[PATCH] cgroup/cpuset: Invalidate remote partition on housekeeping conflict

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

Widening an ancestor's exclusive CPU mask can add a boot-isolated CPU
to a valid remote partition root without touching the partition's own
control files. The partition then load balances that CPU, silently
defeating isolcpus=domain for it.

This can be reproduced on a 32-CPU system booted with
isolcpus=domain,4:

    cd /sys/fs/cgroup
    echo +cpuset > cgroup.subtree_control
    mkdir -p A/B
    echo +cpuset > A/cgroup.subtree_control
    echo 2-4 > A/cpuset.cpus
    echo 2-3 > A/cpuset.cpus.exclusive
    echo 2-4 > A/B/cpuset.cpus
    echo 2-4 > A/B/cpuset.cpus.exclusive
    echo root > A/B/cpuset.cpus.partition
    cat A/B/cpuset.cpus.effective               # 2-3
    echo 2-4 > A/cpuset.cpus.exclusive
    cat A/B/cpuset.cpus.partition               # root
    cat A/B/cpuset.cpus.effective               # 2-4

The last write returns 0 and leaves the hierarchy in this state:

    root (cpuset.cpus.effective=0-1,5-31)
    |
    \-- A (member):           cpuset.cpus=2-4
        |                     cpuset.cpus.exclusive=2-4
        \-- B (root, remote): cpuset.cpus=2-4
                              cpuset.cpus.effective=2-4

B is a remote partition: it takes its CPUs directly from the root
cpuset, and A only passes its exclusive list down. Before the last
write, that list is 2-3, so B holds 2-3 and CPU 4 stays in the
root cpuset as a boot-isolated CPU. The write widens A's exclusive
list to 2-4, which additionally grants CPU 4 to B. Nothing rejects
the grant: B remains a valid root partition, and CPU 4 is still
listed in cpuset.cpus.isolated while sitting in a load-balanced
partition.

remote_partition_enable() and validate_partition() already call
prstate_housekeeping_conflict() before granting CPUs. The ancestor
update path in remote_cpus_update() does not. Add the missing check so
that the existing prs_err path invalidates the remote partition instead
of adding the boot-isolated CPU.

Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/cpuset.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 753aa65afcd7..362e5b5dccaa 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1699,6 +1699,8 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 		else if ((prs == PRS_ISOLATED) &&
 			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
 			WRITE_ONCE(cs->prs_err, PERR_HKEEPING);
+		else if (prstate_housekeeping_conflict(prs, tmp->addmask))
+			WRITE_ONCE(cs->prs_err, PERR_HKEEPING);
 		if (cs->prs_err)
 			goto invalidate;
 	}
-- 
2.43.0