Re: [PATCH v2 1/2] mm: Enable khugepaged anonymous collapse on non-writable regions

["Zach O'Keefe" <zokeefe@google.com>]

On Mon, Sep 8, 2025 at 12:51 AM Dev Jain <dev.jain@arm.com> wrote:
>
> Currently khugepaged does not collapse an anonymous region which does not
> have a single writable pte. This is wasteful since a region mapped with
> non-writable ptes, for example, non-writable VMAs mapped by the
> application, won't benefit from THP collapse.
>
> An additional consequence of this constraint is that MADV_COLLAPSE does not
> perform a collapse on a non-writable VMA, and this restriction is nowhere
> to be found on the manpage - the restriction itself sounds wrong to me
> since the user knows the protection of the memory it has mapped, so
> collapsing read-only memory via madvise() should be a choice of the
> user which shouldn't be overridden by the kernel.

Sorry ; late to the party. Certainly agree wrt MADV_COLLAPSE.

Ditto for khugepaged as well. Check added when support for
non-writable pages were added to khugepaged, though retaining
heuristic that at least one pte should be writable;  10359213d05a
("mm: incorporate read-only pages into transparent huge pages"), which
predates max_ptes_swap.

> Therefore, remove this restriction by not honouring SCAN_PAGE_RO.>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Reviewed-by: Zach O'Keefe <zokeefe@google.com>

> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>  mm/khugepaged.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4ec324a4c1fe..a0f1df2a7ae6 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -676,9 +676,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                         writable = true;
>         }
>
> -       if (unlikely(!writable)) {
> -               result = SCAN_PAGE_RO;
> -       } else if (unlikely(cc->is_khugepaged && !referenced)) {
> +       if (unlikely(cc->is_khugepaged && !referenced)) {
>                 result = SCAN_LACK_REFERENCED_PAGE;
>         } else {
>                 result = SCAN_SUCCEED;
> @@ -1421,9 +1419,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>                      mmu_notifier_test_young(vma->vm_mm, _address)))
>                         referenced++;
>         }
> -       if (!writable) {
> -               result = SCAN_PAGE_RO;
> -       } else if (cc->is_khugepaged &&
> +       if (cc->is_khugepaged &&
>                    (!referenced ||
>                     (unmapped && referenced < HPAGE_PMD_NR / 2))) {
>                 result = SCAN_LACK_REFERENCED_PAGE;
> @@ -2830,7 +2826,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>                 case SCAN_PMD_NULL:
>                 case SCAN_PTE_NON_PRESENT:
>                 case SCAN_PTE_UFFD_WP:
> -               case SCAN_PAGE_RO:
>                 case SCAN_LACK_REFERENCED_PAGE:
>                 case SCAN_PAGE_NULL:
>                 case SCAN_PAGE_COUNT:
> --
> 2.30.2
>
>