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

[David Hildenbrand (Arm)]

On 10/31/26 17:42, Andrew Morton wrote:
> On Sat, 31 Oct 2026 11:52:30 +0100 "David Hildenbrand (Arm)" <david@xxxxxxxxxx> wrote:
> 
>> 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?
> 
> It's almost unneeded in current mainline kernels.  PREEMPT_NONE can
> still be selected by
> 
> 	All supported Alpha configurations
> 
> 	All supported Hexagon configurations
> 
> 	Non-ColdFire configurations (classic m68k)
> 
> However 99% of the world are on 5.x and 6.x kernels and many of those
> machines use PREEMPT_NONE.
> 
> 
> IOW, cond_resched() is not really needed in mainline, needed in
> -stable.

But where did the submitter reproduce this?

Why should we even care reading the above?

I'm all confused now.

-- 
Cheers,

David