Re: [stable-6.6.y] mm: khugepaged refuses to freeze

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2/6/26 4:36 PM, David Hildenbrand (Arm) wrote:
> On 2/6/26 06:12, Baolin Wang wrote:
>>
>>
>> On 2/6/26 12:31 PM, Sergey Senozhatsky wrote:
>>> On (26/02/06 12:38), Sergey Senozhatsky wrote:
>>> [..]
>>>>
>>>> Right, I thought about it but wasn't sure.  Could the inner loop (e.g.
>>>> collapse_file() in this particular case) loop long enough to fail 
>>>> suspend
>>>> w/o ever giving the outer loop (khugepaged_do_scan()) a chance to 
>>>> freeze?
>>
>> Yes, that’s possible. However, if we add a try_to_freeze() check in 
>> the inner loop, we need to consider various scenarios (such as 
>> anonymous folio swap-in and other potential cases?), which feels too 
>> hacky to me.
>>
>>> For inner loops I wondered if cond_resched() could be an indicator of
>>> where try_to_freeze() should be placed.  Those cond_resched() calls
>>> are there for a reason, after all.   E.g. something like:
>>>
>>> ---
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index fa6a018b20a8..cee08466a069 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -2431,6 +2431,9 @@ static unsigned int 
>>> khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>>>           unsigned long hstart, hend;
>>>           cond_resched();
>>> +        if (try_to_freeze())
>>> +            break;
>>> +
>>>           if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
>>>               progress++;
>>>               break;
>>> @@ -2453,6 +2456,9 @@ static unsigned int 
>>> khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>>>               bool mmap_locked = true;
>>>               cond_resched();
>>> +            if (try_to_freeze())
>>> +                goto breakouterloop;
>>> +
>>>               if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
>>>                   goto breakouterloop;
>>
>> This looks better than the previous version. Let’s also wait to see if 
>> others have any better suggestions.
> 
> What prevents other callpaths (faults, read(), write(), etc) from 
> similarly triggering swapin?

Usually it’s just a userspace process triggering one page fault to swap 
a page in, then will return to userspace. There aren’t other kernel 
threads like khugepaged continuously do swap-in in a loop.

> I recall that there is a notifier when the system is preparing to sleep 
> (pm notifier or something). Could we simply hook into that to tell 
> khugepaged to suspend+resume?

Do you mean “struct dev_pm_ops”, which is used to register PM callbacks 
for devices? However, I don’t know how to use it with a kernel thread.

Also look at how kswapd does it, kswapd also uses 
kthread_freezable_should_stop() to check the freeze state.


> Essentially, making hpage_collapse_test_exit_or_disable() break our for us.

Ah, yes, even better:)