[PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[Hui Peng]

Fix two partition validation bugs in kernel/cgroup/cpuset.c:

1. In remote_partition_enable(), check whether the requested effective
   xcpus intersect parent->subpartitions_cpus so a remote partition
   cannot claim CPUs already delegated to a local child partition.
2. In validate_partition(), verify that enabling a local partition does
   not overlap CPUs already allocated to an active remote partition.

Fixes: aa7d3a56a20f ("cpuset: fix warning when disabling remote partition")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 2538faac9aba..13fad096c494 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
 		return true;
 
 	/* Exclusive_cpus cannot intersect */
-	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
+	return cpumask_intersects(trial->exclusive_cpus,
+				  cpumask_empty(sibling->exclusive_cpus)
+				  ? sibling->effective_xcpus
+				  : sibling->exclusive_cpus);
 }
 
 static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
@@ -1591,8 +1594,8 @@ 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_intersects(tmp->new_cpus, subpartitions_cpus) ||
 	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
 		return PERR_INVCPUS;
 	if (((new_prs == PRS_ISOLATED) &&
@@ -2411,6 +2414,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
 	if (cpumask_empty(trialcs->effective_xcpus))
 		return PERR_INVCPUS;
 
+	if ((parent == &top_cpuset) &&
+	    cpumask_intersects(trialcs->effective_xcpus, subpartitions_cpus))
+		return PERR_REMOTE;
+
 	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
 					  trialcs->effective_xcpus))
 		return PERR_HKEEPING;
@@ -2970,7 +2977,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 		 * local or remote partition.
 		 */
 		if ((parent == &top_cpuset) &&
-		    cpumask_intersects(cs->exclusive_cpus, subpartitions_cpus)) {
+		    cpumask_intersects(user_xcpus(cs), subpartitions_cpus)) {
 			err = PERR_REMOTE;
 			goto out;
 		}

Re: [PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[Ridong Chen]

On 9/20/2026 6:17 AM, Hui Peng wrote:
> Fix two partition validation bugs in kernel/cgroup/cpuset.c:
> 
> 1. In remote_partition_enable(), check whether the requested effective
>     xcpus intersect parent->subpartitions_cpus so a remote partition
>     cannot claim CPUs already delegated to a local child partition.
> 2. In validate_partition(), verify that enabling a local partition does
>     not overlap CPUs already allocated to an active remote partition.
> 
> Fixes: aa7d3a56a20f ("cpuset: fix warning when disabling remote partition")
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>

Could you please first describe what the issue is and how it can be triggered?
Or is there any reproducer?

> ---
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2538faac9aba..13fad096c494 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
>   		return true;
>   
>   	/* Exclusive_cpus cannot intersect */
> -	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
> +	return cpumask_intersects(trial->exclusive_cpus,
> +				  cpumask_empty(sibling->exclusive_cpus)
> +				  ? sibling->effective_xcpus
> +				  : sibling->exclusive_cpus);
>   }
>   
>   static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
> @@ -1591,8 +1594,8 @@ 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_intersects(tmp->new_cpus, subpartitions_cpus) ||
>   	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
>   		return PERR_INVCPUS;
>   	if (((new_prs == PRS_ISOLATED) &&
> @@ -2411,6 +2414,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
>   	if (cpumask_empty(trialcs->effective_xcpus))
>   		return PERR_INVCPUS;
>   
> +	if ((parent == &top_cpuset) &&
> +	    cpumask_intersects(trialcs->effective_xcpus, subpartitions_cpus))
> +		return PERR_REMOTE;
> +
>   	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
>   					  trialcs->effective_xcpus))
>   		return PERR_HKEEPING;
> @@ -2970,7 +2977,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   		 * local or remote partition.
>   		 */
>   		if ((parent == &top_cpuset) &&
> -		    cpumask_intersects(cs->exclusive_cpus, subpartitions_cpus)) {
> +		    cpumask_intersects(user_xcpus(cs), subpartitions_cpus)) {
>   			err = PERR_REMOTE;
>   			goto out;
>   		}

-- 
Best regards
Ridong

Re: [PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[Hui Peng]

On Sun, Sep 20, 2026 at 09:22:09AM +0800, Ridong Chen wrote:
> Could you please first describe what the issue is and how it can be triggered?
> Or is there any reproducer?

