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

[Nico Pache]

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@xxxxxxxxxx>
> 
> With David's concern addressed:
> 
> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>
> 
>> ---
>>  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
>>
>