[PATCH v4 5/7] selftests/cgroup: replace hardcoded page size values in test_zswap

Li Wang
2026-03-22T06:11:34Z

test_zswap uses hardcoded values of 4095 and 4096 throughout as page
stride and page size, which are only correct on systems with a 4K page
size. On architectures with larger pages (e.g., 64K on arm64 or ppc64),
these constants cause memory to be touched at sub-page granularity,
leading to inefficient access patterns and incorrect page count
calculations, which can cause test failures.

Replace all hardcoded 4095 and 4096 values with a global pagesize
variable initialized from sysconf(_SC_PAGESIZE) at startup, and remove
the redundant local sysconf() calls scattered across individual
functions. No functional change on 4K page size systems.

Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
---
tools/testing/selftests/cgroup/test_zswap.c | 22 +++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 4982e8c4759c..407cfb6ae34d 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -13,6 +13,8 @@
#include "kselftest.h"
#include "cgroup_util.h"

+static size_t pagesize;
+
static int read_int(const char *path, size_t *value)
{
FILE *file;
@@ -68,11 +70,11 @@ static int allocate_and_read_bytes(const char *cgroup, void *arg)

if (!mem)
return -1;
- for (int i = 0; i < size; i += 4095)
+ for (int i = 0; i < size; i += pagesize)
mem[i] = 'a';

/* Go through the allocated memory to (z)swap in and out pages */
- for (int i = 0; i < size; i += 4095) {
+ for (int i = 0; i < size; i += pagesize) {
if (mem[i] != 'a')
ret = -1;
}
@@ -88,7 +90,7 @@ static int allocate_bytes(const char *cgroup, void *arg)

if (!mem)
return -1;
- for (int i = 0; i < size; i += 4095)
+ for (int i = 0; i < size; i += pagesize)
mem[i] = 'a';
free(mem);
return 0;
@@ -267,7 +269,6 @@ static int test_zswapin(const char *root)
*/
static int attempt_writeback(const char *cgroup, void *arg)
{
- long pagesize = sysconf(_SC_PAGESIZE);
size_t memsize = MB(4);
char buf[pagesize];
long zswap_usage;
@@ -436,7 +437,7 @@ static int test_no_invasive_cgroup_shrink(const char *root)
if (cg_enter_current(control_group))
goto out;
control_allocation = malloc(control_allocation_size);
- for (int i = 0; i < control_allocation_size; i += 4095)
+ for (int i = 0; i < control_allocation_size; i += pagesize)
control_allocation[i] = 'a';
if (cg_read_key_long(control_group, "memory.stat", "zswapped") < 1)
goto out;
@@ -476,7 +477,7 @@ static int no_kmem_bypass_child(const char *cgroup, void *arg)
values->child_allocated = true;
return -1;
}
- for (long i = 0; i < values->target_alloc_bytes; i += 4095)
+ for (long i = 0; i < values->target_alloc_bytes; i += pagesize)
((char *)allocation)[i] = 'a';
values->child_allocated = true;
pause();
@@ -524,7 +525,7 @@ static int test_no_kmem_bypass(const char *root)
min_free_kb_low = sys_info.totalram / 500000;
values->target_alloc_bytes = (sys_info.totalram - min_free_kb_high * 1000) +
sys_info.totalram * 5 / 100;
- stored_pages_threshold = sys_info.totalram / 5 / 4096;
+ stored_pages_threshold = sys_info.totalram / 5 / pagesize;
trigger_allocation_size = sys_info.totalram / 20;

/* Set up test memcg */
@@ -551,7 +552,7 @@ static int test_no_kmem_bypass(const char *root)

if (!trigger_allocation)
break;
- for (int i = 0; i < trigger_allocation_size; i += 4095)
+ for (int i = 0; i < trigger_allocation_size; i += pagesize)
trigger_allocation[i] = 'b';
usleep(100000);
free(trigger_allocation);
@@ -562,8 +563,8 @@ static int test_no_kmem_bypass(const char *root)
/* If memory was pushed to zswap, verify it belongs to memcg */
if (stored_pages > stored_pages_threshold) {
int zswapped = cg_read_key_long(test_group, "memory.stat", "zswapped ");
- int delta = stored_pages * 4096 - zswapped;
- int result_ok = delta < stored_pages * 4096 / 4;
+ int delta = stored_pages * pagesize - zswapped;
+ int result_ok = delta < stored_pages * pagesize / 4;

ret = result_ok ? KSFT_PASS : KSFT_FAIL;
break;
@@ -616,6 +617,7 @@ int main(int argc, char **argv)
char root[PATH_MAX];
int i;

+ pagesize = sysconf(_SC_PAGESIZE);
ksft_print_header();
ksft_set_plan(ARRAY_SIZE(tests));
if (cg_find_unified_root(root, sizeof(root), NULL))
--
2.53.0