Hi Ridong,

Thanks for taking a look. Below is a detailed description of both
scenarios, how each is triggered in the current code, the dmesg
WARNINGs produced on 7.3.0-rc3, and the minimized shell reproducers.

------------------------------------------------------------------------
Scenario 1: Enabling a remote partition underneath an ancestor local
            partition root (remote_partition_enable)
------------------------------------------------------------------------

How it happens:
1. Suppose top-level cgroup "A" is a valid local partition root owning
   CPU 1 (cpuset.cpus = 1, cpuset.cpus.exclusive = 1,
   cpuset.cpus.partition = root). Enabling "A" calls
   partition_xcpus_add(..., &top_cpuset, {1}), which adds CPU 1 to
   subpartitions_cpus.
2. Child "A/B" is a normal non-partition member (cpuset.cpus.partition =
   member) with cpuset.cpus = 1 and cpuset.cpus.exclusive = 1.
3. Grandchild "A/B/D" has cpuset.cpus = 1, cpuset.cpus.exclusive = 1.
   When "root" is written to "A/B/D/cpuset.cpus.partition",
   update_prstate() checks is_partition_valid(parent) on A/B (which is
   false because A/B is PRS_MEMBER) and therefore calls
   remote_partition_enable(D, PRS_ROOT, &tmpmask).
4. In remote_partition_enable(), compute_excpus(D, tmp->new_cpus) walks
   up D -> B -> A and computes tmp->new_cpus = {1}. Because ancestor "A"
   is already a valid local partition root, CPU 1 is already present in
   subpartitions_cpus.
5. remote_partition_enable() hits:
     WARN_ON_ONCE(cpumask_intersects(tmp->new_cpus, subpartitions_cpus));
   at kernel/cgroup/cpuset.c:1594 and continues without returning an
   error, enabling "A/B/D" as a valid remote partition on CPU 1 while
   ancestor "A" is simultaneously a valid local partition on CPU 1,
   which then also triggers a second WARNING in
   rebuild_sched_domains_locked() (kernel/cgroup/cpuset.c:906).

Minimized reproducer (Scenario 1):
  #!/bin/sh
  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

dmesg output on 7.3.0-rc3:
  WARNING: kernel/cgroup/cpuset.c:1594 at update_prstate+0xbef/0xd70, CPU#2
  Call Trace:
   cpuset_partition_write+0x112/0x140
  WARNING: kernel/cgroup/cpuset.c:906 at rebuild_sched_domains_locked+0x4f2/0x710, CPU#2
  Call Trace:
   cpuset_partition_write+0x112/0x140

------------------------------------------------------------------------
Scenario 2: Invalid top-level local partition transitioning to valid
            over an active remote child partition (validate_partition)
------------------------------------------------------------------------

How it happens:
1. Suppose top-level cgroup "A" is configured as a partition root
   (echo root > A/cpuset.cpus.partition) and given all online CPUs
   (e.g., echo 0-3 > A/cpuset.cpus.exclusive on a 4-CPU system), which
   transitions "A" to PRS_INVALID_ROOT ("root invalid (Parent unable to
   distribute cpu downstream)").
2. Because "A" is invalid (!is_partition_valid(A)), creating child "A/R"
   with cpuset.cpus = 1, cpuset.cpus.exclusive = 1, and
   cpuset.cpus.partition = root succeeds via remote_partition_enable(),
   making "A/R" a valid remote partition on CPU 1 (adding CPU 1 to
   subpartitions_cpus and removing CPU 1 from top_cpuset.effective_cpus).
3. Next, shrinking "A/cpuset.cpus.exclusive" from "0-3" to "1" calls
   update_exclusive_cpumask() -> partition_cpus_change(A, trialcs, &tmp)
   -> validate_partition(A, trialcs).
4. Unlike update_prstate() (which checks (parent == &top_cpuset) &&
   cpumask_intersects(..., subpartitions_cpus) and returns PERR_REMOTE),
   validate_partition() omits the subpartitions_cpus check and returns
   PERR_NONE (0).
5. partition_cpus_change() then calls update_parent_effective_cpumask(A,
   partcmd_update, trialcs->effective_xcpus, &tmp) to activate "A" on
   CPU 1, which is already removed from top_cpuset.effective_cpus by
   "A/R", triggering WARN_ON_ONCE(!cpumask_subset(tmp->new_cpus,
   parent->effective_cpus)) at kernel/cgroup/cpuset.c:1943 and
   WARN_ON_ONCE(old_prs < 0) in partition_xcpus_del() at
   kernel/cgroup/cpuset.c:1342.

