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

Lance Yang
2026-03-15T15:16:37Z

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properly indicates that this process is no longer valid to operate on.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