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SubjectRe: [PATCH v9 1/7] cgroup/cpuset: Don't let child cpusets restrict parent in default hierarchy
On Mon, Dec 13, 2021 at 10:41:23AM -1000, Tejun Heo <tj@kernel.org> wrote:
> > To address this issue, the check is now removed for the default hierarchy
> > to free parent cpusets from being restricted by child cpusets. The
> > check will still apply for legacy hierarchy.

I'm trying to find whether something in update_cpumasks_hier() ensures
the constraint is checkd on the legacy hierarchy but it seems to me this
baby was thrown out with the bathwater. How is the legacy check still
applied?

> Applied to cgroup/for-5.17.

It comes out a bit more complex if I want to achieve both variants in
the below followup:

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0dd7d853ed17..8b6e06f504f6 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -590,6 +590,35 @@ static inline void free_cpuset(struct cpuset *cs)
kfree(cs);
}

+/*
+ * validate_change_legacy() - Validate conditions specific to legacy (v1)
+ * behavior.
+ */
+static int validate_change_legacy(struct cpuset *cur, struct cpuset *trial)
+{
+ struct cgroup_subsys_state *css;
+ struct cpuset *c, *par;
+ int ret;
+
+ WARN_ON_ONCE(!rcu_read_lock_held());
+
+ /* Each of our child cpusets must be a subset of us */
+ ret = -EBUSY;
+ cpuset_for_each_child(c, css, cur)
+ if (!is_cpuset_subset(c, trial))
+ goto out;
+
+ /* On legacy hierarchy, we must be a subset of our parent cpuset. */
+ ret = -EACCES;
+ par = parent_cs(cur);
+ if (par && !is_cpuset_subset(trial, par))
+ goto out;
+
+ ret = 0;
+out:
+ return ret;
+}
+
/*
* validate_change() - Used to validate that any proposed cpuset change
* follows the structural rules for cpusets.
@@ -614,20 +643,21 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
{
struct cgroup_subsys_state *css;
struct cpuset *c, *par;
- int ret;
-
- /* The checks don't apply to root cpuset */
- if (cur == &top_cpuset)
- return 0;
+ int ret = 0;

rcu_read_lock();
- par = parent_cs(cur);

- /* On legacy hierarchy, we must be a subset of our parent cpuset. */
- ret = -EACCES;
- if (!is_in_v2_mode() && !is_cpuset_subset(trial, par))
+ ret = validate_change_legacy(cur, trial);
+ if (ret)
+ goto out;
+
+ /* Remaining checks don't apply to root cpuset */
+ ret = 0;
+ if (cur == &top_cpuset)
goto out;

+ par = parent_cs(cur);
+
/*
* If either I or some sibling (!= me) is exclusive, we can't
* overlap
@@ -1175,9 +1205,7 @@ enum subparts_cmd {
*
* Because of the implicit cpu exclusive nature of a partition root,
* cpumask changes that violates the cpu exclusivity rule will not be
- * permitted when checked by validate_change(). The validate_change()
- * function will also prevent any changes to the cpu list if it is not
- * a superset of children's cpu lists.
+ * permitted when checked by validate_change().
*/
static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
struct cpumask *newmask,
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