Re: [PATCH mm-new v5 4/5] mm: khugepaged: skip lazy-free folios

[Vernon Yang <vernon2gm@gmail.com>]

On Sat, Jan 24, 2026 at 12:32 AM Lance Yang <lance.yang@linux.dev> wrote:
>
> On 2026/1/23 23:08, Vernon Yang wrote:
> > On Fri, Jan 23, 2026 at 5:09 PM Lance Yang <lance.yang@linux.dev> wrote:
> >>
> >> On 2026/1/23 16:22, Vernon Yang wrote:
> >>> From: Vernon Yang <yanglincheng@kylinos.cn>
> >>>
>
> [...]
>
> >>> @@ -583,6 +584,11 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >>>                folio = page_folio(page);
> >>>                VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
> >>>
> >>> +             if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {
> >>
> >> I'm wondering if we need "cc->is_khugepaged &&" as well here?
> >>
> >> We should allow users to enforce collapse via the madvise_collapse()
> >> path even if pages are marked lazyfree, IMHO.
> >
> > $ man madvise
> > MADV_COLLAPSE
> >          Perform a best-effort synchronous collapse of the native pages
> >          mapped by the memory range into Transparent Huge Pages (THPs).
> >
> > The semantics of MADV_COLLAPSE are best-effort and do not imply to enforce
> > collapsing, so we don't need "cc->is_khugepaged" here.
> >
> > We can imagine that if a user simultaneously uses MADV_FREE and
> > MADV_COLLAPSE, it indicates a misunderstanding of their semantics.
> > As the kernel, we need to safeguard the baseline.
>
> No. Afraid I don't think so.
>
> To be clear, what I meant by "enforce":
>
> Yep, MADV_COLLAPSE is best-effort - it can fail. But when users
> call MADV_COLLAPSE, they're explicitly asking for collapse.
>
> Compared to khugepaged just scanning around, that's already "enforce"
> - users are actively requesting it, not passively waiting for.
>
> Note that you're *breaking* userspace. Users would not be able
> to collapse the range where there are any lazyfree pages anymore,
> even when they explicitly call MADV_COLLAPSE.
>
> For khugepaged, skipping lazyfree makes sense.

I got your meaning, this is equivalent to two questions:

1. Does the semantics of best-effort imply any "enforce" meaning?
2. When madvise(MADV_FREE| MADV_COLLAPSE), do we want to collapse
   lazyfree folios?

This is a semantic warning, and I'd like to hear others' opinions.

> >
> >>> +                     result = SCAN_PAGE_LAZYFREE;
> >>> +                     goto out;
> >>> +             }
> >>> +
> >>>                /* See hpage_collapse_scan_pmd(). */
> >>>                if (folio_maybe_mapped_shared(folio)) {
> >>>                        ++shared;
> >>> @@ -1330,6 +1336,11 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
> >>>                }
> >>>                folio = page_folio(page);
> >>>
> >>> +             if (!pte_dirty(pteval) && folio_test_lazyfree(folio)) {
> >>
> >> Ditto.
> >>
> >>> +                     result = SCAN_PAGE_LAZYFREE;
> >>> +                     goto out_unmap;
> >>> +             }
> >>> +
> >>>                if (!folio_test_anon(folio)) {
> >>>                        result = SCAN_PAGE_ANON;
> >>>                        goto out_unmap;
> >>
>