selftests/cgroup: test_zswap: wait for cgroup to unpopulate in test_zswap_writeback

Patch series "selftests/cgroup: fixes for test_zswap on single core VM",
v4.

This series fixes two test failures in test_zswap observed when running on
a single-core VM (-smp 1) with 4GB of RAM.

Patch 1 addresses a race condition in test_zswap_writeback() where
waitpid() returns before the exiting child process is switched away by the
kernel, causing an immediate write of "+memory" to cgroup.subtree_control
to fail with -EBUSY.  We fix this by waiting for cgroup.events to report
"populated 0".

Patch 2 fixes an implicit unsigned conversion bug in test_no_kmem_bypass()
where small negative timing differences between debugfs stored_pages and
cgroup zswapped bytes caused the comparison to falsely fail due to
unsigned promotion.


This patch (of 2):

When running test_zswap on a single-core VM (-smp 1) with 4GB of RAM,
test_zswap_writeback intermittently fails on the initial run after boot.

In test_zswap_writeback(), after waitpid() reaps the child process created
by test_zswap_writeback_one(), writing "+memory" to cgroup.subtree_control
can fail with -EBUSY.  Under cgroup v2, enabling domain subtree
controllers is forbidden while any tasks remain in cgroup.procs.

When a child process exits, exit_notify() wakes the parent process,
allowing waitpid() to return immediately.  However, the cgroup populated
task count (nr_populated_csets) is only decremented when the exiting task
is switched away via finish_task_switch() -> cgroup_task_dead().  On
single-core systems, the parent runs before the dead child has been
switched out, causing "+memory" to fail with -EBUSY if written immediately
after waitpid() returns.

Fix this by waiting for cgroup.events to report "populated 0\n" via
cg_read_strcmp_wait() before enabling subtree control.

Link: https://lore.kernel.org/20260828033741.2184560-1-wfelipe@google.com
Link: https://lore.kernel.org/20260828033741.2184560-2-wfelipe@google.com
Signed-off-by: Wilson Felipe Pereira <wfelipe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 8df54b59513a..b1abc94317e4 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -409,6 +409,8 @@ static int test_zswap_writeback(const char *root, bool wb)
 	 * Thus, the parent's setting shall be what's in effect. */
 	if (cg_write(test_group, "memory.zswap.max", "max"))
 		goto out;
+	if (cg_read_strcmp_wait(test_group, "cgroup.events", "populated 0\n"))
+		goto out;
 	if (cg_write(test_group, "cgroup.subtree_control", "+memory"))
 		goto out;