Minimized reproducer (Scenario 2, on a 4-CPU VM):
  #!/bin/sh
  mkdir -p /tmp/cg2
  mount -t cgroup2 none /tmp/cg2
  echo "+cpuset" > /tmp/cg2/cgroup.subtree_control

  mkdir /tmp/cg2/A
  echo root > /tmp/cg2/A/cpuset.cpus.partition
  echo 0-3 > /tmp/cg2/A/cpuset.cpus.exclusive
  echo "+cpuset" > /tmp/cg2/A/cgroup.subtree_control

  mkdir /tmp/cg2/A/R
  echo 1 > /tmp/cg2/A/R/cpuset.cpus
  echo 1 > /tmp/cg2/A/R/cpuset.cpus.exclusive
  echo root > /tmp/cg2/A/R/cpuset.cpus.partition

  # Shrink A's exclusive_cpus to 1
  echo 1 > /tmp/cg2/A/cpuset.cpus.exclusive

dmesg output on 7.3.0-rc3:
  WARNING: kernel/cgroup/cpuset.c:1943 at update_parent_effective_cpumask+0x189b/0x1fd0
  Call Trace:
   cpuset_write_resmask+0xcf2/0x1690
  WARNING: kernel/cgroup/cpuset.c:1342 at partition_xcpus_del+0x15b/0x1b0
  Call Trace:
   update_parent_effective_cpumask+0x118c/0x1fd0
   cpuset_write_resmask+0xcf2/0x1690

Also, in v2 of the patch, I will refine the check in validate_partition()
to exclude cs's own existing effective_xcpus when cs is already a valid
local partition (!is_partition_valid(cs)), and split the two scenarios
into separate patches with these reproducers in the commit messages if
you prefer.

Best regards,
Hui Peng

[PATCH v2 1/2] cgroup/cpuset: return PERR_NOCPUS in remote_partition_enable() on subpartitions_cpus conflict

[Hui Peng]

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")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split into a 2-patch series (1/2 for remote_partition_enable(), 2/2 for
  validate_partition() and cpus_excl_conflict()).
- Return PERR_NOCPUS instead of PERR_INVCPUS when tmp->new_cpus intersects
  subpartitions_cpus to match remote_cpus_update(), as suggested by
  Guopeng Zhang.
- Add regression test to tools/testing/selftests/cgroup/test_cpuset_prs.sh
  and include the step-by-step reproducer in the commit description as
  requested by Ridong Chen.

 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 52eb66e3353e..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"
-- 
2.49.0

[PATCH v2 2/2] cgroup/cpuset: prevent local partition activation over remote partition and sibling xcpus conflict

[Hui Peng]

Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
partition over a remote one") added a check in update_prstate() to reject
creating a top-level local partition whose cs->exclusive_cpus intersects
subpartitions_cpus.

