Re: [PATCH v12 mm-new 02/15] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

Wei Yang
2025-11-08T01:44:29Z

On Wed, Oct 22, 2025 at 12:37:04PM -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properly indicates that this process is no longer valid to operate on.
>
>We also guard the khugepaged_pages_collapsed variable to ensure its only
>incremented for khugepaged.
>
>Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>Acked-by: David Hildenbrand <david@redhat.com>
>Signed-off-by: Nico Pache <npache@redhat.com>

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

One nit below.

>---
> mm/khugepaged.c | 97 ++++++++++++++++++++++++++-----------------------
> 1 file changed, 52 insertions(+), 45 deletions(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index 6c4abc7f45cf..36e31d99e507 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -2370,6 +2370,53 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> return result;
> }
>
>+/*
>+ * Try to collapse a single PMD starting at a PMD aligned addr, and return
>+ * the results.
>+ */
>+static int 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;
>+ int 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;
>+ result = collapse_scan_file(mm, addr, file, pgoff, cc);
>+ fput(file);
>+ if (result != SCAN_PTE_MAPPED_HUGEPAGE)
>+ goto end;
>+
>+ mmap_read_lock(mm);
>+ *mmap_locked = true;
>+ if (collapse_test_exit_or_disable(mm)) {
>+ mmap_read_unlock(mm);
>+ *mmap_locked = false;
>+ return SCAN_ANY_PROCESS;
>+ }
>+ result = collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
>+ if (result == SCAN_PMD_MAPPED)
>+ result = SCAN_SUCCEED;
>+ mmap_read_unlock(mm);
>+ *mmap_locked = false;

For all cases, we would set mmap_locked to false. Not sure it bother to adjust
it.

--
Wei Yang
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