mm/selftests: fix split_huge_page_test failure on systems with 64KB page size

The split_huge_page_test fails on systems with a 64KB base page size. 
This is because the order of a 2MB huge page is different:

On 64KB systems, the order is 5.

On 4KB systems, it's 9.

The test currently assumes a maximum huge page order of 9, which is only
valid for 4KB base page systems.  On systems with 64KB pages, attempting
to split huge pages beyond their actual order (5) causes the test to fail.

In this patch, we calculate the huge page order based on the system's base
page size.  With this change, the tests now run successfully on both 64KB
and 4KB page size systems.

Link: https://lkml.kernel.org/r/20250816040113.760010-5-aboorvad@linux.ibm.com
Fixes: fa6c02315f ("mm: huge_memory: a new debugfs interface for splitting THP tests")
Co-developed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/cow.c

@@ -41,11 +41,6 @@ static size_t hugetlbsizes[10];
 static int gup_fd;
 static bool has_huge_zeropage;
 
-static int sz2ord(size_t size)
-{
-	return __builtin_ctzll(size / pagesize);
-}
-
 static int detect_thp_sizes(size_t sizes[], int max)
 {
 	int count = 0;

tools/testing/selftests/mm/split_huge_page_test.c

@@ -525,6 +525,9 @@ int main(int argc, char **argv)
 	const char *fs_loc;
 	bool created_tmp;
 	int offset;
+	unsigned int max_order;
+	unsigned int nr_pages;
+	unsigned int tests;
 
 	ksft_print_header();
 
@@ -536,35 +539,38 @@ int main(int argc, char **argv)
 	if (argc > 1)
 		optional_xfs_path = argv[1];
 
-	ksft_set_plan(1+8+1+9+9+8*4+2);
-
 	pagesize = getpagesize();
 	pageshift = ffs(pagesize) - 1;
 	pmd_pagesize = read_pmd_pagesize();
 	if (!pmd_pagesize)
 		ksft_exit_fail_msg("Reading PMD pagesize failed\n");
 
+	nr_pages = pmd_pagesize / pagesize;
+	max_order = sz2ord(pmd_pagesize);
+	tests = 2 + (max_order - 1) + (2 * max_order) + (max_order - 1) * 4 + 2;
+	ksft_set_plan(tests);
+
 	fd_size = 2 * pmd_pagesize;
 
 	split_pmd_zero_pages();
 
-	for (i = 0; i < 9; i++)
+	for (i = 0; i < max_order; i++)
 		if (i != 1)
 			split_pmd_thp_to_order(i);
 
 	split_pte_mapped_thp();
-	for (i = 0; i < 9; i++)
+	for (i = 0; i < max_order; i++)
 		split_file_backed_thp(i);
 
 	created_tmp = prepare_thp_fs(optional_xfs_path, fs_loc_template,
 			&fs_loc);
-	for (i = 8; i >= 0; i--)
+	for (i = max_order - 1; i >= 0; i--)
 		split_thp_in_pagecache_to_order_at(fd_size, fs_loc, i, -1);
 
-	for (i = 0; i < 9; i++)
+	for (i = 0; i < max_order; i++)
 		for (offset = 0;
-		     offset < pmd_pagesize / pagesize;
-		     offset += MAX(pmd_pagesize / pagesize / 4, 1 << i))
+		     offset < nr_pages;
+		     offset += MAX(nr_pages / 4, 1 << i))
 			split_thp_in_pagecache_to_order_at(fd_size, fs_loc, i, offset);
 	cleanup_thp_fs(fs_loc, created_tmp);
 

tools/testing/selftests/mm/uffd-wp-mremap.c

@@ -19,11 +19,6 @@ static size_t thpsizes[20];
 static int nr_hugetlbsizes;
 static size_t hugetlbsizes[10];
 
-static int sz2ord(size_t size)
-{
-	return __builtin_ctzll(size / pagesize);
-}
-
 static int detect_thp_sizes(size_t sizes[], int max)
 {
 	int count = 0;

tools/testing/selftests/mm/vm_util.h

@@ -127,6 +127,11 @@ static inline void log_test_result(int result)
 	ksft_test_result_report(result, "%s\n", test_name);
 }
 
+static inline int sz2ord(size_t size)
+{
+	return __builtin_ctzll(size / getpagesize());
+}
+
 void *sys_mremap(void *old_address, unsigned long old_size,
 		 unsigned long new_size, int flags, void *new_address);