Re: [PATCH] mm/migrate: fix softlockup in do_pages_stat() with huge page arrays

[David Hildenbrand (Arm)]

On 10/31/26 04:33, Qi Xi wrote:
> From: Qi Xi <xiqi2@xxxxxxxxxx>
> 
> do_pages_stat() walks the user page array in 16-page chunks but never
> reschedules, so a huge nr_pages can hold a CPU past the watchdog
> threshold on non-preemptible kernels. Add cond_resched() to the loop so
> long move_pages() node-query calls yield periodically.
> 
> With KASAN and function tracing enabled, a move_pages() node query
> (nodes == NULL) over 10 million page entries (~40GB) can trigger the
> softlockup on a PREEMPT_NONE kernel.
> 
> Fixes: 80bba1290ab5 ("mm: no get_user/put_user while holding mmap_sem in do_pages_stat?")
> Signed-off-by: Qi Xi <xiqi2@xxxxxxxxxx>
> ---
>  mm/migrate.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 7bdcdb57652f..6c2371020cc5 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2545,6 +2545,7 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
>  
>  		chunk_offset += chunk_nr;
>  		nr_pages -= chunk_nr;
> +		cond_resched();
>  	}
>  	return nr_pages ? -EFAULT : 0;
>  }

I though we wanted to stop throwing in cond_resched() at this point?

-- 
Cheers,

David