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

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

On 21/05/25 5:36 pm, David Hildenbrand wrote:
> On 19.05.25 09:48, Dev Jain wrote:
>
> Please highlight in the subject that this is only about 
> MM_CP_PROT_NUMA handling.


Sure.


>
>> 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,
>> +        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;
>>   +                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;
>> +                }
>
>
> I suggest
>
> a) not burying that skip_batch label in another if condition
>
> b) looking into factoring out prot_numa handling into a separate 
> function first.
>
>
> Likely we want something like
>
> if (prot_numa) {
>     nr_ptes = prot_numa_pte_range_skip_ptes(vma, addr, oldpte);
>     if (nr_ptes)
>         continue;
> }
>
> ... likely with a better function name,


I want to be able to reuse the folio from vm_normal_folio(), and we also 
need

nr_ptes to know how much to skip, so if there is no objection in passing 
int *nr_ptes,

or struct folio **foliop to this new function, then I'll carry on with 
your suggestion :)


>
>