?");
+					continue;
+				}
+			} else {
 				gopts.page_size = psize();
+			}
 
 			/* Ensure we have at least 2 pages */
 			gopts.nr_pages = MAX(UFFD_TEST_MEM_SIZE, gopts.page_size * 2)
@@ -1776,12 +1781,6 @@ int main(int argc, char *argv[])
 				continue;
 
 			uffd_test_start("%s on %s", test->name, mem_type->name);
-			if ((mem_type->mem_flag == MEM_HUGETLB ||
-			    mem_type->mem_flag == MEM_HUGETLB_PRIVATE) &&
-			    (default_huge_page_size() == 0)) {
-				uffd_test_skip("huge page size is 0, feature missing?");
-				continue;
-			}
 			if (!uffd_feature_supported(test)) {
 				uffd_test_skip("feature missing");
 				continue;