cgroup/cpuset: Don't allow creation of local partition over a remote one

JIRA: https://issues.redhat.com/browse/RHEL-107751

commit 6da580e
Author: Waiman Long <longman@redhat.com>
Date:   Sun, 30 Mar 2025 17:52:43 -0400

    cgroup/cpuset: Don't allow creation of local partition over a remote one

    Currently, we don't allow the creation of a remote partition underneath
    another local or remote partition. However, it is currently possible to
    create a new local partition with an existing remote partition underneath
    it if top_cpuset is the parent. However, the current cpuset code does
    not set the effective exclusive CPUs correctly to account for those
    that are taken by the remote partition.

    Changing the code to properly account for those remote partition CPUs
    under all possible circumstances can be complex. It is much easier to
    not allow such a configuration which is not that useful. So forbid
    that by making sure that exclusive_cpus mask doesn't overlap with
    subpartitions_cpus and invalidate the partition if that happens.

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

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

kernel/cgroup/cpuset-internal.h

@@ -33,6 +33,7 @@ enum prs_errcode {
 	PERR_CPUSEMPTY,
 	PERR_HKEEPING,
 	PERR_ACCESS,
+	PERR_REMOTE,
 };
 
 /* bits in struct cpuset flags field */

kernel/cgroup/cpuset.c

@@ -61,6 +61,7 @@ static const char * const perr_strings[] = {
 	[PERR_CPUSEMPTY] = "cpuset.cpus and cpuset.cpus.exclusive are empty",
 	[PERR_HKEEPING]  = "partition config conflicts with housekeeping setup",
 	[PERR_ACCESS]    = "Enable partition not permitted",
+	[PERR_REMOTE]    = "Have remote partition underneath",
 };
 
 /*
@@ -2888,6 +2889,19 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 			goto out;
 		}
 
+		/*
+		 * We don't support the creation of a new local partition with
+		 * a remote partition underneath it. This unsupported
+		 * setting can happen only if parent is the top_cpuset because
+		 * a remote partition cannot be created underneath an existing
+		 * local or remote partition.
+		 */
+		if ((parent == &top_cpuset) &&
+		    cpumask_intersects(cs->exclusive_cpus, subpartitions_cpus)) {
+			err = PERR_REMOTE;
+			goto out;
+		}
+
 		/*
 		 * If parent is valid partition, enable local partiion.
 		 * Otherwise, enable a remote partition.