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

[David Hildenbrand (Arm)]

On 3/18/26 19:59, Nico Pache wrote:
> 
> 
> 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.

Either spell all out (no default) OR add a default.

I prefer to just ... use the default :)

-- 
Cheers,

David