Re: [PATCH v7 2/9] cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()

[Gregory Price <gourry@gourry.net>]

On Sat, Jun 20, 2026 at 11:28:09PM -0400, 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 mostly be useful for v1
> where mems_allowed is the same as effective_mems. With v2, the memory
> nodes in effective_mems should be a subset of node_states[N_MEMORY]
> except 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
> the effective_mems's as cpuset_handle_hotplug() has not been called
> from cpuset_track_online_nodes().
> 
> 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 most of the time. For
> v1, mpol_rebind_mm() uses mems_allowed which may differ from what
> guarantee_online_mems() returns, but it conforms to what the cpuset v1
> documentation says with respect to setting memory policy.
> 
> Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)
> 

Reviewed-by: Gregory Price <gourry@gourry.net>