Re: [PATCH v3 2/3] cgroup/cpuset: prevent activating local partition over remote one

[Waiman Long <longman@redhat.com>]

On 9/20/26 2:14 PM, Hui Peng wrote:
> Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
> partition over a remote one") added a check in update_prstate() to reject
> creating a top-level local partition whose cs->exclusive_cpus intersects
> subpartitions_cpus.
>
> However, if a top-level cgroup A1 is first set to "root" with all CPUs
> (CX0-7:P1, becoming "root invalid" because no CPUs would remain in
> top_cpuset), has a child A1/A2 enabled as a remote partition on a subset
> of those exclusive CPUs (CX2-3:P2), and A1 is then updated to a valid CPU
> subset (CX1-3), update_exclusive_cpumask() / update_cpumask() calls
> validate_partition() without going through update_prstate(). Because
> validate_partition() does not check trialcs->exclusive_cpus against
> subpartitions_cpus, A1 transitions from "root invalid" to a valid local
> partition over the existing remote partition A1/A2 and triggers:
>
>    WARNING: kernel/cgroup/cpuset.c:1943 at update_parent_effective_cpumask+0x189b/0x1fd0
>
> Check trialcs->exclusive_cpus against subpartitions_cpus in
> validate_partition() when !is_partition_valid(cs), and add a regression
> test case to tools/testing/selftests/cgroup/test_cpuset_prs.sh.
>
> Tested in QEMU on Linux 7.3.0-rc3 using
> tools/testing/selftests/cgroup/test_cpuset_prs.sh.
>
> Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v3:
> - Split the validate_partition() fix (Fixes: 6da580ec656a) and the
>    cpus_excl_conflict() fix (Fixes: 2a3602030d80) into separate patches.
>
>   kernel/cgroup/cpuset.c                        | 4 ++++
>   tools/testing/selftests/cgroup/test_cpuset_prs.sh | 2 ++
>   2 files changed, 6 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 7381fa8502e7..7c5b4a8f1f3a 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2415,6 +2415,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
>   	if (cpumask_empty(trialcs->effective_xcpus))
>   		return PERR_INVCPUS;
>   
> +	if ((parent == &top_cpuset) && !is_partition_valid(cs) &&
> +	    cpumask_intersects(trialcs->exclusive_cpus, subpartitions_cpus))
> +		return PERR_REMOTE;
> +
This additional check looks good.
>   	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
>   					  trialcs->effective_xcpus))
>   		return PERR_HKEEPING;
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index b732078bf319..f4d1822d4e21 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -321,6 +321,8 @@ TEST_MATRIX=(
>   	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
>   	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
>   	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
> +	" CX0-7:P1 CX2-3:P2 .      .    CX1-3    .      .      .     0 A1:1|A2:2-3|XA2:2-3 \
> +								       A1:P-1|A2:P2 2-3"

This test will not work because you are assuming the test system only 
have 8 CPUs. That may not be the case and 2 local partitions may be 
created instead.

Cheers,
Longman

>   
>   	# Local partition invalidation tests
>   	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \