[PATCH v7 6/8] selftest/cgroup: fix zswap test_no_invasive_cgroup_shrink on large pagesize system

Li Wang
2026-04-24T04:02:34Z

test_no_invasive_cgroup_shrink sets up two cgroups: wb_group, which is
expected to trigger zswap writeback, and a control group (renamed to
zw_group), which should only have pages sitting in zswap without any
writeback.

There are two problems with the current test:

1) The data patterns are reversed. wb_group uses allocate_bytes(), which
writes only a single byte per page — trivially compressible,
especially by zstd — so compressed pages fit within zswap.max and
writeback is never triggered. Meanwhile, the control group uses
getrandom() to produce hard-to-compress data, but it is the group
that does *not* need writeback.

2) The test uses fixed sizes (10K zswap.max, 10MB allocation) that are
too small on systems with large PAGE_SIZE (e.g. 64K), failing to
build enough memory pressure to trigger writeback reliably.

Fix both issues by:
- Swapping the data patterns: fill wb_group pages with partially
random data (getrandom for page_size/4 bytes) to resist compression
and trigger writeback, and fill zw_group pages with simple repeated
data to stay compressed in zswap.
- Making all size parameters PAGE_SIZE-aware: set allocation size to
PAGE_SIZE * 1024, memory.zswap.max to PAGE_SIZE, and memory.max to
allocation_size / 2 for both cgroups.
- Allocating memory inline instead of via cg_run() so the pages
remain resident throughout the test.

=== Error Log ===
# getconf PAGESIZE
65536

# ./test_zswap
TAP version 13
...
ok 5 test_zswap_writeback_disabled
ok 6 # SKIP test_no_kmem_bypass
not ok 7 test_no_invasive_cgroup_shrink

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>
Cc: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
---
tools/testing/selftests/cgroup/test_zswap.c | 70 ++++++++++++++-------
1 file changed, 49 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 23ff11390a3..8f0478923bd 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -11,6 +11,7 @@
#include <string.h>
#include <sys/wait.h>
#include <sys/mman.h>
+#include <sys/random.h>

#include "kselftest.h"
#include "cgroup_util.h"
@@ -426,44 +427,71 @@ static int test_zswap_writeback_disabled(const char *root)
static int test_no_invasive_cgroup_shrink(const char *root)
{
int ret = KSFT_FAIL;
- size_t control_allocation_size = MB(10);
- char *control_allocation = NULL, *wb_group = NULL, *control_group = NULL;
+ unsigned int off;
+ size_t allocation_size = page_size * 1024;
+ unsigned int nr_pages = allocation_size / page_size;
+ char zswap_max_buf[32], mem_max_buf[32];
+ char *zw_allocation = NULL, *wb_allocation = NULL;
+ char *zw_group = NULL, *wb_group = NULL;
+
+ snprintf(zswap_max_buf, sizeof(zswap_max_buf), "%d", page_size);
+ snprintf(mem_max_buf, sizeof(mem_max_buf), "%zu", allocation_size / 2);

wb_group = setup_test_group_1M(root, "per_memcg_wb_test1");
if (!wb_group)
return KSFT_FAIL;
- if (cg_write(wb_group, "memory.zswap.max", "10K"))
+ if (cg_write(wb_group, "memory.zswap.max", zswap_max_buf))
+ goto out;
+ if (cg_write(wb_group, "memory.max", mem_max_buf))
+ goto out;
+
+ zw_group = setup_test_group_1M(root, "per_memcg_wb_test2");
+ if (!zw_group)
goto out;
- control_group = setup_test_group_1M(root, "per_memcg_wb_test2");
- if (!control_group)
+ if (cg_write(zw_group, "memory.max", mem_max_buf))
goto out;

- /* Push some test_group2 memory into zswap */
- if (cg_enter_current(control_group))
+ /* Push some zw_group memory into zswap (simple data, easy to compress) */
+ if (cg_enter_current(zw_group))
goto out;
- control_allocation = malloc(control_allocation_size);
- 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)
+ zw_allocation = malloc(allocation_size);
+ for (int i = 0; i < nr_pages; i++) {
+ off = (unsigned long)i * page_size;
+ memset(&zw_allocation[off], 0, page_size);
+ memset(&zw_allocation[off], 'a', page_size/4);
+ }
+ if (cg_read_key_long(zw_group, "memory.stat", "zswapped") < 1)
goto out;

- /* Allocate 10x memory.max to push wb_group memory into zswap and trigger wb */
- if (cg_run(wb_group, allocate_bytes, (void *)MB(10)))
+ /* Push wb_group memory into zswap with hard-to-compress data to trigger wb */
+ if (cg_enter_current(wb_group))
+ goto out;
+ wb_allocation = malloc(allocation_size);
+ if (!wb_allocation)
goto out;
+ for (int i = 0; i < nr_pages; i++) {
+ off = (unsigned long)i * page_size;
+ memset(&wb_allocation[off], 0, page_size);
+ getrandom(&wb_allocation[off], page_size/4, 0);
+ }

/* Verify that only zswapped memory from gwb_group has been written back */
- if (get_cg_wb_count(wb_group) > 0 && get_cg_wb_count(control_group) == 0)
+ if (get_cg_wb_count(wb_group) > 0 && get_cg_wb_count(zw_group) == 0)
ret = KSFT_PASS;
out:
cg_enter_current(root);
- if (control_group) {
- cg_destroy(control_group);
- free(control_group);
+ if (zw_group) {
+ cg_destroy(zw_group);
+ free(zw_group);
+ }
+ if (wb_group) {
+ cg_destroy(wb_group);
+ free(wb_group);
}
- cg_destroy(wb_group);
- free(wb_group);
- if (control_allocation)
- free(control_allocation);
+ if (zw_allocation)
+ free(zw_allocation);
+ if (wb_allocation)
+ free(wb_allocation);
return ret;
}

--
2.53.0