linux-kernel - Re: [PATCH v2 2/4] mm: khugepaged: just skip when the memory has been collapsed

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Message-ID: <CACZaFFMb_HY3-FwbFZQ6QvSSN72FYggeP-DOyCX4QU4gAFPBmg@mail.gmail.com>
Date: Tue, 30 Dec 2025 23:46:01 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: akpm@...ux-foundation.org, david@...nel.org, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev, 
	richard.weiyang@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH v2 2/4] mm: khugepaged: just skip when the memory has been collapsed

On Mon, Dec 29, 2025 at 1:52 PM Vernon Yang <vernon2gm@...il.com> wrote:
>
> The following data is traced by bpftrace on a desktop system. After
> the system has been left idle for 10 minutes upon booting, a lot of
> SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan
> by khugepaged.
>
> @scan_pmd_status[1]: 1           ## SCAN_SUCCEED
> @scan_pmd_status[6]: 2           ## SCAN_EXCEED_SHARED_PTE
> @scan_pmd_status[3]: 142         ## SCAN_PMD_MAPPED
> @scan_pmd_status[2]: 178         ## SCAN_NO_PTE_TABLE
> total progress size: 674 MB
> Total time         : 419 seconds ## include khugepaged_scan_sleep_millisecs
>
> The khugepaged_scan list save all task that support collapse into hugepage,
> as long as the task is not destroyed, khugepaged will not remove it from
> the khugepaged_scan list. This exist a phenomenon where task has already
> collapsed all memory regions into hugepage, but khugepaged continues to
> scan it, which wastes CPU time and invalid, and due to
> khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
> scanning a large number of invalid task, so scanning really valid task
> is later.
>
> After applying this patch, when the memory is either SCAN_PMD_MAPPED or
> SCAN_NO_PTE_TABLE, just skip it, as follow:
>
> @scan_pmd_status[6]: 2
> @scan_pmd_status[3]: 147
> @scan_pmd_status[2]: 173
> total progress size: 45 MB
> Total time         : 20 seconds
>
> Signed-off-by: Vernon Yang <yanglincheng@...inos.cn>
> ---
>  mm/khugepaged.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9f99f61689f8..2b3685b195f5 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -66,7 +66,10 @@ enum scan_result {
>  static struct task_struct *khugepaged_thread __read_mostly;
>  static DEFINE_MUTEX(khugepaged_mutex);
>
> -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
> +/*
> + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
> + * every 10 second.
> + */
>  static unsigned int khugepaged_pages_to_scan __read_mostly;
>  static unsigned int khugepaged_pages_collapsed;
>  static unsigned int khugepaged_full_scans;
> @@ -2487,12 +2490,22 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>                                         khugepaged_scan.address, &mmap_locked, cc);
>                         }
>
> -                       if (*result == SCAN_SUCCEED)
> -                               ++khugepaged_pages_collapsed;
> -
>                         /* move to next address */
>                         khugepaged_scan.address += HPAGE_PMD_SIZE;
> -                       progress += HPAGE_PMD_NR;
> +
> +                       switch (*result) {
> +                       case SCAN_NO_PTE_TABLE:
> +                       case SCAN_PMD_MAPPED:
> +                       case SCAN_PTE_MAPPED_HUGEPAGE:
> +                               progress++;
> +                               break;
> +                       case SCAN_SUCCEED:
> +                               ++khugepaged_pages_collapsed;
> +                               fallthrough;
> +                       default:
> +                               progress += HPAGE_PMD_NR;
> +                       }
> +
>                         if (!mmap_locked)
>                                 /*
>                                  * We released mmap_lock so break loop.  Note
> --
> 2.51.0
>

Sorry, this coding has warnings at SCAN_PTE_MAPPED_HUGEPAGE, and the code
will be refactored in the next version. But the core idea is unchanged.