linux-kernel - Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <20251109024013.fzt7xxpmxwi75xgr@master>
Date: Sun, 9 Nov 2025 02:40:13 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
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Subject: Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP
 collapse attempts

On Wed, Oct 22, 2025 at 12:37:15PM -0600, Nico Pache wrote:
>There are cases where, if an attempted collapse fails, all subsequent
>orders are guaranteed to also fail. Avoid these collapse attempts by
>bailing out early.
>
>Signed-off-by: Nico Pache <npache@...hat.com>
>---
> mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
> 1 file changed, 30 insertions(+), 1 deletion(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index e2319bfd0065..54f5c7888e46 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -1431,10 +1431,39 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address,
> 			ret = collapse_huge_page(mm, address, referenced,
> 						 unmapped, cc, mmap_locked,
> 						 order, offset);
>-			if (ret == SCAN_SUCCEED) {
>+
>+			/*
>+			 * Analyze failure reason to determine next action:
>+			 * - goto next_order: try smaller orders in same region
>+			 * - continue: try other regions at same order
>+			 * - break: stop all attempts (system-wide failure)
>+			 */
>+			switch (ret) {
>+			/* Cases were we should continue to the next region */
>+			case SCAN_SUCCEED:
> 				collapsed += 1UL << order;
>+				fallthrough;
>+			case SCAN_PTE_MAPPED_HUGEPAGE:
> 				continue;
>+			/* Cases were lower orders might still succeed */
>+			case SCAN_LACK_REFERENCED_PAGE:
>+			case SCAN_EXCEED_NONE_PTE:
>+			case SCAN_EXCEED_SWAP_PTE:
>+			case SCAN_EXCEED_SHARED_PTE:
>+			case SCAN_PAGE_LOCK:
>+			case SCAN_PAGE_COUNT:
>+			case SCAN_PAGE_LRU:
>+			case SCAN_PAGE_NULL:
>+			case SCAN_DEL_PAGE_LRU:
>+			case SCAN_PTE_NON_PRESENT:
>+			case SCAN_PTE_UFFD_WP:
>+			case SCAN_ALLOC_HUGE_PAGE_FAIL:
>+				goto next_order;
>+			/* All other cases should stop collapse attempts */
>+			default:
>+				break;
> 			}
>+			break;

One question here:

Suppose we have iterated several orders and not collapse successfully yet. So
the mthp_bitmap_stack[] would look like this:

[8 7 6 6]
       ^
       |

Now we found this one pass the threshold check, but it fails with other
result.

Current code looks it would give up at all, but we may still have a chance to
collapse the above 3 range?

> 		}
> 
> next_order:
>-- 
>2.51.0

-- 
Wei Yang
Help you, Help me