Re: [PATCH v7 1/9] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

[Gregory Price]

On Sat, Jun 20, 2026 at 11:28:08PM -0400, Waiman Long wrote:
> From: Farhad Alemi <farhad.alemi@xxxxxxxxxxxx>
> 
> Creating a child cpuset where cpuset.mems is never set leads to a div/0
> when a VMA mempolicy with MPOL_F_RELATIVE_NODES rebinds in response to a
> CPU hotplug event.
> 
> Reproduction steps:
>  1) Create a cgroup w/ cpuset controls (do not set cpuset.mems)
>  2) Move the task into the child cpuset
>  3) Create a VMA mempolicy for that task with MPOL_F_RELATIVE_NODES
>  4) unplug and hotplug a cpu
>       echo 0 > /sys/devices/system/cpu/cpu1/online
>       echo 1 > /sys/devices/system/cpu/cpu1/online
>  5) mempolicy rebind does a div/0 in mpol_relative_nodemask on the
>     call to __nodes_fold()
> 
> The cpuset code passes (cs->mems_allowed) which is not guaranteed to have
> nodes to the rebind routine.  Use cs->effective_mems instead, which is
> guaranteed to have a non-empty nodemask.
> 
> Closes: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@xxxxxxxxxxxxxx/
> Link: https://lore.kernel.org/all/CA+0ovCiEz6SP_sn3kN4Tb+_oC=eHMXy_Ffj=usV3wREdQrUtww@xxxxxxxxxxxxxx/
> Fixes: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}")
> Suggested-by: Gregory Price <gourry@xxxxxxxxxx>
> Suggested-by: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Farhad Alemi <farhad.alemi@xxxxxxxxxxxx>
> Acked-by: Waiman Long <longman@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>

> ---
>  kernel/cgroup/cpuset.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 591e3aa487fc..b21c31650583 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2653,7 +2653,7 @@ 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
> -- 
> 2.54.0
>