Re: [PATCH mm-unstable v18 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts
From: Lance Yang <lance.yang@linux.dev>
Date: 2026-05-31 07:31:31
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On Fri, May 22, 2026 at 09:00:07AM -0600, Nico Pache wrote:
quoted hunk ↗ jump to hunk
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. Reviewed-by: Lorenzo Stoakes <ljs@kernel.org> Acked-by: Usama Arif <usama.arif@linux.dev> Acked-by: David Hildenbrand (Arm) <david@kernel.org> Signed-off-by: Nico Pache <redacted> --- mm/khugepaged.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-)diff --git a/mm/khugepaged.c b/mm/khugepaged.c index d3d7db8be26c..15b7298bc225 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c@@ -1535,9 +1535,31 @@ static int mthp_collapse(struct mm_struct *mm, struct vm_area_struct *vma,collapse_address = address + offset * PAGE_SIZE; ret = collapse_huge_page(mm, collapse_address, referenced, unmapped, cc, order); - if (ret == SCAN_SUCCEED) { + + switch (ret) { + /* Cases where we continue to next collapse candidate */ + case SCAN_SUCCEED: collapsed += nr_ptes; + fallthrough; + case SCAN_PTE_MAPPED_HUGEPAGE: continue; + /* Cases where 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_NULL: + case SCAN_DEL_PAGE_LRU: + case SCAN_PTE_NON_PRESENT: + case SCAN_PTE_UFFD_WP: + case SCAN_ALLOC_HUGE_PAGE_FAIL:
Nit: shouldn't SCAN_CGROUP_CHARGE_FAIL go with SCAN_ALLOC_HUGE_PAGE_FAIL here? If charging the current order fails, a smaller order might still fit :) Cheers, Lance
+ case SCAN_PAGE_LAZYFREE: + goto next_order; + /* Cases where no further collapse is possible */ + default: + return collapsed; } } -- 2.54.0