selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"

Patch series "selftests/mm: reuse FORCE_READ to replace "asm volatile("" :
"+r" (XXX));" and some cleanup", v2.

This series introduces a common FORCE_READ() macro to replace the cryptic
asm volatile("" : "+r" (variable)); construct used in several mm
selftests.  This improves code readability and maintainability by removing
duplicated, hard-to-understand code.


This patch (of 2):

Several mm selftests use the `asm volatile("" : "+r" (variable));`
construct to force a read of a variable, preventing the compiler from
optimizing away the memory access.  This idiom is cryptic and duplicated
across multiple test files.

Following a suggestion from David[1], this patch refactors this common
pattern into a FORCE_READ() macro

Link: https://lkml.kernel.org/r/20250717131857.59909-1-lianux.mm@gmail.com
Link: https://lkml.kernel.org/r/20250717131857.59909-2-lianux.mm@gmail.com
Link: https://lore.kernel.org/lkml/4a3e0759-caa1-4cfa-bc3f-402593f1eee3@redhat.com/ [1]
Signed-off-by: wang lian <lianux.mm@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/cow.c

@@ -1534,7 +1534,7 @@ static void test_ro_fast_pin(char *mem, const char *smem, size_t size)
 
 static void run_with_zeropage(non_anon_test_fn fn, const char *desc)
 {
-	char *mem, *smem, tmp;
+	char *mem, *smem;
 
 	log_test_start("%s ... with shared zeropage", desc);
 
@@ -1554,8 +1554,8 @@ static void run_with_zeropage(non_anon_test_fn fn, const char *desc)
 	}
 
 	/* Read from the page to populate the shared zeropage. */
-	tmp = *mem + *smem;
-	asm volatile("" : "+r" (tmp));
+	FORCE_READ(mem);
+	FORCE_READ(smem);
 
 	fn(mem, smem, pagesize);
 munmap:
@@ -1566,7 +1566,7 @@ munmap:
 
 static void run_with_huge_zeropage(non_anon_test_fn fn, const char *desc)
 {
-	char *mem, *smem, *mmap_mem, *mmap_smem, tmp;
+	char *mem, *smem, *mmap_mem, *mmap_smem;
 	size_t mmap_size;
 	int ret;
 
@@ -1617,8 +1617,8 @@ static void run_with_huge_zeropage(non_anon_test_fn fn, const char *desc)
 	 * the first sub-page and test if we get another sub-page populated
 	 * automatically.
 	 */
-	tmp = *mem + *smem;
-	asm volatile("" : "+r" (tmp));
+	FORCE_READ(mem);
+	FORCE_READ(smem);
 	if (!pagemap_is_populated(pagemap_fd, mem + pagesize) ||
 	    !pagemap_is_populated(pagemap_fd, smem + pagesize)) {
 		ksft_test_result_skip("Did not get THPs populated\n");
@@ -1634,7 +1634,7 @@ munmap:
 
 static void run_with_memfd(non_anon_test_fn fn, const char *desc)
 {
-	char *mem, *smem, tmp;
+	char *mem, *smem;
 	int fd;
 
 	log_test_start("%s ... with memfd", desc);
@@ -1668,8 +1668,8 @@ static void run_with_memfd(non_anon_test_fn fn, const char *desc)
 	}
 
 	/* Fault the page in. */
-	tmp = *mem + *smem;
-	asm volatile("" : "+r" (tmp));
+	FORCE_READ(mem);
+	FORCE_READ(smem);
 
 	fn(mem, smem, pagesize);
 munmap:
@@ -1682,7 +1682,7 @@ close:
 
 static void run_with_tmpfile(non_anon_test_fn fn, const char *desc)
 {
-	char *mem, *smem, tmp;
+	char *mem, *smem;
 	FILE *file;
 	int fd;
 
@@ -1724,8 +1724,8 @@ static void run_with_tmpfile(non_anon_test_fn fn, const char *desc)
 	}
 
 	/* Fault the page in. */
-	tmp = *mem + *smem;
-	asm volatile("" : "+r" (tmp));
+	FORCE_READ(mem);
+	FORCE_READ(smem);
 
 	fn(mem, smem, pagesize);
 munmap:
@@ -1740,7 +1740,7 @@ static void run_with_memfd_hugetlb(non_anon_test_fn fn, const char *desc,
 				   size_t hugetlbsize)
 {
 	int flags = MFD_HUGETLB;
-	char *mem, *smem, tmp;
+	char *mem, *smem;
 	int fd;
 
 	log_test_start("%s ... with memfd hugetlb (%zu kB)", desc,
@@ -1778,8 +1778,8 @@ static void run_with_memfd_hugetlb(non_anon_test_fn fn, const char *desc,
 	}
 
