Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been collapsed

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Dec 15, 2025 at 05:04:17PM +0800, 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
>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.

Two thing s come up my mind:

  * what happens if we split the huge page under memory pressure?
  * would this interfere with mTHP collapse?

>
>Signed-off-by: Vernon Yang <yanglincheng@kylinos.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;
> };
> 
> 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;

If collapse successfully, we don't need to set maybe_collapse to true?

>+			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;
> }
>-- 
>2.51.0
>

-- 
Wei Yang
Help you, Help me