Re: [PATCH v3 1/5] mm: Optimize mprotect() by batch-skipping PTEs

[Dev Jain <dev.jain@arm.com>]

On 21/05/25 5:28 pm, Ryan Roberts wrote:
> On 19/05/2025 08:48, Dev Jain wrote:
>> In case of prot_numa, there are various cases in which we can skip to the
>> next iteration. Since the skip condition is based on the folio and not
>> the PTEs, we can skip a PTE batch.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   mm/mprotect.c | 36 +++++++++++++++++++++++++++++-------
>>   1 file changed, 29 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 88608d0dc2c2..1ee160ed0b14 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -83,6 +83,18 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>   	return pte_dirty(pte);
>>   }
>>   
>> +static int mprotect_batch(struct folio *folio, unsigned long addr, pte_t *ptep,
> Perhaps it should be called mprotect_folio_pte_batch() to match the existing
> madvise_folio_pte_batch()?


Thanks, this is better.


>
>> +		pte_t pte, int max_nr_ptes)
>> +{
>> +	const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +
>> +	if (!folio_test_large(folio) || (max_nr_ptes == 1))
>> +		return 1;
>> +
>> +	return folio_pte_batch(folio, addr, ptep, pte, max_nr_ptes, flags,
>> +			       NULL, NULL, NULL);
>> +}
>> +
>>   static long change_pte_range(struct mmu_gather *tlb,
>>   		struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
>>   		unsigned long end, pgprot_t newprot, unsigned long cp_flags)
>> @@ -94,6 +106,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
>>   	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
>>   	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
>> +	int nr_ptes;
>>   
>>   	tlb_change_page_size(tlb, PAGE_SIZE);
>>   	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>> @@ -108,8 +121,10 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   	flush_tlb_batched_pending(vma->vm_mm);
>>   	arch_enter_lazy_mmu_mode();
>>   	do {
>> +		nr_ptes = 1;
>>   		oldpte = ptep_get(pte);
>>   		if (pte_present(oldpte)) {
>> +			int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
>>   			pte_t ptent;
>>   
>>   			/*
>> @@ -126,15 +141,18 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   					continue;
>>   
>>   				folio = vm_normal_folio(vma, addr, oldpte);
>> -				if (!folio || folio_is_zone_device(folio) ||
>> -				    folio_test_ksm(folio))
>> +				if (!folio)
>>   					continue;
> You modify mprotect_batch() to handle folio == NULL later, perhaps just add that
> here, then you don't need to unpick this conditional and can just goto
> skip_branch, even for the !folio case.


I'll check this.


>
> Thanks,
> Ryan
>
>>   
>> +				if (folio_is_zone_device(folio) ||
>> +				    folio_test_ksm(folio))
>> +					goto skip_batch;
>> +
>>   				/* Also skip shared copy-on-write pages */
>>   				if (is_cow_mapping(vma->vm_flags) &&
>>   				    (folio_maybe_dma_pinned(folio) ||
>>   				     folio_maybe_mapped_shared(folio)))
>> -					continue;
>> +					goto skip_batch;
>>   
>>   				/*
>>   				 * While migration can move some dirty pages,
>> @@ -143,7 +161,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   				 */
>>   				if (folio_is_file_lru(folio) &&
>>   				    folio_test_dirty(folio))
>> -					continue;
>> +					goto skip_batch;
>>   
>>   				/*
>>   				 * Don't mess with PTEs if page is already on the node
>> @@ -151,7 +169,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   				 */
>>   				nid = folio_nid(folio);
>>   				if (target_node == nid)
>> -					continue;
>> +					goto skip_batch;
>>   				toptier = node_is_toptier(nid);
>>   
>>   				/*
>> @@ -159,8 +177,12 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   				 * balancing is disabled
>>   				 */
>>   				if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_NORMAL) &&
>> -				    toptier)
>> +				    toptier) {
>> +skip_batch:
>> +					nr_ptes = mprotect_batch(folio, addr, pte,
>> +								 oldpte, max_nr_ptes);
>>   					continue;
>> +				}
>>   				if (folio_use_access_time(folio))
>>   					folio_xchg_access_time(folio,
>>   						jiffies_to_msecs(jiffies));
>> @@ -280,7 +302,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   				pages++;
>>   			}
>>   		}
>> -	} while (pte++, addr += PAGE_SIZE, addr != end);
>> +	} while (pte += nr_ptes, addr += nr_ptes * PAGE_SIZE, addr != end);
>>   	arch_leave_lazy_mmu_mode();
>>   	pte_unmap_unlock(pte - 1, ptl);
>>