 	/* Fault the page in. */
-	tmp = *mem + *smem;
-	asm volatile("" : "+r" (tmp));
+	FORCE_READ(mem);
+	FORCE_READ(smem);
 
 	fn(mem, smem, hugetlbsize);
 munmap:

tools/testing/selftests/mm/guard-regions.c

@@ -35,13 +35,6 @@
 static volatile sig_atomic_t signal_jump_set;
 static sigjmp_buf signal_jmp_buf;
 
-/*
- * Ignore the checkpatch warning, we must read from x but don't want to do
- * anything with it in order to trigger a read page fault. We therefore must use
- * volatile to stop the compiler from optimising this away.
- */
-#define FORCE_READ(x) (*(volatile typeof(x) *)x)
-
 /*
  * How is the test backing the mapping being tested?
  */

tools/testing/selftests/mm/hugetlb-madvise.c

@@ -47,14 +47,11 @@ void write_fault_pages(void *addr, unsigned long nr_pages)
 
 void read_fault_pages(void *addr, unsigned long nr_pages)
 {
-	volatile unsigned long dummy = 0;
 	unsigned long i;
 
 	for (i = 0; i < nr_pages; i++) {
-		dummy += *((unsigned long *)(addr + (i * huge_page_size)));
-
 		/* Prevent the compiler from optimizing out the entire loop: */
-		asm volatile("" : "+r" (dummy));
+		FORCE_READ(((unsigned long *)(addr + (i * huge_page_size))));
 	}
 }
 

tools/testing/selftests/mm/migration.c

@@ -16,6 +16,7 @@
 #include <sys/types.h>
 #include <signal.h>
 #include <time.h>
+#include "vm_util.h"
 
 #define TWOMEG		(2<<20)
 #define RUNTIME		(20)
@@ -103,15 +104,13 @@ int migrate(uint64_t *ptr, int n1, int n2)
 
 void *access_mem(void *ptr)
 {
-	volatile uint64_t y = 0;
-	volatile uint64_t *x = ptr;
-
 	while (1) {
 		pthread_testcancel();
-		y += *x;
-
-		/* Prevent the compiler from optimizing out the writes to y: */
-		asm volatile("" : "+r" (y));
+		/* Force a read from the memory pointed to by ptr. This ensures
+		 * the memory access actually happens and prevents the compiler
+		 * from optimizing away this entire loop.
+		 */
+		FORCE_READ((uint64_t *)ptr);
 	}
 
 	return NULL;

tools/testing/selftests/mm/pagemap_ioctl.c

@@ -1525,9 +1525,7 @@ void zeropfn_tests(void)
 
 	ret = madvise(mem, hpage_size, MADV_HUGEPAGE);
 	if (!ret) {
-		char tmp = *mem;
-
-		asm volatile("" : "+r" (tmp));
+		FORCE_READ(mem);
 
 		ret = pagemap_ioctl(mem, hpage_size, &vec, 1, 0,
 				    0, PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);

tools/testing/selftests/mm/split_huge_page_test.c

@@ -398,7 +398,6 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 		char **addr)
 {
 	size_t i;
-	int dummy = 0;
 	unsigned char buf[1024];
 
 	srand(time(NULL));
@@ -440,8 +439,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 	madvise(*addr, fd_size, MADV_HUGEPAGE);
 
 	for (size_t i = 0; i < fd_size; i++)
-		dummy += *(*addr + i);
-	asm volatile("" : "+r" (dummy));
+		FORCE_READ((*addr + i));
 
 	if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) {
 		ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n");

tools/testing/selftests/mm/vm_util.h

@@ -18,6 +18,13 @@
 #define PM_SWAP                       BIT_ULL(62)
 #define PM_PRESENT                    BIT_ULL(63)
 
+/*
+ * Ignore the checkpatch warning, we must read from x but don't want to do
+ * anything with it in order to trigger a read page fault. We therefore must use
+ * volatile to stop the compiler from optimising this away.
+ */
+#define FORCE_READ(x) (*(volatile typeof(x) *)x)
+
 extern unsigned int __page_size;
 extern unsigned int __page_shift;