Re: [PATCH v3 2/3] mm: mprotect: avoid unnecessary struct page accessing if pte_protnone()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2025/10/16 9:28, Zi Yan wrote:
> On 15 Oct 2025, at 8:35, Kefeng Wang wrote:
> 
>> If the pte_protnone() is true, we could avoid unnecessary struct page
>> accessing and reduce cache footprint when scanning page tables for prot
>> numa, the performance test of pmbench memory accessing benchmark
>> should be benifit, see more commit a818f5363a0e ("autonuma: reduce cache
>> footprint when scanning page tables").
>>
>> Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/mprotect.c | 30 +++++++++++++-----------------
>>   1 file changed, 13 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index bb59a42809b8..ed44aadb7aaa 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -118,18 +118,13 @@ static int mprotect_folio_pte_batch(struct folio *folio, pte_t *ptep,
>>   	return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr_ptes, flags);
>>   }
>>
>> -static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
>> -			   pte_t oldpte, pte_t *pte, int target_node,
>> -			   struct folio *folio)
>> +static bool prot_numa_skip(struct vm_area_struct *vma, int target_node,
>> +		struct folio *folio)
>>   {
>>   	bool ret = true;
>>   	bool toptier;
>>   	int nid;
>>
>> -	/* Avoid TLB flush if possible */
>> -	if (pte_protnone(oldpte))
>> -		goto skip;
>> -
>>   	if (!folio)
>>   		goto skip;
>>
>> @@ -307,23 +302,24 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   			struct page *page;
>>   			pte_t ptent;
>>
>> +			/* Already in the desired state. */
>> +			if (prot_numa && pte_protnone(oldpte))
>> +				continue;
> 
> For a folio pte batch, instead of calculating nr_ptes to skip, this just
> skip PTEs one by one. Looking at folio_pte_batch_flags(), this way looks
> better.

folio_pte_batch_flags need a folio, we want to avoid it,
maybe use pte_batch_hint() here.

> 
>> +
>>   			page = vm_normal_page(vma, addr, oldpte);
>>   			if (page)
>>   				folio = page_folio(page);
>> +
>>   			/*
>>   			 * Avoid trapping faults against the zero or KSM
>>   			 * pages. See similar comment in change_huge_pmd.
>>   			 */
>> -			if (prot_numa) {
>> -				int ret = prot_numa_skip(vma, addr, oldpte, pte,
>> -							 target_node, folio);
>> -				if (ret) {
>> -
>> -					/* determine batch to skip */
>> -					nr_ptes = mprotect_folio_pte_batch(folio,
>> -						  pte, oldpte, max_nr_ptes, /* flags = */ 0);
>> -					continue;
>> -				}
>> +			if (prot_numa && prot_numa_skip(vma, target_node,
>> +							folio)) {
>> +				/* determine batch to skip */
>> +				nr_ptes = mprotect_folio_pte_batch(folio,
>> +					  pte, oldpte, max_nr_ptes, /* flags = */ 0);
>> +				continue;
>>   			}
>>
>>   			nr_ptes = mprotect_folio_pte_batch(folio, pte, oldpte, max_nr_ptes, flags);
> 
> Acked-by: Zi Yan <ziy@nvidia.com>

Thanks.
> 
> --
> Best Regards,
> Yan, Zi
>