Re: [PATCH v4 1/3] cgroup/cpuset: return PERR_NOCPUS in remote_partition_enable() on subpartitions_cpus conflict

[Ridong Chen <ridong.chen@linux.dev>]

On 9/24/2026 12:27 PM, Hui Peng wrote:
> When a remote partition is created underneath an existing local partition
> via a non-partition (PRS_MEMBER) intermediate cgroup, update_prstate() sees
> parent->partition_root_state == PRS_MEMBER and calls
> remote_partition_enable().
> 
> Commit 86888c7bd117 ("cgroup/cpuset: Add warnings to catch inconsistency
> in exclusive CPUs") replaced the cpumask_intersects(tmp->new_cpus,
> subpartitions_cpus) error check in remote_partition_enable() with
> WARN_ON_ONCE(). As a result, remote_partition_enable() emits a warning
> and proceeds to enable the remote partition on CPUs that are already
> owned by the ancestor local partition in subpartitions_cpus.
> 
> This can be reproduced on Linux 7.3.0-rc3 with:
> 
>    mkdir -p /tmp/cg1
>    mount -t cgroup2 none /tmp/cg1
>    echo "+cpuset" > /tmp/cg1/cgroup.subtree_control
> 
>    mkdir /tmp/cg1/A
>    echo 1 > /tmp/cg1/A/cpuset.cpus
>    echo 1 > /tmp/cg1/A/cpuset.cpus.exclusive
>    echo root > /tmp/cg1/A/cpuset.cpus.partition
>    echo "+cpuset" > /tmp/cg1/A/cgroup.subtree_control
> 
>    mkdir /tmp/cg1/A/B
>    echo 1 > /tmp/cg1/A/B/cpuset.cpus
>    echo 1 > /tmp/cg1/A/B/cpuset.cpus.exclusive
>    echo "+cpuset" > /tmp/cg1/A/B/cgroup.subtree_control
> 
>    mkdir /tmp/cg1/A/B/D
>    echo 1 > /tmp/cg1/A/B/D/cpuset.cpus
>    echo 1 > /tmp/cg1/A/B/D/cpuset.cpus.exclusive
>    echo root > /tmp/cg1/A/B/D/cpuset.cpus.partition
> 
> which triggers:
> 
>    WARNING: kernel/cgroup/cpuset.c:1594 at remote_partition_enable+0x1c1/0x300
> 
> and leaves both /tmp/cg1/A and /tmp/cg1/A/B/D as active root partitions
> claiming exclusive CPU 1.
> 
> Fix this by returning PERR_NOCPUS when tmp->new_cpus intersects
> subpartitions_cpus in remote_partition_enable(), matching the error code
> used by remote_cpus_update() for the same subpartitions_cpus conflict, 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 the reproducer above and
> tools/testing/selftests/cgroup/test_cpuset_prs.sh.
> 
> Fixes: 86888c7bd117 ("cgroup/cpuset: Add warnings to catch inconsistency in exclusive CPUs")
> Suggested-by: Guopeng Zhang <guopeng.zhang@linux.dev>
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
>   kernel/cgroup/cpuset.c                            | 3 ++-
>   tools/testing/selftests/cgroup/test_cpuset_prs.sh | 2 ++
>   2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2538faac9aba..7381fa8502e7 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1591,10 +1591,11 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>   	 * above it or remote partition root underneath it is not allowed.
>   	 */
>   	compute_excpus(cs, tmp->new_cpus);
> -	WARN_ON_ONCE(cpumask_intersects(tmp->new_cpus, subpartitions_cpus));
>   	if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
>   	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
>   		return PERR_INVCPUS;
> +	if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus))
> +		return PERR_NOCPUS;
>   	if (((new_prs == PRS_ISOLATED) &&
>   	     !isolated_cpus_can_update(tmp->new_cpus, NULL)) ||
>   	    prstate_housekeeping_conflict(new_prs, tmp->new_cpus))
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 3f7e81709365..b732078bf319 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -298,6 +298,8 @@ TEST_MATRIX=(
>   	" C0-4:X2-4 C1-4:X2-4:P2 C2-4:X4:P1 \
>   				   .      .      .      X1      .    0 A1:0-1|A2:2-4|A3:2-4 \
>   								       A1:P0|A2:P2|A3:P-1 2-4"
> +	" CX1-3:P1 CX1-3  CX1-3    .      .      .      P1      .    0 A1:1-3|A2:1-3|A3:1-3 \
> +								       A1:P1|A2:P0|A3:P-1"
>   
>   	# Remote partition offline tests
>   	"   C0-3    C1-3  C2-3     .    X2-3   X2-3 X2-3:P2:O2=0 .   0 A1:0-1|A2:1|A3:3 A1:P0|A3:P2 2-3"

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Thanks.
-- 
Best regards
Ridong