--- a/tools/testing/selftests/vm/ksm_tests.c~selftest-vm-add-utilh-and-and-move-helper-functions-there
+++ a/tools/testing/selftests/vm/ksm_tests.c
@@ -12,6 +12,7 @@
 
 #include "../kselftest.h"
 #include "../../../../include/vdso/time64.h"
+#include "util.h"
 
 #define KSM_SYSFS_PATH "/sys/kernel/mm/ksm/"
 #define KSM_FP(s) (KSM_SYSFS_PATH s)
@@ -22,15 +23,6 @@
 #define KSM_MERGE_ACROSS_NODES_DEFAULT true
 #define MB (1ul << 20)
 
-#define PAGE_SHIFT 12
-#define HPAGE_SHIFT 21
-
-#define PAGE_SIZE (1 << PAGE_SHIFT)
-#define HPAGE_SIZE (1 << HPAGE_SHIFT)
-
-#define PAGEMAP_PRESENT(ent)	(((ent) & (1ull << 63)) != 0)
-#define PAGEMAP_PFN(ent)	((ent) & ((1ull << 55) - 1))
-
 struct ksm_sysfs {
 	unsigned long max_page_sharing;
 	unsigned long merge_across_nodes;
@@ -456,34 +448,6 @@ err_out:
 	return KSFT_FAIL;
 }
 
-int64_t allocate_transhuge(void *ptr, int pagemap_fd)
-{
-	uint64_t ent[2];
-
-	/* drop pmd */
-	if (mmap(ptr, HPAGE_SIZE, PROT_READ | PROT_WRITE,
-				MAP_FIXED | MAP_ANONYMOUS |
-				MAP_NORESERVE | MAP_PRIVATE, -1, 0) != ptr)
-		errx(2, "mmap transhuge");
-
-	if (madvise(ptr, HPAGE_SIZE, MADV_HUGEPAGE))
-		err(2, "MADV_HUGEPAGE");
-
-	/* allocate transparent huge page */
-	*(volatile void **)ptr = ptr;
-
-	if (pread(pagemap_fd, ent, sizeof(ent),
-			(uintptr_t)ptr >> (PAGE_SHIFT - 3)) != sizeof(ent))
-		err(2, "read pagemap");
-
-	if (PAGEMAP_PRESENT(ent[0]) && PAGEMAP_PRESENT(ent[1]) &&
-	    PAGEMAP_PFN(ent[0]) + 1 == PAGEMAP_PFN(ent[1]) &&
-	    !(PAGEMAP_PFN(ent[0]) & ((1 << (HPAGE_SHIFT - PAGE_SHIFT)) - 1)))
-		return PAGEMAP_PFN(ent[0]);
-
-	return -1;
-}
-
 static int ksm_merge_hugepages_time(int mapping, int prot, int timeout, size_t map_size)
 {
 	void *map_ptr, *map_ptr_orig;
--- a/tools/testing/selftests/vm/transhuge-stress.c~selftest-vm-add-utilh-and-and-move-helper-functions-there
+++ a/tools/testing/selftests/vm/transhuge-stress.c
@@ -15,48 +15,12 @@
 #include <fcntl.h>
 #include <string.h>
 #include <sys/mman.h>
+#include "util.h"
 
-#define PAGE_SHIFT 12
-#define HPAGE_SHIFT 21
-
-#define PAGE_SIZE (1 << PAGE_SHIFT)
-#define HPAGE_SIZE (1 << HPAGE_SHIFT)
-
-#define PAGEMAP_PRESENT(ent)	(((ent) & (1ull << 63)) != 0)
-#define PAGEMAP_PFN(ent)	((ent) & ((1ull << 55) - 1))
-
-int pagemap_fd;
 int backing_fd = -1;
 int mmap_flags = MAP_ANONYMOUS | MAP_NORESERVE | MAP_PRIVATE;
 #define PROT_RW (PROT_READ | PROT_WRITE)
 
