Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

[Yang Shi]

On Thu, Feb 1, 2024 at 4:53 AM Lance Yang <ioworker0@xxxxxxxxx> wrote:
>
> The collapsing behavior of khugepaged with pages
> marked using MADV_FREE might cause confusion
> among users.
>
> For instance, allocate a 2MB chunk using mmap and
> later release it by MADV_FREE. Khugepaged will not
> collapse this chunk. From the user's perspective,
> it treats lazyfree pages as pte_none. However,
> for some pages marked as lazyfree with MADV_FREE,
> khugepaged might collapse this chunk and copy
> these pages to a new huge page. This inconsistency
> in behavior could be confusing for users.
>
> After a successful MADV_FREE operation, if there is
> no subsequent write, the kernel can free the pages
> at any time. Therefore, in my opinion, counting
> lazyfree pages in max_pte_none seems reasonable.
>
> Perhaps treating MADV_FREE like MADV_DONTNEED, not
> copying lazyfree pages when khugepaged collapses
> huge pages in the background better aligns with
> user expectations.
>
> Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> ---
>  mm/khugepaged.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 2b219acb528e..6cbf46d42c6a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -777,6 +777,7 @@ static int __collapse_huge_page_copy(pte_t *pte,
>                                      pmd_t orig_pmd,
>                                      struct vm_area_struct *vma,
>                                      unsigned long address,
> +                                    struct collapse_control *cc,
>                                      spinlock_t *ptl,
>                                      struct list_head *compound_pagelist)
>  {
> @@ -797,6 +798,13 @@ static int __collapse_huge_page_copy(pte_t *pte,
>                         continue;
>                 }
>                 src_page = pte_page(pteval);
> +
> +               if (cc->is_khugepaged
> +                               && !folio_test_swapbacked(page_folio(src_page))) {
> +                       clear_user_highpage(page, _address);
> +                       continue;

If the page was written before khugepaged collapsed it, and khugepaged
collapsed the page before memory reclaim kicked in, didn't this
somehow cause data corruption?

> +               }
> +
>                 if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
>                         result = SCAN_COPY_MC;
>                         break;
> @@ -1205,7 +1213,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>         anon_vma_unlock_write(vma->anon_vma);
>
>         result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> -                                          vma, address, pte_ptl,
> +                                          vma, address, cc, pte_ptl,
>                                            &compound_pagelist);
>         pte_unmap(pte);
>         if (unlikely(result != SCAN_SUCCEED))
> --
> 2.33.1
>