However, two paths still allow overlapping exclusive CPU ownership between
a top-level local partition and a remote partition or sibling:

1. If a top-level cgroup A1 has cpuset.cpus.exclusive set, is first set to
   "root" while cpuset.cpus is empty (becoming "root invalid"), has a
   child A1/A2 enabled as a remote partition on those exclusive CPUs, and
   then receives CPUs via A1/cpuset.cpus, update_cpumasks_hier() calls
   validate_partition() without going through update_prstate(). Because
   validate_partition() does not check trialcs->exclusive_cpus against
   subpartitions_cpus, A1 transitions from "root invalid" to a valid local
   partition over the existing remote partition A1/A2 and triggers:

     WARNING: kernel/cgroup/cpuset.c:1866 at update_parent_effective_cpumask+0x54a/0x8a0

2. In cpus_excl_conflict(), when a valid local partition A1 uses implicit
   exclusive CPUs (cpuset.cpus set without cpuset.cpus.exclusive, so
   sibling->exclusive_cpus is empty while sibling->effective_xcpus is
   populated), a sibling cgroup B1 can still set cpuset.cpus.exclusive on
   the same CPUs because cpus_excl_conflict() only checks
   sibling->exclusive_cpus.

Fix these issues by:
- Checking trialcs->exclusive_cpus against subpartitions_cpus in
  validate_partition() when !is_partition_valid(cs).
- Falling back to sibling->effective_xcpus in cpus_excl_conflict() when
  sibling->exclusive_cpus is empty.
- Adding regression test cases for both paths to
  tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Tested in QEMU on Linux 7.3.0-rc3 using
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split from the remote_partition_enable() fix into patch 2/2.
- Check trialcs->exclusive_cpus (rather than effective_xcpus) in
  validate_partition() when !is_partition_valid(cs) so sibling local
  partitions with overlapping cpuset.cpus continue to share remaining
  available CPUs as expected by test_cpuset_prs.sh.
- Add regression tests to tools/testing/selftests/cgroup/test_cpuset_prs.sh.

 kernel/cgroup/cpuset.c                        | 9 ++++++++-
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7381fa8502e7..f46a51f07938 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
 		return true;
 
 	/* Exclusive_cpus cannot intersect */
-	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
+	return cpumask_intersects(trial->exclusive_cpus,
+				  cpumask_empty(sibling->exclusive_cpus)
+				  ? sibling->effective_xcpus
+				  : sibling->exclusive_cpus);
 }
 
 static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
@@ -2415,6 +2418,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
 	if (cpumask_empty(trialcs->effective_xcpus))
 		return PERR_INVCPUS;
 
+	if ((parent == &top_cpuset) && !is_partition_valid(cs) &&
+	    cpumask_intersects(trialcs->exclusive_cpus, subpartitions_cpus))
+		return PERR_REMOTE;
+
 	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
 					  trialcs->effective_xcpus))
 		return PERR_HKEEPING;
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index b732078bf319..f62155cf6162 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -321,6 +321,8 @@ TEST_MATRIX=(
 	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
 	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
 	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
+	" X1-3:P1 CX2-3:P2  .      .    C1-3     .      .      .     0 A1:1|A2:2-3|XA2:2-3 \
+								       A1:P-1|A2:P2 2-3"
 
 	# Local partition invalidation tests
 	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
@@ -443,6 +445,7 @@ TEST_MATRIX=(
 
 	# Changes to cpuset.cpus.exclusive that violate exclusivity rule is rejected
 	"   C0-3     .      .    C4-5   X0-3     .      .     X3-5   1 A1:0-3|B1:4-5"
+	"  C0-3:P1   .      .    C4-5     .      .      .     X3-5   1 A1:0-3|B1:4-5 A1:P1|B1:P0"
 
 	# cpuset.cpus.exclusive cannot be set to a superset of sibling's cpuset.cpus
 	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"
-- 
2.49.0

Re: [PATCH v2 2/2] cgroup/cpuset: prevent local partition activation over remote partition and sibling xcpus conflict

[Guopeng Zhang]

