[PATCH v4 0/2] selftests/cgroup: fixes for test_zswap on single core VM

[Wilson Felipe Pereira]

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.

v3 -> v4:
- Patch 2: Use `long long` for `delta` and `max_delta` to avoid integer
  overflow on 32-bit platforms (Andrew Morton, Sashiko).

v2 -> v3:
- Patch 1: Collect Acked-by from Michal Koutný.
- Patch 2: Explicitly cast stored_pages to (long) in delta calculation and
  inline the return comparison (Michal Koutný).

v1 -> v2:
- Patch 1: Replace EBUSY retry loop with cg_read_strcmp_wait() waiting for
  cgroup.events "populated 0" (Michal Koutný).
- Patch 1: Clarify task lifecycle in commit description (Yosry Ahmed).
- Patch 2: Remove abs() and declare delta/zswapped as signed longs with a
  signed threshold comparison (Michal Koutný).
- Patch 2: Add Fixes tag (Michal Koutný).

v3: https://lore.kernel.org/all/20260827034807.2822234-1-wfelipe@xxxxxxxxxx/
v2: https://lore.kernel.org/all/20260824033533.2147900-1-wfelipe@xxxxxxxxxx/
v1: https://lore.kernel.org/all/20260804042053.56940-1-wfelipe@xxxxxxxxxx/

Wilson Felipe Pereira (2):
  selftests/cgroup: test_zswap: wait for cgroup to unpopulate in
    test_zswap_writeback
  selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in
    test_no_kmem_bypass

 tools/testing/selftests/cgroup/test_zswap.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

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