Re: [PATCH mm-unstable v3 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Mar 11, 2026 at 03:13:15PM -0600, Nico Pache wrote:
[..]
>@@ -2823,46 +2855,20 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> 			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> 		}
> 		mmap_assert_locked(mm);
>-		if (!vma_is_anonymous(vma)) {
>-			struct file *file = get_file(vma->vm_file);
>-			pgoff_t pgoff = linear_page_index(vma, addr);
> 
>-			mmap_read_unlock(mm);
>-			mmap_locked = false;
>-			*lock_dropped = true;
>-			result = collapse_scan_file(mm, addr, file, pgoff, cc);
>-
>-			if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
>-			    mapping_can_writeback(file->f_mapping)) {
>-				loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
>-				loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
>+		result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
> 
>-				filemap_write_and_wait_range(file->f_mapping, lstart, lend);
>-				triggered_wb = true;
>-				fput(file);
>-				goto retry;
>-			}
>-			fput(file);
>-		} else {
>-			result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
>-		}
> 		if (!mmap_locked)
> 			*lock_dropped = true;
> 
>-handle_result:
> 		switch (result) {
> 		case SCAN_SUCCEED:
> 		case SCAN_PMD_MAPPED:
> 			++thps;
> 			break;
>-		case SCAN_PTE_MAPPED_HUGEPAGE:
>-			BUG_ON(mmap_locked);
>-			mmap_read_lock(mm);
>-			result = try_collapse_pte_mapped_thp(mm, addr, true);
>-			mmap_read_unlock(mm);
>-			goto handle_result;
> 		/* Whitelisted set of results where continuing OK */
> 		case SCAN_NO_PTE_TABLE:
>+		case SCAN_PTE_MAPPED_HUGEPAGE:

It looks we won't have this case after refactor?

Current code flow is like this:

  result = collapse_single_pmd()
      result = collapse_scan_file()
          result = collapse_file()

      if (result == SCAN_PTE_MAPPED_HUGEPAGE) {            --- (1)
          result = SCAN_ANY_PROCESS;
	  or 
	  result = try_collapse_pte_mapped_thp();
      }

Only collapse_scan_file() and collapse_file() may return
SCAN_PTE_MAPPED_HUGEPAGE, and then handled in (1). After this, result is set
to another value to indicate whether we collapse it or not.

So I am afraid we don't expect to see SCAN_PTE_MAPPED_HUGEPAGE here. Do I miss
something?

> 		case SCAN_PTE_NON_PRESENT:
> 		case SCAN_PTE_UFFD_WP:
> 		case SCAN_LACK_REFERENCED_PAGE:
>-- 
>2.53.0
>

-- 
Wei Yang
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