在 2026/9/20 16:23, Hui Peng 写道:
> Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
> partition over a remote one") added a check in update_prstate() to reject
> creating a top-level local partition whose cs->exclusive_cpus intersects
> subpartitions_cpus.
> 
> However, two paths still allow overlapping exclusive CPU ownership between
> a top-level local partition and a remote partition or sibling:
> 
> 1. If a top-level cgroup A1 has cpuset.cpus.exclusive set, is first set to
>    "root" while cpuset.cpus is empty (becoming "root invalid"), has a
>    child A1/A2 enabled as a remote partition on those exclusive CPUs, and
>    then receives CPUs via A1/cpuset.cpus, update_cpumasks_hier() calls
>    validate_partition() without going through update_prstate(). Because
>    validate_partition() does not check trialcs->exclusive_cpus against
>    subpartitions_cpus, A1 transitions from "root invalid" to a valid local
>    partition over the existing remote partition A1/A2 and triggers:
> 
>      WARNING: kernel/cgroup/cpuset.c:1866 at update_parent_effective_cpumask+0x54a/0x8a0
> 
> 2. In cpus_excl_conflict(), when a valid local partition A1 uses implicit
>    exclusive CPUs (cpuset.cpus set without cpuset.cpus.exclusive, so
>    sibling->exclusive_cpus is empty while sibling->effective_xcpus is
>    populated), a sibling cgroup B1 can still set cpuset.cpus.exclusive on
>    the same CPUs because cpus_excl_conflict() only checks
>    sibling->exclusive_cpus.
> 
> Fix these issues by:
> - Checking trialcs->exclusive_cpus against subpartitions_cpus in
>   validate_partition() when !is_partition_valid(cs).
> - Falling back to sibling->effective_xcpus in cpus_excl_conflict() when
>   sibling->exclusive_cpus is empty.
> - Adding regression test cases for both paths to
>   tools/testing/selftests/cgroup/test_cpuset_prs.sh.
> 
> Tested in QEMU on Linux 7.3.0-rc3 using
> tools/testing/selftests/cgroup/test_cpuset_prs.sh.
> 
> Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")

I think this patch is still missing a Fixes: tag for the
cpus_excl_conflict() change.

The validate_partition() change can be covered by 6da580ec656a.

However, the sibling CPU ownership issue fixed in cpus_excl_conflict()
was introduced by the following commit:
Fixes: 2a3602030d80 ("cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict")

Thanks,
Guopeng

> Cc: stable@vger.kernel.org
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v2:
> - Split from the remote_partition_enable() fix into patch 2/2.
> - Check trialcs->exclusive_cpus (rather than effective_xcpus) in
>   validate_partition() when !is_partition_valid(cs) so sibling local
>   partitions with overlapping cpuset.cpus continue to share remaining
>   available CPUs as expected by test_cpuset_prs.sh.
> - Add regression tests to tools/testing/selftests/cgroup/test_cpuset_prs.sh.
> 
>  kernel/cgroup/cpuset.c                        | 9 ++++++++-
>  tools/testing/selftests/cgroup/test_cpuset_prs.sh | 3 +++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 7381fa8502e7..f46a51f07938 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
>  		return true;
>  
>  	/* Exclusive_cpus cannot intersect */
> -	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
> +	return cpumask_intersects(trial->exclusive_cpus,
> +				  cpumask_empty(sibling->exclusive_cpus)
> +				  ? sibling->effective_xcpus
> +				  : sibling->exclusive_cpus);
>  }
>  
>  static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
> @@ -2415,6 +2418,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
>  	if (cpumask_empty(trialcs->effective_xcpus))
>  		return PERR_INVCPUS;
>  
> +	if ((parent == &top_cpuset) && !is_partition_valid(cs) &&
> +	    cpumask_intersects(trialcs->exclusive_cpus, subpartitions_cpus))
> +		return PERR_REMOTE;
> +
>  	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
>  					  trialcs->effective_xcpus))
>  		return PERR_HKEEPING;
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index b732078bf319..f62155cf6162 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -321,6 +321,8 @@ TEST_MATRIX=(
>  	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
>  	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
>  	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
> +	" X1-3:P1 CX2-3:P2  .      .    C1-3     .      .      .     0 A1:1|A2:2-3|XA2:2-3 \
> +								       A1:P-1|A2:P2 2-3"
>  
>  	# Local partition invalidation tests
>  	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
> @@ -443,6 +445,7 @@ TEST_MATRIX=(
>  
>  	# Changes to cpuset.cpus.exclusive that violate exclusivity rule is rejected
>  	"   C0-3     .      .    C4-5   X0-3     .      .     X3-5   1 A1:0-3|B1:4-5"
> +	"  C0-3:P1   .      .    C4-5     .      .      .     X3-5   1 A1:0-3|B1:4-5 A1:P1|B1:P0"
>  
>  	# cpuset.cpus.exclusive cannot be set to a superset of sibling's cpuset.cpus
>  	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"

