Re: [PATCH mm-new 2/2] mm/khugepaged: merge PTE scanning logic into a new helper

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

On 2025/10/4 01:11, Zi Yan wrote:
> On 2 Oct 2025, at 3:32, Lance Yang wrote:
> 
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd()
>> and __collapse_huge_page_isolate() was almost duplicated.
>>
>> This patch cleans things up by moving all the common PTE checking logic
>> into a new shared helper, thp_collapse_check_pte().
>>
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/khugepaged.c | 167 ++++++++++++++++++++++++++++++------------------
>>   1 file changed, 104 insertions(+), 63 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 808523f92c7b..2a897cfb1d03 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -61,6 +61,12 @@ enum scan_result {
>>   	SCAN_PAGE_FILLED,
>>   };
>>
>> +enum pte_check_result {
>> +	PTE_CHECK_SUCCEED,
>> +	PTE_CHECK_CONTINUE,
>> +	PTE_CHECK_FAIL,
>> +};
>> +
>>   #define CREATE_TRACE_POINTS
>>   #include <trace/events/huge_memory.h>
>>
>> @@ -533,6 +539,87 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
>>   	}
>>   }
>>
>> +/*
>> + * thp_collapse_check_pte - Check if a PTE is suitable for THP collapse
>> + * @pte:           PTE to check
>> + * @vma:           VMA the PTE belongs to
>> + * @cc:            Collapse control settings
>> + * @scan_swap_pte: Allow scanning of swap PTEs if true
>> + * @none_or_zero:  Counter for none/zero PTEs (must be non-NULL)
>> + * @unmapped:      Counter for swap PTEs (must be non-NULL if scan_swap_pte
>> + *                 is true)
>> + * @scan_result:   Used to return the failure reason (SCAN_*) on a
>> + *                 PTE_CHECK_FAIL return. Must be non-NULL
>> + *
>> + * Returns:
>> + *   PTE_CHECK_SUCCEED  - Valid PTE, proceed with collapse
>> + *   PTE_CHECK_CONTINUE - Skip this none/zero PTE but continue scanning
>> + *   PTE_CHECK_FAIL     - Abort collapse scan
>> + */
>> +static inline int thp_collapse_check_pte(pte_t pte, struct vm_area_struct *vma,
>> +		struct collapse_control *cc, bool scan_swap_pte,
>> +		int *none_or_zero, int *unmapped, int *scan_result)
>> +{
>> +	VM_BUG_ON(!none_or_zero || !scan_result);
>> +	VM_BUG_ON(scan_swap_pte && !unmapped);
> 
> Please use VM_WARN_ONCE_ON and just return. Or put all stats in a struct,
> pass and check the struct pointer instead.

Yes, will use VM_WARN_ONCE_ON instead of VM_BUG_ON, and just return 
PTE_CHECK_FAIL.

Thanks,
Lance

> 
> 
> Best Regards,
> Yan, Zi