Re: [PATCH] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure

[Dev Jain <dev.jain@arm.com>]

On 01/05/26 6:54 pm, Lance Yang wrote:
> 
> On Fri, May 01, 2026 at 04:10:58PM +0530, Dev Jain wrote:
>>
>>
>> On 01/05/26 1:27 pm, Ye Liu wrote:
>>> From: Ye Liu <liuye@kylinos.cn>
>>>
>>> __khugepaged_enter() sets MMF_VM_HUGEPAGE before allocating the
>>> corresponding mm_slot.  If mm_slot_alloc() fails, the function
>>> returns with the flag set but without inserting the mm into the
>>> khugepaged tracking structures.
>>>
>>> This leaves the mm in an inconsistent state: it is marked as
>>> registered (MMF_VM_HUGEPAGE set), but will never be scanned by
>>> khugepaged.  Future attempts to register the mm are skipped since
>>> khugepaged_enter_vma() checks the flag and returns early.
>>>
>>> Fix this by clearing MMF_VM_HUGEPAGE when mm_slot_alloc() fails,
>>> restoring the ability to retry registration later.
>>>
>>> Signed-off-by: Ye Liu <liuye@kylinos.cn>
>>> ---
>>>  mm/khugepaged.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 7d48d4fbd5f3..60ab7c1b61dd 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -559,8 +559,10 @@ void __khugepaged_enter(struct mm_struct *mm)
>>>  		return;
>>>  
>>>  	slot = mm_slot_alloc(mm_slot_cache);
>>> -	if (!slot)
>>> +	if (!slot) {
>>> +		mm_flags_clear(MMF_VM_HUGEPAGE, mm);
>>>  		return;
>>> +	}
>>
>> You could just move the test_and_set() after this no?
> 
> Yep, that sounds better :)
> 
> Just one small thing: if we move the test_and_set(), after
> mm_slot_alloc(), we need to free the slot when test_and_set() says that
> the flag was already set. Otherwise the racing caller that loses would
> leak the slot :)
> 
> Something like:
> 
> slot = mm_slot_alloc(mm_slot_cache);
> if (!slot)
> 	return;
> 
> if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm))) {
> 	mm_slot_free(mm_slot_cache, slot);
> 	return;
> }

My bad, I missed the fact that test-and-set may also fail.
So I think Ye's patch is also fine.
> 
>> So if slot allocation fails then MMF_VM_HUGEPAGE is never set.
>>
>> Fixes tag is probably required but not sure about Ccing stable.
> 
> Yes, Fixes sounds good. khugepaged collapse is best-effort though,
> anyway :)
> 
> So I'm also not sure about Ccing stable.
> 
>>>  
>>>  	spin_lock(&khugepaged_mm_lock);
>>>  	mm_slot_insert(mm_slots_hash, mm, slot);
> 
> Cheers, Lance