[PATCH v6 0/3] 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 standardizes cgroup key-value stat and event parsers on 64-bit
types by replacing cg_read_key_long() with cg_read_key_s64() across the
entire cgroup selftest suite, avoiding 32-bit integer overflows on systems
with large memory or high counter values.

Patch 3 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.

v5 -> v6:
- Now it is a 3-patch series.
- Patch 1: wait for cgroup to unpopulate in test_zswap_writeback
  (unchanged, retains Michal's Acked-by).
- Patch 2: convert cg_read_key_*() helpers to s64, define s64 as
  signed long long in cgroup_util.h, update poll/reporting helpers
  (values_close, values_close_report with llabs), helper functions,
  and all callers across cgroup selftests to avoid 32-bit
  overflow/truncation (suggested by Michal Koutný).
  - signed long long for s64 was based on powerpc/include/utils.h
- Patch 3: test_zswap: fix implicit unsigned promotion bug in
  test_no_kmem_bypass, updated to use s64 for delta and max_delta.
- Verify warning-free builds on both 64-bit and 32-bit (gcc -m32).

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 an 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).

v5: https://lore.kernel.org/all/20260901052250.154532-1-wfelipe@google.com/
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 (3):
  selftests/cgroup: test_zswap: wait for cgroup to unpopulate in
    test_zswap_writeback
  selftests/cgroup: convert cg_read_key_*() helpers to s64
  selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in
    test_no_kmem_bypass

 .../selftests/cgroup/lib/cgroup_util.c        | 14 ++--
 .../cgroup/lib/include/cgroup_util.h          | 22 +++---
 tools/testing/selftests/cgroup/test_cpu.c     | 47 ++++++-------
 tools/testing/selftests/cgroup/test_freezer.c | 16 ++---
 tools/testing/selftests/cgroup/test_kmem.c    | 56 ++++++++-------
 .../selftests/cgroup/test_memcontrol.c        | 69 ++++++++++---------
 tools/testing/selftests/cgroup/test_pids.c    |  5 +-
 tools/testing/selftests/cgroup/test_zswap.c   | 63 +++++++++--------
 8 files changed, 154 insertions(+), 138 deletions(-)

--
2.55.0.979.g7e5102b832-goog