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

[Lance Yang <lance.yang@linux.dev>]

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;
}

>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