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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 9 Jun 2026 19:57:41 -0400 Farhad Alemi <farhad.alemi@berkeley.edu> wrote:

> cpuset_update_tasks_nodemask() rebinds a task's own mempolicy to the
> cpuset's effective, online mems (newmems, from guarantee_online_mems()),
> but rebinds that task's VMA mempolicies to the *configured* mask instead:

Hard to understand.  Was "rebinds" supposed to be "is supposed to
rebind"?

> 	cpuset_change_task_nodemask(task, &newmems);
> 	...
> 	mpol_rebind_mm(mm, &cs->mems_allowed);
> 
> On the default (v2) hierarchy a cpuset that has never had cpuset.mems
> written keeps mems_allowed empty while effective_mems is inherited
> non-empty from the parent, and tasks may be attached to it (the
> empty-mems attach check is v1-only).  A subsequent rebind -- e.g. from a
> CPU hotplug event walking the cpuset -- then calls mpol_rebind_mm() with
> an empty mask.  For a VMA policy created with MPOL_F_RELATIVE_NODES this
> reaches mpol_relative_nodemask() ->
> nodes_fold(..., nodes_weight(cs->mems_allowed) == 0) -> bitmap_fold(),
> whose set_bit(oldbit % sz, dst) divides by zero:
> 
>   Oops: divide error: 0000 [#1] SMP KASAN NOPTI
>   RIP: 0010:bitmap_fold+0x5e/0xb0
>    mpol_rebind_nodemask
>    mpol_rebind_mm
>    cpuset_update_tasks_nodemask
>    cpuset_handle_hotplug
>    sched_cpu_deactivate
>    cpuhp_thread_fun
> 
> cs->mems_allowed is the only nodemask in this function that is not the
> effective set: the task-policy rebind, the page-migration target and
> cs->old_mems_allowed all use newmems.  The sibling cpuset_attach() path
> already rebinds VMA policies against the effective mems
> (cpuset_attach_nodemask_to = cs->effective_mems) and explicitly notes
> that mems_allowed can be empty under hotplug.  Rebind the VMA policies to
> newmems too: it is guaranteed non-empty by guarantee_online_mems(), which
> fixes the divide-by-zero, and it makes the VMA policies consistent with
> the task policy and with the nodes the task is actually allowed to use.

How is this bug triggered?