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Message-ID: <20250716123126.3851-1-lianux.mm@gmail.com>
Date: Wed, 16 Jul 2025 20:31:26 +0800
From: wang lian <lianux.mm@...il.com>
To: akpm@...ux-foundation.org,
broonie@...nel.org,
david@...hat.com,
lorenzo.stoakes@...cle.com,
sj@...nel.org,
ziy@...dia.com,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: brauner@...nel.org,
jannh@...gle.com,
Liam.Howlett@...cle.com,
shuah@...nel.org,
vbabka@...e.cz,
ludovico.zy.wu@...il.com,
gkwang@...x-info.com,
p1ucky0923@...il.com,
ryncsn@...il.com,
zijing.zhang@...ton.me,
lianux.mm@...il.com
Subject: [PATCH] selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"
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
[1] https://lore.kernel.org/lkml/4a3e0759-caa1-4cfa-bc3f-402593f1eee3@redhat.com/
Signed-off-by: wang lian <lianux.mm@...il.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
tools/testing/selftests/mm/cow.c | 30 +++++++++----------
tools/testing/selftests/mm/hugetlb-madvise.c | 5 +---
tools/testing/selftests/mm/migration.c | 13 ++++----
tools/testing/selftests/mm/pagemap_ioctl.c | 4 +--
.../selftests/mm/split_huge_page_test.c | 4 +--
5 files changed, 24 insertions(+), 32 deletions(-)
diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
index 788e82b00b75..d30625c18259 100644
--- a/tools/testing/selftests/mm/cow.c
+++ b/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 @@ static void run_with_zeropage(non_anon_test_fn fn, const char *desc)
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 @@ static void run_with_huge_zeropage(non_anon_test_fn fn, const char *desc)
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 @@ static void run_with_memfd(non_anon_test_fn fn, const char *desc)
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:
diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
index e74107185324..1afe14b9dc0c 100644
--- a/tools/testing/selftests/mm/hugetlb-madvise.c
+++ b/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))));
}
}
diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
index a306f8bab087..c5a73617796a 100644
--- a/tools/testing/selftests/mm/migration.c
+++ b/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;
diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c
index c2dcda78ad31..0d4209eef0c3 100644
--- a/tools/testing/selftests/mm/pagemap_ioctl.c
+++ b/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);
diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index f0d9c035641d..05de1fc0005b 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -399,7 +399,6 @@ 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));
@@ -441,8 +440,7 @@ 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");
--
2.43.0
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