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/
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>
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);
}
/* 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:
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;
* 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");
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);
}
/* Fault the page in. */
- tmp = *mem + *smem;
- asm volatile("" : "+r" (tmp));
+ FORCE_READ(mem);
+ FORCE_READ(smem);
fn(mem, smem, pagesize);
munmap:
static void run_with_tmpfile(non_anon_test_fn fn, const char *desc)
{
- char *mem, *smem, tmp;
+ char *mem, *smem;
FILE *file;
int fd;
}
/* Fault the page in. */
- tmp = *mem + *smem;
- asm volatile("" : "+r" (tmp));
+ FORCE_READ(mem);
+ FORCE_READ(smem);
fn(mem, smem, pagesize);
munmap:
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,
}
/* Fault the page in. */
- tmp = *mem + *smem;
- asm volatile("" : "+r" (tmp));
+ FORCE_READ(mem);
+ FORCE_READ(smem);
fn(mem, smem, hugetlbsize);
munmap:
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?
*/
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))));
}
}
#include <sys/types.h>
#include <signal.h>
#include <time.h>
+#include "vm_util.h"
#define TWOMEG (2<<20)
#define RUNTIME (20)
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;
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);
char **addr)
{
size_t i;
- int dummy = 0;
unsigned char buf[1024];
srand(time(NULL));
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");
#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;
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