Re: [PATCH V3 2/5] mm/khugepaged: count small VMAs towards scan limit

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

On 2026/1/22 15:32, Dev Jain wrote:
> 
> On 19/01/26 12:52 am, 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).
>> 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
> 
> Won't such a VMA be skipped by thp_vma_allowable_order()? That internally
> checks, apart from eligibility by sysfs, that the extent of the VMA can
> map a hugepage.

Ah, you're right!

I was worrying about a case that doesn't actually happen.

Thanks,
Lance

> 
>>
>> 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@gmail.com>
>> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
>>
>> Incorporate comment feedback from Lance:
>> https://lore.kernel.org/linux-mm/6b408736-978a-4d40-adfc-97819951c3a6@linux.dev
>>
>>   mm/khugepaged.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 984294a16861..93ce39915f4a 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 +2447,8 @@ 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) {
>> +		if (khugepaged_scan.address > hend || hend <= hstart) {
>> +			/* VMA already scanned or too small/unaligned for hugepage. */
>>   			progress++;
>>   			continue;
>>   		}