[PATCH v4 0/3] cgroup/cpuset: fix exclusive CPU conflict checks for local and remote partitions

[Hui Peng <benquike@gmail.com>]

This series fixes three exclusive CPU validation gaps across local and
remote partitions in kernel/cgroup/cpuset.c and adds corresponding
regression tests to tools/testing/selftests/cgroup/test_cpuset_prs.sh:

1. Patch 1/3 ("cgroup/cpuset: return PERR_NOCPUS in
   remote_partition_enable() on subpartitions_cpus conflict"):
   Restores the subpartitions_cpus conflict error check in
   remote_partition_enable() (returning PERR_NOCPUS to match
   remote_cpus_update()) so a remote partition cannot be enabled
   underneath an ancestor local partition via a non-partition
   intermediate cgroup.

2. Patch 2/3 ("cgroup/cpuset: prevent activating local partition over
   remote one"):
   Checks trialcs->exclusive_cpus against subpartitions_cpus in
   validate_partition() when !is_partition_valid(cs) so an initially
   invalid top-level partition cannot later transition to a valid local
   partition over an active remote partition via cpuset.cpus /
   cpuset.cpus.exclusive updates.

3. Patch 3/3 ("cgroup/cpuset: check sibling effective_xcpus in
   cpus_excl_conflict()"):
   Falls back to sibling->effective_xcpus in cpus_excl_conflict() when
   sibling->exclusive_cpus is empty so a sibling cgroup cannot claim
   cpuset.cpus.exclusive over a valid local partition's implicit
   exclusive CPUs.

Changes in v4:
- Send as a standalone series with a cover letter (0/3) instead of
  replying in-thread to the previous version, as requested by Ridong
  Chen and Waiman Long.
- Restore the exact v2 diff and commit description for patch 1/3
  (which was accidentally corrupted when splitting v2 into 3 patches in
  v3), fixing git apply against mainline.
- Make the test_cpuset_prs.sh test case in patch 2/3 independent of the
  system CPU count (using X2-3 / CX2-3:P2 / P1:CX1-3 instead of
  CX0-7:P1), as pointed out by Waiman Long.

Changes in v3:
- Split patch 2/2 from v2 into patch 2/3 (validate_partition(),
  Fixes: 6da580ec656a) and patch 3/3 (cpus_excl_conflict(),
  Fixes: 2a3602030d80), as pointed out by Guopeng Zhang.
- Add Assisted-by: LLM tag.

Changes in v2:
- Split the remote_partition_enable() fix into patch 1/2 and return
  PERR_NOCPUS instead of PERR_INVCPUS to match remote_cpus_update(), as
  suggested by Guopeng Zhang.
- Add step-by-step reproducer and test_cpuset_prs.sh regression tests,
  as requested by Ridong Chen.

Hui Peng (3):
  cgroup/cpuset: return PERR_NOCPUS in remote_partition_enable() on subpartitions_cpus conflict
  cgroup/cpuset: prevent activating local partition over remote one
  cgroup/cpuset: check sibling effective_xcpus in cpus_excl_conflict()

 kernel/cgroup/cpuset.c                            | 12 ++++++++++--
 tools/testing/selftests/cgroup/test_cpuset_prs.sh |  5 +++++
 2 files changed, 15 insertions(+), 2 deletions(-)

-- 
2.49.0