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

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

On Wed, Mar 11, 2026 at 03:13:15PM -0600, Nico Pache wrote:
>The khugepaged daemon and madvise_collapse have two different
>implementations that do almost the same thing. Create collapse_single_pmd
>to increase code reuse and create an entry point to these two users.
>
>Refactor madvise_collapse and collapse_scan_mm_slot to use the new
>collapse_single_pmd function. This introduces a minor behavioral change
>that is most likely an undiscovered bug. The current implementation of
>khugepaged tests collapse_test_exit_or_disable before calling
>collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
>case. By unifying these two callers madvise_collapse now also performs
>this check. We also modify the return value to be SCAN_ANY_PROCESS which
>properly indicates that this process is no longer valid to operate on.
>
>By moving the madvise_collapse writeback-retry logic into the helper
>function we can also avoid having to revalidate the VMA.
>
>We also guard the khugepaged_pages_collapsed variable to ensure its only
>incremented for khugepaged.
>
>Signed-off-by: Nico Pache <npache@redhat.com>
>---
> mm/khugepaged.c | 120 +++++++++++++++++++++++++-----------------------
> 1 file changed, 63 insertions(+), 57 deletions(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 33ae56e313ed..733c4a42c2ce 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -2409,6 +2409,65 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> 	return result;
> }
> 
>+/*
>+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
>+ * the results.
>+ */
>+static enum scan_result collapse_single_pmd(unsigned long addr,
>+		struct vm_area_struct *vma, bool *mmap_locked,
>+		struct collapse_control *cc)
>+{
>+	struct mm_struct *mm = vma->vm_mm;
>+	bool triggered_wb = false;
>+	enum scan_result result;
>+	struct file *file;
>+	pgoff_t pgoff;
>+
>+	if (vma_is_anonymous(vma)) {
>+		result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
>+		goto end;
>+	}
>+
>+	file = get_file(vma->vm_file);
>+	pgoff = linear_page_index(vma, addr);
>+
>+	mmap_read_unlock(mm);
>+	*mmap_locked = false;
>+retry:
>+	result = collapse_scan_file(mm, addr, file, pgoff, cc);
>+
>+	/*
>+	 * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
>+	 * then retry the collapse one time.
>+	 */
>+	if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
>+	    !triggered_wb && mapping_can_writeback(file->f_mapping)) {
>+		const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
>+		const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
>+
>+		filemap_write_and_wait_range(file->f_mapping, lstart, lend);
>+		triggered_wb = true;
>+		goto retry;

While the old retry path did go back through hugepage_vma_revalidate(),
the retry itself is not relying on the original VMA remaining unchanged
IIUC.

After dropping mmap_lock, the code still holds a reference to the file,
so no lifetime issue should arise here :)

So, LGTM!
Reviewed-by: Lance Yang <lance.yang@linux.dev>

Cheers,
Lance