Re: [PATCH mm-unstable v18 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[David Hildenbrand (Arm)]

On 5/31/26 09:31, Lance Yang wrote:
> 
> On Fri, May 22, 2026 at 09:00:07AM -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.
>>
>> Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
>> Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
>> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
>> ---
>> 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 :)

I think the reasoning was here, that if we are already that close to our mem
limit, we should just give up instead of trying to squeeze it in .. :)

-- 
Cheers,

David