[PATCH-next v3 6/9] cgroup/cpuset: Don't allow a child root partition under an isolated parent

[Waiman Long <longman@redhat.com>]

Although it is not typical for real users to create a complicated nested
partition hierarchy, it is currently allowed to create nested root and
isolated partitions. In addition, disabling or invalidating a partition
should always be allowed especially when the cpuset is being deleted.

With a nested root and isolated partition hierarchy, it is possible
that all active housekeeping CPUs can be exhausted when disabling or
invalidating a root partition and returning its CPUs to the isolated
parent.

One way to avoid this problem is to disallow the creation of a root
partition under an isolated parent. It is indeed a new limitation imposed
on the existing cpuset partition functionality. However an isolated
partition is usually a left node where scheduling and some other kernel
noises are disabled. It doesn't make much sense to create a child root
partition under an isolated parent and remove some CPUs from the set
of isolated CPUs again. Creation of a child isolated partition under a
root or isolated parent is still allowed.

The cgroup-v2 documentation is also updated to reflect this new
limitation. New test cases are added to test_cpuset_prs.sh to verify
this new behavior. Some existing test cases are also updated accordingly.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/cgroup-v2.rst           |  4 +++-
 kernel/cgroup/cpuset-internal.h                   |  1 +
 kernel/cgroup/cpuset.c                            | 13 +++++++++++++
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 14 ++++++++------
 4 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index bd8c80f3bbc9..2c445829e24d 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2767,7 +2767,9 @@ Cpuset Interface Files
 	be retained, but behaves more like a "member".
 
 	All possible state transitions among "member", "root" and
-	"isolated" are allowed.
+	"isolated" are allowed with the exception that a local child
+	"root" partition is not allowed to be created under an "isolated"
+	parent partition.
 
 	On read, the "cpuset.cpus.partition" file can show the following
 	values.
diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 5da9c91295b0..2e44dc8634dc 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -35,6 +35,7 @@ enum prs_errcode {
 	PERR_HKEEPING,
 	PERR_ACCESS,
 	PERR_REMOTE,
+	PERR_ISOPARENT,
 };
 
 /* bits in struct cpuset flags field */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 40c004386af0..681701ba46e1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -60,6 +60,7 @@ static const char * const perr_strings[] = {
 	[PERR_HKEEPING]  = "partition config conflicts with housekeeping setup",
 	[PERR_ACCESS]    = "Enable partition not permitted",
 	[PERR_REMOTE]    = "Have remote partition underneath",
+	[PERR_ISOPARENT] = "Root partition is not allowed with isolated parent",
 };
 
 /*
@@ -2865,6 +2866,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	int err = PERR_NONE, old_prs = cs->partition_root_state;
 	struct cpuset *parent = parent_cs(cs);
 	struct tmpmasks tmpmask;
+	int parent_prs = parent->partition_root_state;
 	bool isolcpus_updated = false;
 	bool disable_partition = false;
 
@@ -2910,6 +2912,14 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 			enum partition_cmd cmd = (new_prs == PRS_ROOT)
 					       ? partcmd_enable : partcmd_enablei;
 
+			/*
+			 * It is not allowed to create a root partition under
+			 * an isolated parent partition.
+			 */
+			if ((new_prs == PRS_ROOT) && (parent_prs == PRS_ISOLATED)) {
+				err = PERR_ISOPARENT;
+				goto out;
+			}
 			err = update_parent_effective_cpumask(cs, cmd, NULL, &tmpmask);
 		} else {
 			err = remote_partition_enable(cs, new_prs, &tmpmask);
@@ -2922,6 +2932,9 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 		if (prstate_housekeeping_conflict(new_prs, cs->effective_xcpus)) {
 			disable_partition = true;
 			err = PERR_HKEEPING;
+		} else if (parent_prs == PRS_ISOLATED) {
+			disable_partition = true;
+			err = PERR_ISOPARENT;
 		} else {
 			isolcpus_updated = true;
 		}
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 874a8a161236..85a4f87840b4 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -288,16 +288,12 @@ TEST_MATRIX=(
 								       A1:P0|A2:P1|A3:P0|B1:P1"
 	"   C0-3    C1-3   C3     C4    X2-3  X2-3:P1   P2     P1    0 A1:0-1|A2:2|A3:3|B1:4 \
 								       A1:P0|A2:P1|A3:P2|B1:P1 2-4|3"
-	"   C0-4    C1-4  C2-4     .    X2-4  X2-4:P2  X4:P1    .    0 A1:0-1|A2:2-3|A3:4 \
-								       A1:P0|A2:P2|A3:P1 2-4|2-3"
-	"   C0-4    C1-4  C2-4     .    X2-4  X2-4:P2 X3-4:P1   .    0 A1:0-1|A2:2|A3:3-4 \
-								       A1:P0|A2:P2|A3:P1 2"
 	" C0-4:X2-4 C1-4:X2-4:P1 C2-4:X4:P2 \
 				   .      .      X5      .      .    0 A1:0-4|A2:1-4|A3:2-4 \
 								       A1:P0|A2:P-1|A3:P-2 ."
-	" C0-4:X2-4 C1-4:X2-4:P2 C2-4:X4:P1 \
+	" C0-4:X2-4 C1-4:X2-4:P1 C2-4:X4:P2 \
 				   .      .      .      X1      .    0 A1:0-1|A2:2-4|A3:2-4 \
-								       A1:P0|A2:P2|A3:P-1 2-4"
+								       A1:P0|A2:P1|A3:P-2 ."
 	" 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"
 
@@ -430,6 +426,12 @@ TEST_MATRIX=(
 	# cpuset.cpus can become empty with task in it as it inherits parent's effective CPUs
 	"   C1-3    C2      .      .       .    T:C     .      .     0 A1:1-3|A2:1-3"
 
+	# Creation of root partition under an isolated partition is not allowed
+	"  C1-3:P2  C2      .      .       .     P1     .      .     0 A1:1-3|A2:2 A1:P2|A2:P-1 1-3"
+	"  C1-3:P2  C2      .      .       .     P2     .      .     0 A1:1,3|A2:2 A1:P2|A2:P2  1-3"
+	"  C1-3:P2 C2:P2    .      .       .     P1     .      .     0 A1:1-3|A2:2 A1:P2|A2:P-1 1-3"
+
+	#
 	#  old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
 	#  ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------
 	# Failure cases:
-- 
2.55.0