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

Wei Yang
2025-11-09T02:40:16Z

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@redhat.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
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