Re: [RFC PATCH v2 04/11] mm: khugepaged: add shmem mTHP collapse support

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

On Wed, Jun 10, 2026 at 06:29:12PM +0800, Baolin Wang wrote:
[...]
>@@ -1512,8 +1517,12 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
> 			enum scan_result ret;
> 
> 			collapse_address = address + offset * PAGE_SIZE;
>-			ret = collapse_huge_page(mm, collapse_address, referenced,
>-						 unmapped, cc, order);
>+			if (file)
>+				ret = collapse_file(mm, collapse_address, file,
>+						start + offset, cc, order);
>+			else
>+				ret = collapse_huge_page(mm, collapse_address,
>+						referenced, unmapped, cc, order);
> 
> 			switch (ret) {
> 			/* Cases where we continue to next collapse candidate */
>@@ -1521,6 +1530,7 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
> 				collapsed += nr_ptes;
> 				fallthrough;
> 			case SCAN_PTE_MAPPED_HUGEPAGE:

Looks like SCAN_PTE_MAPPED_HUGEPAGE from collapse_file() get lost for
the PMD-order case.

Previously, collapse_file() returned it straight back to
collapse_single_pmd(), so we would run try_collapse_pte_mapped_thp().

Now it hits mthp_collapse() fitst, and that case just goes to
next_offset ...

>+			case SCAN_PAGE_COMPOUND:
> 				goto next_offset;
> 			/* Cases where lower orders might still succeed */
> 			case SCAN_ALLOC_HUGE_PAGE_FAIL:
[...]

Cheers, Lance