Re: [PATCH] mm: khugepaged: fix the incorrect statistics when collapsing large file folios

[Barry Song]

On Mon, Oct 14, 2024 at 6:25 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> Khugepaged already supports collapsing file large folios (including shmem mTHP)
> by commit 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse"), and the
> control parameters in khugepaged: 'khugepaged_max_ptes_swap' and 'khugepaged_max_ptes_none',
> still compare based on PTE granularity to determine whether a file collapse is
> needed. However, the statistics for 'present' and 'swap' in hpage_collapse_scan_file()
> do not take into account the large folios, which may lead to incorrect judgments
> regarding the khugepaged_max_ptes_swap/none parameters, resulting in unnecessary
> file collapses.
>
> To fix this issue, take into account the large folios' statistics for 'present'
> and 'swap' variables in the hpage_collapse_scan_file().
>
> Fixes: 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse")
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

> ---
>  mm/khugepaged.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index ba28ba09fe89..6f8d46d107b4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2267,7 +2267,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>                         continue;
>
>                 if (xa_is_value(folio)) {
> -                       ++swap;
> +                       swap += 1 << xas_get_order(&xas);
>                         if (cc->is_khugepaged &&
>                             swap > khugepaged_max_ptes_swap) {
>                                 result = SCAN_EXCEED_SWAP_PTE;
> @@ -2314,7 +2314,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>                  * is just too costly...
>                  */
>
> -               present++;
> +               present += folio_nr_pages(folio);
>
>                 if (need_resched()) {
>                         xas_pause(&xas);
> --
> 2.39.3
>