+ cgroup-cpuset-rebind-mm-mempolicy-to-effective_mems-not-mems_allowed.patch added to mm-hotfixes-unstable branch

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

The patch titled
     Subject: cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     cgroup-cpuset-rebind-mm-mempolicy-to-effective_mems-not-mems_allowed.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/cgroup-cpuset-rebind-mm-mempolicy-to-effective_mems-not-mems_allowed.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Farhad Alemi <farhad.alemi@berkeley.edu>
Subject: cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed
Date: Sun, 14 Jun 2026 06:25:55 -0700

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.

Link: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/
Link: https://lore.kernel.org/all/CA+0ovCiEz6SP_sn3kN4Tb+_oC=eHMXy_Ffj=usV3wREdQrUtww@mail.gmail.com/
Link: https://lore.kernel.org/CA+0ovCgfHJHv5d1mzapWWvF-LhjppzDX8NPPLvCPZxPKg8RiYw@mail.gmail.com
Fixes: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}")
Signed-off-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Suggested-by: Gregory Price <gourry@gourry.net>
Suggested-by: Waiman Long <longman@redhat.com>
Closes: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/
Acked-by: Waiman Long <longman@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/cgroup/cpuset.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/cgroup/cpuset.c~cgroup-cpuset-rebind-mm-mempolicy-to-effective_mems-not-mems_allowed
+++ a/kernel/cgroup/cpuset.c
@@ -2653,7 +2653,7 @@ void cpuset_update_tasks_nodemask(struct
 
 		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
_

Patches currently in -mm which might be from farhad.alemi@berkeley.edu are

cgroup-cpuset-rebind-mm-mempolicy-to-effective_mems-not-mems_allowed.patch