Re: [PATCH V2 2/5] mm/khugepaged: count small VMAs towards scan limit

[Wei Yang]

On Wed, Dec 24, 2025 at 07:51:36PM +0800, Lance Yang wrote:
>
>
>On 2025/12/24 19:13, Shivank Garg wrote:
>> The khugepaged_scan_mm_slot() uses a 'progress' counter to limit the
>> amount of work performed and consists of three components:
>> 1. Transitioning to a new mm (+1).

Hmm... maybe not only a new mm, but also we start another scan from last mm.

Since default khugepaged_pages_to_scan is 8 PMD, it looks very possible.

>> 2. Skipping an unsuitable VMA (+1).
>> 3. Scanning a PMD-sized range (+HPAGE_PMD_NR).
>> 
>> Consider a 1MB VMA sitting between two 2MB alignment boundaries:
>> 
>>       vma1       vma2   vma3
>>      +----------+------+----------+
>>      |2M        |1M    |2M        |
>>      +----------+------+----------+
>>                 ^      ^
>>                 start  end
>>                 ^
>>            hstart,hend
>> 
>> In this case, for vma2:
>>    hstart = round_up(start, HPAGE_PMD_SIZE)  -> Next 2MB alignment
>>    hend   = round_down(end, HPAGE_PMD_SIZE) -> Prev 2MB alignment
>> 
>> Currently, since `hend <= hstart`, VMAs that are too small or unaligned
>> to contain a hugepage are skipped without incrementing 'progress'.
>> A process containing a large number of such small VMAs will unfairly
>> consume more CPU cycles before yielding compared to a process with
>> fewer, larger, or aligned VMAs.
>> 
>> Fix this by incrementing progress when the `hend <= hstart` condition
>> is met.
>> 
>> Additionally, change 'progress' type to `unsigned int` to match both
>> the 'pages' type and the function return value.
>> 
>> Suggested-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
>> ---
>>   mm/khugepaged.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 107146f012b1..0b549c3250f9 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2403,7 +2403,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>>   	struct mm_slot *slot;
>>   	struct mm_struct *mm;
>>   	struct vm_area_struct *vma;
>> -	int progress = 0;
>> +	unsigned int progress = 0;
>>   	VM_BUG_ON(!pages);
>>   	lockdep_assert_held(&khugepaged_mm_lock);
>> @@ -2447,7 +2447,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>>   		}
>>   		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>>   		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
>> -		if (khugepaged_scan.address > hend) {
>
>Maybe add a short comment explaining why we increment progress for small VMAs
>;)
>
>Something like this:
>
>		/* Count small VMAs that can't hold a hugepage towards scan limit */
>> +		if (khugepaged_scan.address > hend || hend <= hstart) {
>>   			progress++;
>>   			continue;
>>   		}
>
>Otherwise, looks good to me.
>
>Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
>

The code change LGTM.

Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>

-- 
Wei Yang
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