Re: [PATCH v2 2/2] cgroup/cpuset: prevent local partition activation over remote partition and sibling xcpus conflict

[Hui Peng]

On Sun, Sep 20, 2026 at 04:57:39PM +0800, Guopeng Zhang wrote:
> I think this patch is still missing a Fixes: tag for the
> cpus_excl_conflict() change.
>
> The validate_partition() change can be covered by 6da580ec656a.
>
> However, the sibling CPU ownership issue fixed in cpus_excl_conflict()
> was introduced by the following commit:
> Fixes: 2a3602030d80 ("cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict")

Thanks for pointing this out, Guopeng!

In v3, I have split the validate_partition() change (with
Fixes: 6da580ec656a) and the cpus_excl_conflict() change (with
Fixes: 2a3602030d80) into separate patches ([PATCH v3 2/3] and
[PATCH v3 3/3]) so that each patch fixes a single logical issue with its
corresponding Fixes: tag.

Best regards,
Hui

[PATCH v3 1/3] cgroup/cpuset: return PERR_NOCPUS in remote_partition_enable()

[Hui Peng]

Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
partition over a remote one") updated update_prstate() to prevent creating
a top-level local partition whose exclusive CPUs conflict with an existing
remote partition.

However, if a top-level local partition A1 is created on CPUs 1-3, has a
non-partition member child A1/A2 with cpuset.cpus.exclusive = 1-3, and a
grandchild A1/A2/A3 is then enabled as a partition root on 1-3,
update_prstate() sees !is_partition_valid(parent) (since A1/A2 is a member)
and calls remote_partition_enable(). Because remote_partition_enable() does
not check whether tmp->new_cpus intersects subpartitions_cpus, A1/A2/A3 is
enabled as a remote partition on CPUs already owned by A1, triggering:

  WARNING: kernel/cgroup/cpuset.c:1594 at update_prstate+0xbef/0xd70

Return PERR_NOCPUS in remote_partition_enable() when tmp->new_cpus
intersects subpartitions_cpus, matching the error code used by
remote_cpus_update() for subpartitions_cpus conflicts, and add a
corresponding regression test case to
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Tested in QEMU on Linux 7.3.0-rc3 using
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")
Suggested-by: Guopeng Zhang <guopeng.zhang@linux.dev>
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v3:
- Split the series into 3 patches (one per logical change and Fixes: tag).

 kernel/cgroup/cpuset.c                        | 4 +++-
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c8ab89cfbe3d..7381fa8502e7 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1678,8 +1678,10 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	 * Callers must have validated that effective_xcpus is not empty.
 	 */
 	spin_lock_irq(&callback_lock);
-	if (cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus)) {
+	if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus) ||
+	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus)) {
 		spin_unlock_irq(&callback_lock);
+		if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus))
+			return PERR_NOCPUS;
 		return PERR_HKEEPING;
 	}
 
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index a1f42ef64bc1..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"
-- 
2.49.0

[PATCH v3 2/3] cgroup/cpuset: prevent activating local partition over remote one

[Hui Peng]

Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
partition over a remote one") added a check in update_prstate() to reject
creating a top-level local partition whose cs->exclusive_cpus intersects
subpartitions_cpus.

