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

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

On 2/6/26 12:31 PM, Sergey Senozhatsky wrote:
> On (26/02/06 12:38), Sergey Senozhatsky wrote:
> [..]
>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>> index eff9e3061925..fa6a018b20a8 100644
>>>> --- a/mm/khugepaged.c
>>>> +++ b/mm/khugepaged.c
>>>> @@ -1894,6 +1894,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>>>>    		xas_set(&xas, index);
>>>>    		folio = xas_load(&xas);
>>>> +		if (try_to_freeze())
>>>> +			goto xa_unlocked;
>>>> +
>>>>    		VM_BUG_ON(index != xas.xa_index);
>>>>    		if (is_shmem) {
>>>>    			if (!folio) {
>>>
>>> Your analysis is reasonable. When the system is freezing, khugepaged is
>>> still trying to swap-in shmem to collapse, which prevents the system from
>>> entering suspend state. However, it’s not only shmem that will swap in,
>>> collapsing anonymous folios may also trigger swap-in operations.
>>
>> 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.