Re: [PATCH 2/2] kthread: Drop warning on cpumask allocation failure in kthread_affine_node()

[Waiman Long <longman@redhat.com>]

cc: Frederic Weisbecker <frederic@kernel.org> for his input.

-Longman

On 9/14/26 8:09 AM, Quchaosheng wrote:
> kthread_affine_node() warns when zalloc_cpumask_var() fails:
>
>    if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
>            WARN_ON_ONCE(1);
>            return;
>    }
>
> The allocation uses GFP_KERNEL, so it can fail under memory pressure or
> fault injection.  A failed allocation is a recoverable condition and not a
> kernel bug, so the warning is noise.  syzbot reports it for a WireGuard
> NAPI thread:
>
>    WARNING: kernel/kthread.c:359 at kthread_affine_node+0x200/0x2e8
>    CPU: 0 PID: 5207 Comm: napi/wg2-0
>    Call Trace:
>     alloc_cpumask_var_node+0xfc/0x138
>     zalloc_cpumask_var
>     kthread_affine_node+0x148/0x2e8
>     kthread+0x29c/0x3d4
>     ret_from_fork+0x10/0x20
>
> The other two callers of zalloc_cpumask_var() in this file, in the kthread
> preferred affinity and kthreads_online_cpu() paths, return -ENOMEM without
> a warning.  kthread_affine_node() returns void, so it cannot report the
> error either, and it only skips the affinity setup for this thread.  Return
> early without warning, matching those callers.
>
> Reported-by: syzbot+37ca7ae3e98cb65c3209@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=37ca7ae3e98cb65c3209
> Signed-off-by: Quchaosheng <quchaosheng000406@163.com>
> ---
>   kernel/kthread.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index a3f95c904..f735a74b6 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -355,10 +355,8 @@ static void kthread_affine_node(void)
>   	if (WARN_ON_ONCE(kthread_is_per_cpu(current)))
>   		return;
>   
> -	if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
> -		WARN_ON_ONCE(1);
> +	if (!zalloc_cpumask_var(&affinity, GFP_KERNEL))
>   		return;
> -	}
>   
>   	mutex_lock(&kthread_affinity_lock);
>   	WARN_ON_ONCE(!list_empty(&kthread->affinity_node));
>