However, if a top-level cgroup A1 is first set to "root" with all CPUs
(CX0-7:P1, becoming "root invalid" because no CPUs would remain in
top_cpuset), has a child A1/A2 enabled as a remote partition on a subset
of those exclusive CPUs (CX2-3:P2), and A1 is then updated to a valid CPU
subset (CX1-3), update_exclusive_cpumask() / update_cpumask() calls
validate_partition() without going through update_prstate(). Because
validate_partition() does not check trialcs->exclusive_cpus against
subpartitions_cpus, A1 transitions from "root invalid" to a valid local
partition over the existing remote partition A1/A2 and triggers:

  WARNING: kernel/cgroup/cpuset.c:1943 at update_parent_effective_cpumask+0x189b/0x1fd0

Check trialcs->exclusive_cpus against subpartitions_cpus in
validate_partition() when !is_partition_valid(cs), 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
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v3:
- Split the validate_partition() fix (Fixes: 6da580ec656a) and the
  cpus_excl_conflict() fix (Fixes: 2a3602030d80) into separate patches.

 kernel/cgroup/cpuset.c                        | 4 ++++
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7381fa8502e7..7c5b4a8f1f3a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2415,6 +2415,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
 	if (cpumask_empty(trialcs->effective_xcpus))
 		return PERR_INVCPUS;
 
+	if ((parent == &top_cpuset) && !is_partition_valid(cs) &&
+	    cpumask_intersects(trialcs->exclusive_cpus, subpartitions_cpus))
+		return PERR_REMOTE;
+
 	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
 					  trialcs->effective_xcpus))
 		return PERR_HKEEPING;
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index b732078bf319..f4d1822d4e21 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -321,6 +321,8 @@ TEST_MATRIX=(
 	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
 	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
 	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
+	" CX0-7:P1 CX2-3:P2 .      .    CX1-3    .      .      .     0 A1:1|A2:2-3|XA2:2-3 \
+								       A1:P-1|A2:P2 2-3"
 
 	# Local partition invalidation tests
 	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
-- 
2.49.0

[PATCH v3 3/3] cgroup/cpuset: check sibling effective_xcpus in cpus_excl_conflict()

[Hui Peng]

In cpus_excl_conflict(), when a valid local partition A1 uses implicit
exclusive CPUs (cpuset.cpus set without cpuset.cpus.exclusive, so
sibling->exclusive_cpus is empty while sibling->effective_xcpus is
populated), a sibling cgroup B1 can still set cpuset.cpus.exclusive on
the same CPUs because cpus_excl_conflict() only checks
sibling->exclusive_cpus.

Fall back to sibling->effective_xcpus in cpus_excl_conflict() when
sibling->exclusive_cpus is empty, 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
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 2a3602030d80 ("cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v3:
- Split out from the validate_partition() fix into patch 3/3 and add the
  Fixes tag for commit 2a3602030d80, as pointed out by Guopeng Zhang.

 kernel/cgroup/cpuset.c                        | 5 ++++-
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7c5b4a8f1f3a..f46a51f07938 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
 		return true;
 
 	/* Exclusive_cpus cannot intersect */
-	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
+	return cpumask_intersects(trial->exclusive_cpus,
+				  cpumask_empty(sibling->exclusive_cpus)
+				  ? sibling->effective_xcpus
+				  : sibling->exclusive_cpus);
 }
 
 static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index f4d1822d4e21..f62155cf6162 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -445,6 +445,7 @@ TEST_MATRIX=(
 
 	# Changes to cpuset.cpus.exclusive that violate exclusivity rule is rejected
 	"   C0-3     .      .    C4-5   X0-3     .      .     X3-5   1 A1:0-3|B1:4-5"
+	"  C0-3:P1   .      .    C4-5     .      .      .     X3-5   1 A1:0-3|B1:4-5 A1:P1|B1:P0"
 
 	# cpuset.cpus.exclusive cannot be set to a superset of sibling's cpuset.cpus
 	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"
-- 
2.49.0

Re: [PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[Guopeng Zhang]

在 2026/9/20 06:17, Hui Peng 写道:

