Re: [PATCH v2] cgroup/cpuset: use mems_allowed_seq when checking a policy nodemask

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 10/19/26 18:21, Gregory Price (Meta) wrote:
> A cpuset update changes the task's mems_allowed and rebinds its
> mempolicy in separate steps under mems_allowed_seq, but the check
> reads both without it.  This can cause a spurious numa placement
> miss under some cpuset/mempolicy combinations.
> 
> A reader that loads one mask before the update and the other after
> it can see a spurious empty intersection and drop the nodemask.
> 
> For example, a task with cpuset.mems={0} and MPOL_BIND={0}.  If the
> cpuset moves to mems={1,2}, the policy is rebound to {1}.
> 
> The allocator can race and compare the old mems_allowed {0} against
> the new policy {1} - which will result in a dropped nodemask and a
> misplaced allocation on node 2.

And since this can have only performance implications and not cause a
functional issue, we don't need Cc: stable.

(unlike violating a __GFP_THISNODE allocation which could be quite bad and
has happened in the past)

> Simply retry the intersection until mems_allowed_seq is stable.
> 
> This covers the task policy, which is rebound under mems_allowed_seq.
> VMA policies are rebound afterwards by mpol_rebind_mm(), outside
> mems_allowed_seq, and are not covered by this change.
> 
> Fixes: cc9a6c877661 ("cpuset: mm: reduce large amounts of memory barrier related damage v3")
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Reviewed-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> ---
> v2: there is no real update here, i'm breaking it off of the page_alloc
>     series so the page_alloc fixes backport collectively cleaner.
> 
>  kernel/cgroup/cpuset.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 3f52717c19654..8ee239d5c6b48 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4354,10 +4354,21 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk)
>   * @nodemask: the nodemask to be checked
>   *
>   * Are any of the nodes in the nodemask allowed in current->mems_allowed?
> + *
> + * A cpuset update rebinds the task's mempolicy and changes mems_allowed in
> + * separate steps, so retry if an update raced with the check.
>   */
>  int cpuset_nodemask_valid_mems_allowed(const nodemask_t *nodemask)
>  {
> -	return nodes_intersects(*nodemask, current->mems_allowed);
> +	unsigned int seq;
> +	int ret;
> +
> +	do {
> +		seq = read_mems_allowed_begin();
> +		ret = nodes_intersects(*nodemask, current->mems_allowed);
> +	} while (read_mems_allowed_retry(seq));
> +
> +	return ret;
>  }
>  
>  /*