-int64_t allocate_transhuge(void *ptr)
-{
-	uint64_t ent[2];
-
-	/* drop pmd */
-	if (mmap(ptr, HPAGE_SIZE, PROT_RW, MAP_FIXED | mmap_flags,
-		 backing_fd, 0) != ptr)
-		errx(2, "mmap transhuge");
-
-	if (madvise(ptr, HPAGE_SIZE, MADV_HUGEPAGE))
-		err(2, "MADV_HUGEPAGE");
-
-	/* allocate transparent huge page */
-	*(volatile void **)ptr = ptr;
-
-	if (pread(pagemap_fd, ent, sizeof(ent),
-			(uintptr_t)ptr >> (PAGE_SHIFT - 3)) != sizeof(ent))
-		err(2, "read pagemap");
-
-	if (PAGEMAP_PRESENT(ent[0]) && PAGEMAP_PRESENT(ent[1]) &&
-	    PAGEMAP_PFN(ent[0]) + 1 == PAGEMAP_PFN(ent[1]) &&
-	    !(PAGEMAP_PFN(ent[0]) & ((1 << (HPAGE_SHIFT - PAGE_SHIFT)) - 1)))
-		return PAGEMAP_PFN(ent[0]);
-
-	return -1;
-}
-
 int main(int argc, char **argv)
 {
 	size_t ram, len;
@@ -67,6 +31,7 @@ int main(int argc, char **argv)
 	double s;
 	uint8_t *map;
 	size_t map_len;
+	int pagemap_fd;
 
 	ram = sysconf(_SC_PHYS_PAGES);
 	if (ram > SIZE_MAX / sysconf(_SC_PAGESIZE) / 4)
@@ -122,7 +87,7 @@ int main(int argc, char **argv)
 		for (p = ptr; p < ptr + len; p += HPAGE_SIZE) {
 			int64_t pfn;
 
-			pfn = allocate_transhuge(p);
+			pfn = allocate_transhuge(p, pagemap_fd);
 
 			if (pfn < 0) {
 				nr_failed++;
--- /dev/null
+++ a/tools/testing/selftests/vm/util.h
@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __KSELFTEST_VM_UTIL_H
+#define __KSELFTEST_VM_UTIL_H
+
+#include <stdint.h>
+#include <sys/mman.h>
+#include <err.h>
+
+#define PAGE_SHIFT	12
+#define HPAGE_SHIFT	21
+
+#define PAGE_SIZE (1 << PAGE_SHIFT)
+#define HPAGE_SIZE (1 << HPAGE_SHIFT)
+
+#define PAGEMAP_PRESENT(ent)	(((ent) & (1ull << 63)) != 0)
+#define PAGEMAP_PFN(ent)	((ent) & ((1ull << 55) - 1))
+
+
+static inline int64_t allocate_transhuge(void *ptr, int pagemap_fd)
+{
+	uint64_t ent[2];
+
+	/* drop pmd */
+	if (mmap(ptr, HPAGE_SIZE, PROT_READ | PROT_WRITE,
+		 MAP_FIXED | MAP_ANONYMOUS |
+		 MAP_NORESERVE | MAP_PRIVATE, -1, 0) != ptr)
+		errx(2, "mmap transhuge");
+
+	if (madvise(ptr, HPAGE_SIZE, MADV_HUGEPAGE))
+		err(2, "MADV_HUGEPAGE");
+
+	/* allocate transparent huge page */
+	*(volatile void **)ptr = ptr;
+
+	if (pread(pagemap_fd, ent, sizeof(ent),
+		  (uintptr_t)ptr >> (PAGE_SHIFT - 3)) != sizeof(ent))
+		err(2, "read pagemap");
+
+	if (PAGEMAP_PRESENT(ent[0]) && PAGEMAP_PRESENT(ent[1]) &&
+	    PAGEMAP_PFN(ent[0]) + 1 == PAGEMAP_PFN(ent[1]) &&
+	    !(PAGEMAP_PFN(ent[0]) & ((1 << (HPAGE_SHIFT - PAGE_SHIFT)) - 1)))
+		return PAGEMAP_PFN(ent[0]);
+
+	return -1;
+}
+
+#endif