Hello Hui,

> Fix two partition validation bugs in kernel/cgroup/cpuset.c:
> 
> 1. In remote_partition_enable(), check whether the requested effective
>    xcpus intersect parent->subpartitions_cpus so a remote partition
>    cannot claim CPUs already delegated to a local child partition.
> 2. In validate_partition(), verify that enabling a local partition does
>    not overlap CPUs already allocated to an active remote partition.
> 
> Fixes: aa7d3a56a20f ("cpuset: fix warning when disabling remote partition")
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2538faac9aba..13fad096c494 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
>  		return true;
>  
>  	/* Exclusive_cpus cannot intersect */
> -	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
> +	return cpumask_intersects(trial->exclusive_cpus,
> +				  cpumask_empty(sibling->exclusive_cpus)
> +				  ? sibling->effective_xcpus
> +				  : sibling->exclusive_cpus);
>  }
>  
>  static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
> @@ -1591,8 +1594,8 @@ 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_intersects(tmp->new_cpus, subpartitions_cpus) ||
>  	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
>  		return PERR_INVCPUS;

I have been going through this part of the code repeatedly recently, and I
would like to share one thought.

Should this return PERR_NOCPUS instead?

+	if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus))
+		return PERR_NOCPUS;

In the first reproducer you described in your reply to Ridong:

Minimized reproducer (Scenario 1):
#!/bin/sh
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

CPU 1 is active, and the requested CPU mask itself is valid. The failure
happens because CPU 1 has already been allocated to partition A, so the
top cpuset can no longer distribute it to the remote partition D.

remote_cpus_update() uses PERR_NOCPUS for the same kind of
subpartitions_cpus conflict:

else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
         cpumask_subset(top_cpuset.effective_cpus,
                        tmp->addmask))
        WRITE_ONCE(cs->prs_err, PERR_NOCPUS);

Would using PERR_NOCPUS here be more appropriate? This would keep the
initial remote-partition enable path consistent with the CPU update path
for an existing remote partition, and it also seems to describe the
failure more accurately: the CPU itself is not invalid, but has already
been allocated to another partition, so the parent can no longer
distribute it downstream.

Thanks,
Guopeng

>  	if (((new_prs == PRS_ISOLATED) &&
> @@ -2411,6 +2414,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
>  	if (cpumask_empty(trialcs->effective_xcpus))
>  		return PERR_INVCPUS;
>  
> +	if ((parent == &top_cpuset) &&
> +	    cpumask_intersects(trialcs->effective_xcpus, subpartitions_cpus))
> +		return PERR_REMOTE;
> +
>  	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
>  					  trialcs->effective_xcpus))
>  		return PERR_HKEEPING;
> @@ -2970,7 +2977,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  		 * local or remote partition.
>  		 */
>  		if ((parent == &top_cpuset) &&
> -		    cpumask_intersects(cs->exclusive_cpus, subpartitions_cpus)) {
> +		    cpumask_intersects(user_xcpus(cs), subpartitions_cpus)) {
>  			err = PERR_REMOTE;
>  			goto out;
>  		}

Re: [PATCH] cgroup/cpuset: prevent overlapping local and remote partition creation

[Hui Peng]

Hi Guopeng,

On Sun, Sep 20, 2026 at 02:30:23PM +0800, Guopeng Zhang wrote:
> Should this return PERR_NOCPUS instead?
>
> +	if (cpumask_intersects(tmp->new_cpus, subpartitions_cpus))
> +		return PERR_NOCPUS;
>
...
> Would using PERR_NOCPUS here be more appropriate? This would keep the
> initial remote-partition enable path consistent with the CPU update path
> for an existing remote partition, and it also seems to describe the
> failure more accurately: the CPU itself is not invalid, but has already
> been allocated to another partition, so the parent can no longer
> distribute it downstream.

Agreed, returning PERR_NOCPUS when tmp->new_cpus intersects
subpartitions_cpus is much more accurate and keeps remote_partition_enable()
consistent with remote_cpus_update(). I have updated this in v2.

Thanks,
Hui