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

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

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.

>
> 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;
>  		}