[PATCH v5 0/2] selftests/cgroup: fixes for test_zswap on single core VM

[Wilson Felipe Pereira <wfelipe@google.com>]

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.

v4 -> v5:
- Patch 2: Introduce `cg_read_key_long_long()` to return a `long long`
  value, and have `cg_read_key_long` use it and convert to `long`.
- Patch 2: Make `zswapped` a `long long`. This prevents a overflow on
  32-bit platforms (Andrew Morton, Sashiko).

v3 -> v4:
- Patch 2: Use `long long` for `delta` and `max_delta` and explicitly cast
  `stored_pages` to avoid integer overflow and implicit unsigned promotion
  warnings 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).

v4: https://lore.kernel.org/all/20260828033741.2184560-1-wfelipe@google.com/
v3: https://lore.kernel.org/all/20260827034807.2822234-1-wfelipe@google.com/
v2: https://lore.kernel.org/all/20260824033533.2147900-1-wfelipe@google.com/
v1: https://lore.kernel.org/all/20260804042053.56940-1-wfelipe@google.com/

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/lib/cgroup_util.c       | 17 ++++++++++++-----
 tools/testing/selftests/cgroup/lib/include/cgroup_util.h |  1 +
 tools/testing/selftests/cgroup/test_zswap.c            | 13 ++++++++-----
 3 files changed, 21 insertions(+), 10 deletions(-)

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