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
>
