Re: [RFC PATCH v2 07/11] mm: khugepaged: skip large folios that don't need to be collapsed

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

On Wed, Jun 10, 2026 at 06:29:15PM +0800, Baolin Wang wrote:
[...]
>@@ -2843,6 +2853,11 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> 		goto out;
> 	}
> 
>+	if (bitmap_empty(cc->mthp_present_ptes, MAX_PTRS_PER_PTE)) {
>+		result = SCAN_FAIL;
>+		goto out;
>+	}

This check seems to jump the gun a bit for MADV_COLLAPSE ...

shmem swap entries only bump "swap" during the scan in
collapse_scan_file():

		if (xa_is_value(folio)) {
			swap += 1 << xas_get_order(&xas);
			if (swap > max_ptes_swap) {
				result = SCAN_EXCEED_SWAP_PTE;
				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
				break;
			}
			continue;
		}

Holes don't set the bitmap either. They just show up later through the
"present" check:

	if (present < HPAGE_PMD_NR - max_ptes_none) {
		result = SCAN_EXCEED_NONE_PTE;
		count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
		count_mthp_stat(HPAGE_PMD_ORDER,
				MTHP_STAT_COLLAPSE_EXCEED_NONE);
		goto out;
	}

For MADV_COLLAPSE, both max_ptes_none and max_ptes_swap are HPAGE_PMD_NR,
so those checks still let this through.

So now we bail with SCAN_FAIL before collapse_file() ever sees the case,
even though that code still handles both: swap via shmem_get_folio(),
holes via nr_none.

Am I reading that right? If so, looks like a regression ...

>+
> 	result = mthp_collapse(mm, file, start, addr, 0, 0, cc, enabled_orders);
> 	if (result == SCAN_SUCCEED && !cc->is_khugepaged) {
> 		/* If MADV_COLLAPSE, adjust result to call collapse_pte_mapped_thp(). */
>-- 
>2.47.3
>

Cheers, Lance