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

["Gregory Price (Meta)" <gourry@gourry.net>]

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: 8ca1b5a49885 ("mm/page_alloc: detect allocation forbidden by cpuset and bail out early")
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 kernel/cgroup/cpuset.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8ee239d5c6b48..70fbece1b9399 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2795,6 +2795,8 @@ 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);
@@ -2848,8 +2850,6 @@ 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);
-- 
2.53.0-Meta