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

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

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).
> 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@gmail.com>
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>   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@linux.dev>