[PATCH v7 5/8] selftests/cgroup: replace hardcoded page size values in test_zswap

Li Wang
2026-04-24T04:02:14Z

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 <li.wang@linux.dev>
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>
Acked-by: Yosry Ahmed <yosry@kernel.org>
---
tools/testing/selftests/cgroup/test_zswap.c | 45 ++++++++++++---------
1 file changed, 25 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 37aa83c2f1b..23ff11390a3 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -15,6 +15,8 @@
#include "kselftest.h"
#include "cgroup_util.h"

+static int page_size;
+
#define PATH_ZSWAP "/sys/module/zswap"
#define PATH_ZSWAP_ENABLED "/sys/module/zswap/parameters/enabled"

@@ -73,11 +75,11 @@ 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 += page_size)
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 += page_size) {
if (mem[i] != 'a')
ret = -1;
}
@@ -93,7 +95,7 @@ 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 += page_size)
mem[i] = 'a';
free(mem);
return 0;
@@ -167,7 +169,7 @@ static int test_swapin_nozswap(const char *root)
int ret = KSFT_FAIL;
char *test_group, mem_max_buf[32];
long swap_peak, zswpout, min_swap;
- size_t allocation_size = sysconf(_SC_PAGESIZE) * 512;
+ size_t allocation_size = page_size * 512;

min_swap = allocation_size / 4;
snprintf(mem_max_buf, sizeof(mem_max_buf), "%zu", allocation_size * 3/4);
@@ -245,7 +247,7 @@ static int test_zswapin(const char *root)
goto out;
}

- if (zswpin < MB(24) / sysconf(_SC_PAGESIZE)) {
+ if (zswpin < MB(24) / page_size) {
ksft_print_msg("at least 24MB should be brought back from zswap\n");
goto out;
}
@@ -272,9 +274,8 @@ 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];
+ char buf[page_size];
long zswap_usage;
bool wb_enabled = *(bool *) arg;
int ret = -1;
@@ -289,11 +290,11 @@ static int attempt_writeback(const char *cgroup, void *arg)
* half empty, this will result in data that is still compressible
* and ends up in zswap, with material zswap usage.
*/
- for (int i = 0; i < pagesize; i++)
- buf[i] = i < pagesize/2 ? (char) i : 0;
+ for (int i = 0; i < page_size; i++)
+ buf[i] = i < page_size/2 ? (char) i : 0;

- for (int i = 0; i < memsize; i += pagesize)
- memcpy(&mem[i], buf, pagesize);
+ for (int i = 0; i < memsize; i += page_size)
+ memcpy(&mem[i], buf, page_size);

/* Try and reclaim allocated memory */
if (cg_write_numeric(cgroup, "memory.reclaim", memsize)) {
@@ -304,8 +305,8 @@ static int attempt_writeback(const char *cgroup, void *arg)
zswap_usage = cg_read_long(cgroup, "memory.zswap.current");

/* zswpin */
- for (int i = 0; i < memsize; i += pagesize) {
- if (memcmp(&mem[i], buf, pagesize)) {
+ for (int i = 0; i < memsize; i += page_size) {
+ if (memcmp(&mem[i], buf, page_size)) {
ksft_print_msg("invalid memory\n");
goto out;
}
@@ -441,7 +442,7 @@ 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 += page_size)
control_allocation[i] = 'a';
if (cg_read_key_long(control_group, "memory.stat", "zswapped") < 1)
goto out;
@@ -481,7 +482,7 @@ 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 += page_size)
((char *)allocation)[i] = 'a';
values->child_allocated = true;
pause();
@@ -529,7 +530,7 @@ 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 / page_size;
trigger_allocation_size = sys_info.totalram / 20;

/* Set up test memcg */
@@ -556,7 +557,7 @@ 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 += page_size)
trigger_allocation[i] = 'b';
usleep(100000);
free(trigger_allocation);
@@ -567,8 +568,8 @@ 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 * page_size - zswapped;
+ int result_ok = delta < stored_pages * page_size / 4;

ret = result_ok ? KSFT_PASS : KSFT_FAIL;
break;
@@ -622,7 +623,7 @@ static int allocate_random_and_wait(const char *cgroup, void *arg)
close(fd);

/* Touch all pages to ensure they're faulted in */
- for (size_t i = 0; i < size; i += PAGE_SIZE)
+ for (size_t i = 0; i < size; i += page_size)
mem[i] = mem[i];

/* Use MADV_PAGEOUT to push pages into zswap */
@@ -752,6 +753,10 @@ int main(int argc, char **argv)
char root[PATH_MAX];
int i;

+ page_size = sysconf(_SC_PAGE_SIZE);
+ if (page_size <= 0)
+ page_size = BUF_SIZE;
+
ksft_print_header();
ksft_set_plan(ARRAY_SIZE(tests));
if (cg_find_unified_root(root, sizeof(root), NULL))
--
2.53.0