FAILED: patch "[PATCH] cgroup/cpuset: Return PERR_NOCPUS in" failed to apply to 6.18-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 31c88350b7dd1522792f726f79607f31bb55c50f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026092918-captive-docile-ef46@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 31c88350b7dd1522792f726f79607f31bb55c50f Mon Sep 17 00:00:00 2001
From: Hui Peng <benquike@gmail.com>
Date: Thu, 24 Sep 2026 04:27:27 +0000
Subject: [PATCH] cgroup/cpuset: Return PERR_NOCPUS in
 remote_partition_enable() on subpartitions_cpus conflict

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>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8f24171b6055..1fcec89a28b9 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 131d8b4551ef..7efd5e645767 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"