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cgroup/cpuset: check effective mems for a movable-only configuration

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check_insane_mems_config() warns and enables cpusets_insane_config_key
when a cpuset's tasks can allocate only from movable memory.

In cgroup v2 the request may name a memoryless node, which has no zones
and cpusets reports it as a movable-only nodemask.

For example, with node 2 a CPU-only node:

  # echo +cpuset > /sys/fs/cgroup/cgroup.subtree_control
  # mkdir /sys/fs/cgroup/test
  # echo 2 > /sys/fs/cgroup/test/cpuset.mems
  # cat /sys/fs/cgroup/test/cpuset.mems.effective
    0-1
  # dmesg
    Unsupported (movable nodes only) cpuset configuration detected (nmask=2)!

This is a bit confusing, especially consider mems.effective ends up with
a valid nodemask by-design (cpuset.mems is treated as-if it were empty).

Check each new effective mask in update_nodemasks_hier() instead, as the
hotplug path already does. The allocator's bail-out tests the task's
mems_allowed, which comes from the effective mask.

A request naming a movable-only not present in the parent's nodemask
no longer warns (since, again, the cpuset.mems gets overridden and
treated as empty). However, it will warn if the parent's mems include
the node later.

For Example, with node 3 a movable-only node:

  # echo 0 > /sys/fs/cgroup/test/cpuset.mems
  # echo 3 > /sys/fs/cgroup/test/child/cpuset.mems
  # cat /sys/fs/cgroup/test/child/cpuset.mems.effective
    0
  # dmesg
    { empty }
  # echo 0,3 > /sys/fs/cgroup/test/cpuset.mems
  # cat /sys/fs/cgroup/test/child/cpuset.mems.effective
    3
  # dmesg
    Unsupported (movable nodes only) cpuset configuration detected (nmask=3)!

Fixes: 8ca1b5a498 ("mm/page_alloc: detect allocation forbidden by cpuset and bail out early")
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Gregory Price (Meta) 2026-10-22 00:39:43 -04:00 committed by Tejun Heo
parent f5b5cf2be5
commit 4c047bd049
1 changed files with 2 additions and 2 deletions
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4
kernel/cgroup/cpuset.c
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@ -2851,6 +2851,8 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
continue;
rcu_read_unlock();
check_insane_mems_config(new_mems);
spin_lock_irq(&callback_lock);
cp->effective_mems = *new_mems;
spin_unlock_irq(&callback_lock);
@ -2904,8 +2906,6 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
if (retval < 0)
return retval;
check_insane_mems_config(&trialcs->mems_allowed);
spin_lock_irq(&callback_lock);
cs->mems_allowed = trialcs->mems_allowed;
spin_unlock_irq(&callback_lock);