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

[Waiman Long]

On 10/15/26 11:30 PM, Guopeng Zhang wrote:
> 在 2026/10/14 22:05, Waiman Long 写道:
> > 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@xxxxxxxxxx>
> > ---
> >   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;
> > +			}
> I tested the following case. Writing root initially leaves C invalid, but
> updating its cpuset.cpus makes it valid again.
>
> CPUs 0-15 are online, with nohz_full=2-15 and no isolcpus:
>
>      # cd /sys/fs/cgroup
>      # echo +cpuset > cgroup.subtree_control
>      # mkdir A
>      # echo 1-2 > A/cpuset.cpus
>      # echo isolated > A/cpuset.cpus.partition
>      # echo +cpuset > A/cgroup.subtree_control
>      # mkdir A/C
>      # echo 1 > A/C/cpuset.cpus
>      # echo root > A/C/cpuset.cpus.partition
>      # cat A/C/cpuset.cpus.partition
>      root invalid (Root partition is not allowed with isolated parent)
>      # echo 1-2 > A/C/cpuset.cpus
>      # cat A/C/cpuset.cpus.partition
>      root
>
> C becomes a valid root partition.

Thank for finding this bug. Yes, the patch is missing check in some 
places. Will fix in the next version.

Cheers,
Longman