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Message-ID: <d03d6ea7-68fe-4eb5-aa28-1020f3382f4d@linux.dev>
Date: Mon, 15 Dec 2025 19:52:41 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Vernon Yang <vernon2gm@...il.com>
Cc: ziy@...dia.com, npache@...hat.com, baohua@...nel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>,
akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, david@...nel.org
Subject: Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been
collapsed
Hi Vernon,
Thanks for the patches!
On 2025/12/15 17:04, Vernon Yang 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_PMD_NONE are observed during a full scan by
> khugepaged.
>
> @scan_pmd_status[1]: 1 ## SCAN_SUCCEED
> @scan_pmd_status[4]: 158 ## SCAN_PMD_MAPPED
> @scan_pmd_status[3]: 174 ## SCAN_PMD_NONE
> total progress size: 701 MB
> Total time : 440 seconds ## include khugepaged_scan_sleep_millisecs
>
> The khugepaged_scan list save all task that support collapse into hugepage,
> as long as the take is not destroyed, khugepaged will not remove it from
Nit: s/take/task/
> 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 all memory is either SCAN_PMD_MAPPED or
> SCAN_PMD_NONE, the mm is automatically removed from khugepaged's scan
> list. If the page fault or MADV_HUGEPAGE again, it is added back to
> khugepaged.
>
> Signed-off-by: Vernon Yang <yanglincheng@...inos.cn>
> ---
> mm/khugepaged.c | 35 +++++++++++++++++++++++++----------
> 1 file changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0598a19a98cc..1ec1af5be3c8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -115,6 +115,7 @@ struct khugepaged_scan {
> struct list_head mm_head;
> struct mm_slot *mm_slot;
> unsigned long address;
> + bool maybe_collapse;
At a quick glance, the name of "maybe_collapse" is a bit ambiguous ...
Perhaps "scan_needed" or "collapse_possible" would be clearer to
indicate that the mm should be kept in the scan list?
> };
>
> static struct khugepaged_scan khugepaged_scan = {
> @@ -1420,22 +1421,19 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> return result;
> }
>
> -static void collect_mm_slot(struct mm_slot *slot)
> +static void collect_mm_slot(struct mm_slot *slot, bool maybe_collapse)
> {
> struct mm_struct *mm = slot->mm;
>
> lockdep_assert_held(&khugepaged_mm_lock);
>
> - if (hpage_collapse_test_exit(mm)) {
> + if (hpage_collapse_test_exit(mm) || !maybe_collapse) {
> /* free mm_slot */
> hash_del(&slot->hash);
> list_del(&slot->mm_node);
>
> - /*
> - * Not strictly needed because the mm exited already.
> - *
> - * mm_flags_clear(MMF_VM_HUGEPAGE, mm);
> - */
> + if (!maybe_collapse)
> + mm_flags_clear(MMF_VM_HUGEPAGE, mm);
>
> /* khugepaged_mm_lock actually not necessary for the below */
> mm_slot_free(mm_slot_cache, slot);
> @@ -2397,6 +2395,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> struct mm_slot, mm_node);
> khugepaged_scan.address = 0;
> khugepaged_scan.mm_slot = slot;
> + khugepaged_scan.maybe_collapse = false;
> }
> spin_unlock(&khugepaged_mm_lock);
>
> @@ -2470,8 +2469,18 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> khugepaged_scan.address, &mmap_locked, cc);
> }
>
> - if (*result == SCAN_SUCCEED)
> + switch (*result) {
> + case SCAN_PMD_NULL:
> + case SCAN_PMD_NONE:
> + case SCAN_PMD_MAPPED:
> + case SCAN_PTE_MAPPED_HUGEPAGE:
> + break;
> + case SCAN_SUCCEED:
> ++khugepaged_pages_collapsed;
> + fallthrough;
> + default:
> + khugepaged_scan.maybe_collapse = true;
> + }
>
> /* move to next address */
> khugepaged_scan.address += HPAGE_PMD_SIZE;
> @@ -2500,6 +2509,11 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> * if we scanned all vmas of this mm.
> */
> if (hpage_collapse_test_exit(mm) || !vma) {
> + bool maybe_collapse = khugepaged_scan.maybe_collapse;
> +
> + if (mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm))
> + maybe_collapse = true;
> +
> /*
> * Make sure that if mm_users is reaching zero while
> * khugepaged runs here, khugepaged_exit will find
> @@ -2508,12 +2522,13 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> if (!list_is_last(&slot->mm_node, &khugepaged_scan.mm_head)) {
> khugepaged_scan.mm_slot = list_next_entry(slot, mm_node);
> khugepaged_scan.address = 0;
> + khugepaged_scan.maybe_collapse = false;
> } else {
> khugepaged_scan.mm_slot = NULL;
> khugepaged_full_scans++;
> }
>
> - collect_mm_slot(slot);
> + collect_mm_slot(slot, maybe_collapse);
> }
>
> trace_mm_khugepaged_scan(mm, progress, khugepaged_scan.mm_slot == NULL);
> @@ -2616,7 +2631,7 @@ static int khugepaged(void *none)
> slot = khugepaged_scan.mm_slot;
> khugepaged_scan.mm_slot = NULL;
> if (slot)
> - collect_mm_slot(slot);
> + collect_mm_slot(slot, true);
> spin_unlock(&khugepaged_mm_lock);
> return 0;
> }
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