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

[Lance Yang <lance.yang@linux.dev>]

On 2026/6/1 04:02, David Hildenbrand (Arm) wrote:
> 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@kernel.org>
>>> Acked-by: Usama Arif <usama.arif@linux.dev>
>>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>>> Signed-off-by: Nico Pache <npache@redhat.com>
>>> ---
>>> 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 .. :)

Fair point. Just a nit, nevermind :)