linux-kernel - [PATCH 05/10] cgroup/cpuset: Don't allow creation of local partition over a remote one

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Message-ID: <20250330215248.3620801-6-longman@redhat.com>
Date: Sun, 30 Mar 2025 17:52:43 -0400
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Waiman Long <longman@...hat.com>
Subject: [PATCH 05/10] 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@...hat.com>
---
 kernel/cgroup/cpuset-internal.h |  1 +
 kernel/cgroup/cpuset.c          | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 976a8bc3ff60..383963e28ac6 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/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 */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index ffa85d34ba51..f26f791e9323 100644
--- a/kernel/cgroup/cpuset.c
+++ b/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",
 };
 
 /*
@@ -2855,6 +2856,19 @@ 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.
-- 
2.48.1