Re: [PATCH-next v5 1/6] cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()

[Ridong Chen <ridong.chen@linux.dev>]

On 2026/6/2 10:31, Waiman Long wrote:
> Whenever memory node mask is changed, there are 4 places where the node
> mask has to be updated or used.
>  1) task's node mask via cpuset_change_task_nodemask()
>  2) memory policy binding via mpol_rebind_mm()
>  3) if memory migration is enabled, migrate from old_mems_allowed to
>     the new node mask via cpuset_migrate_mm().
>  4) setting old_mems_allowed
> 
> These memory actions are done in cpuset_update_tasks_nodemask() and
> cpuset_attach(). However there are inconsistencies in what node masks
> are being used in these 2 functions.
> 
> In cpuset_update_tasks_nodemask(),
>  - cpuset_change_task_nodemask(): guarantee_online_mems()
>  - mpol_rebind_mm(): mems_allowed
>  - cpuset_migrate_mm(): guarantee_online_mems()
>  - old_mems_allowed: guarantee_online_mems()
> 
> In cpuset_attach(),
>  - cpuset_change_task_nodemask(): guarantee_online_mems()
>  - mpol_rebind_mm(): effective_mems
>  - cpuset_migrate_mm(): effective_mems
>  - old_mems_allowed: effective_mems
> 
> These inconsistencies dates back to quite a long time ago and it is
> hard to say what should be the correct values.
> 
> The guarantee_online_mems() function returns a node mask from current or
> an ancestor cpuset that is a subset of node_states[N_MEMORY]. Nodes in
> node_states[N_MEMORY] are all online, i.e. in node_states[N_ONLINE].
> However, node in node_states[N_ONLINE] may not have memory. So
> node_states[N_MEMORY] should be a subset of node_states[N_ONLINE].
> 
> The guarantee_online_mems() function should only be useful for v1 where
> mems_allowed is the same as effective_mems. With v2, the memory nodes
> in effective_mems should always be a subset of node_states[N_MEMORY].
> The only time that may not be true is when a memory hot-unplug operation
> is in progress and a memory node is removed from node_states[N_MEMORY]
> but not yet reflected in effective_mems as cpuset_handle_hotplug()
> has not yet been called from cpuset_track_online_nodes(). When
> cpuset_handle_hotplug() is called later, the memory node setting
> of the relevant cpusets and tasks will be updated. So replacing the
> guarantee_online_mems() call by just using cs->effective_mems should
> be fine.
> 

I noticed this pattern in several places:

```
if (cpuset_v2())
	newmems = cs->effective_mems;
else
	guarantee_online_mems(cs, &newmems);
```

Would it be simpler to move the v2 logic into guarantee_online_mems?

```
static void guarantee_online_mems(struct cpuset *cs, nodemask_t *pmask)
{
	if (cpuset_v2()) {
		*pmask = cs->effective_mems;
		return;
	}
	while (!nodes_and(*pmask, cs->effective_mems, node_states[N_MEMORY]))
		cs = parent_cs(cs);
}
```

> Let use the following setup for both of them and make them consistent.
>  - cpuset_change_task_nodemask(): guarantee_online_mems()
>  - mpol_rebind_mm(): effective_mems
>  - cpuset_migrate_mm(): guarantee_online_mems()
>  - old_mems_allowed: guarantee_online_mems()
> 
> So for v2, it is effectively all effective_mems. For v1, mpol_rebind_mm()
> uses cpus_allowed which may differ from what guarantee_online_mems()
	^
mems_allowed?
> returns.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 32 +++++++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6bdb68689c24..987456b6d879 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2616,6 +2616,13 @@ static void *cpuset_being_rebound;
>   * Iterate through each task of @cs updating its mems_allowed to the
>   * effective cpuset's.  As this function is called with cpuset_mutex held,
>   * cpuset membership stays stable.
> + *
> + * - cpuset_change_task_nodemask(): guarantee_online_mems()
> + * - mpol_rebind_mm(): effective_mems
> + * - cpuset_migrate_mm(): guarantee_online_mems()
> + * - old_mems_allowed: guarantee_online_mems()
> + *
> + * For v2, guarantee_online_mems() should just return effective_mems.

I agree, but the implementation is not as simple as what I mentioned above.

>   */
>  void cpuset_update_tasks_nodemask(struct cpuset *cs)
>  {
> @@ -2625,7 +2632,10 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
>  
>  	cpuset_being_rebound = cs;		/* causes mpol_dup() rebind */
>  
> -	guarantee_online_mems(cs, &newmems);
> +	if (cpuset_v2())
> +		newmems = cs->effective_mems;
> +	else
> +		guarantee_online_mems(cs, &newmems);
>  
>  	/*
>  	 * The mpol_rebind_mm() call takes mmap_lock, which we couldn't
> @@ -2650,7 +2660,7 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
>  
>  		migrate = is_memory_migrate(cs);
>  
> -		mpol_rebind_mm(mm, &cs->mems_allowed);
> +		mpol_rebind_mm(mm, &cs->effective_mems);
>  		if (migrate)
>  			cpuset_migrate_mm(mm, &cs->old_mems_allowed, &newmems);
>  		else
> @@ -3148,17 +3158,18 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>  
>  	/*
>  	 * In the default hierarchy, enabling cpuset in the child cgroups
> -	 * will trigger a number of cpuset_attach() calls with no change
> -	 * in effective cpus and mems. In that case, we can optimize out
> -	 * by skipping the task iteration and update.
> +	 * will trigger a cpuset_attach() call with no change in effective cpus
> +	 * and mems. In that case, we can optimize out by skipping the task
> +	 * iteration and update.
>  	 */
> -	if (cpuset_v2() && !cpus_updated && !mems_updated) {
> +	if (cpuset_v2()) {
>  		cpuset_attach_nodemask_to = cs->effective_mems;
> -		goto out;
> +		if (!cpus_updated && !mems_updated)
> +			goto out;
> +	} else {
> +		guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
>  	}
>  
> -	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
> -
>  	cgroup_taskset_for_each(task, css, tset)
>  		cpuset_attach_task(cs, task);
>  
> @@ -3168,7 +3179,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>  	 * if there is no change in effective_mems and CS_MEMORY_MIGRATE is
>  	 * not set.
>  	 */
> -	cpuset_attach_nodemask_to = cs->effective_mems;
>  	if (!is_memory_migrate(cs) && !mems_updated)
>  		goto out;
>  
> @@ -3176,7 +3186,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>  		struct mm_struct *mm = get_task_mm(leader);
>  
>  		if (mm) {
> -			mpol_rebind_mm(mm, &cpuset_attach_nodemask_to);
> +			mpol_rebind_mm(mm, &cs->effective_mems);
>  
>  			/*
>  			 * old_mems_allowed is the same with mems_allowed

-- 
Best regards,
Ridong