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

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Message-ID: <CAGsJ_4ybLyd3rHQgfySuiXQt1c7gS-w-2bK6xQ0Tuapt6qLwRA@mail.gmail.com>
Date: Mon, 14 Oct 2024 20:09:49 +0800
From: Barry Song <21cnbao@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, willy@...radead.org, 
	david@...hat.com, ryan.roberts@....com, shy828301@...il.com, ziy@...dia.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: khugepaged: fix the incorrect statistics when
 collapsing large file folios

On Mon, Oct 14, 2024 at 6:25 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> 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@...ux.alibaba.com>

Reviewed-by: Barry Song <baohua@...nel.org>

> ---
>  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
>