Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts

Nico Pache
2026-03-18T18:59:43Z

On 3/17/26 4:35 AM, Lorenzo Stoakes (Oracle) wrote:
> On Wed, Feb 25, 2026 at 08:26:31PM -0700, 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>
>
> With David's concern addressed:
>
> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
>
>> ---
>> mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++-
>> 1 file changed, 34 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 1c3711ed4513..388d3f2537e2 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1492,9 +1492,42 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
>> ret = collapse_huge_page(mm, collapse_address, referenced,
>> unmapped, cc, mmap_locked,
>> order);
>> - if (ret == SCAN_SUCCEED) {
>> +
>> + switch (ret) {
>> + /* Cases were we continue to next collapse candidate */
>> + case SCAN_SUCCEED:
>> collapsed += nr_pte_entries;
>> + 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;
>> + /* Cases were no further collapse is possible */
>> + case SCAN_CGROUP_CHARGE_FAIL:
>> + case SCAN_COPY_MC:
>> + case SCAN_ADDRESS_RANGE:
>> + case SCAN_NO_PTE_TABLE:
>> + case SCAN_ANY_PROCESS:
>> + case SCAN_VMA_NULL:
>> + case SCAN_VMA_CHECK:
>> + case SCAN_SCAN_ABORT:
>> + case SCAN_PAGE_ANON:
>> + case SCAN_PMD_MAPPED:
>> + case SCAN_FAIL:
>> + default:
>
> Agree with david, let's spell them out please :)

I believe David is arguing for the opposite. To drop all these spelt out cases
and just leave the default case.

@david is that correct or did I misunderstand that.

-- Nico

>
>> + return collapsed;
>> }
>> }
>>
>> --
>> 2.53.0
>>
>