Re: [PATCH 2/3] mm: Add generic helper to hint a large folio

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

On 07/05/25 3:33 pm, David Hildenbrand wrote:
> On 06.05.25 07:00, Dev Jain wrote:
>> To use PTE batching, we want to determine whether the folio mapped by
>> the PTE is large, thus requiring the use of vm_normal_folio(). We want
>> to avoid the cost of vm_normal_folio() if the code path doesn't already
>> require the folio. For arm64, pte_batch_hint() does the job. To 
>> generalize
>> this hint, add a helper which will determine whether two consecutive PTEs
>> point to consecutive PFNs, in which case there is a high probability that
>> the underlying folio is large.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   include/linux/pgtable.h | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index b50447ef1c92..28e21fcc7837 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -369,6 +369,22 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
>>   }
>>   #endif
>> +/* Caller must ensure that ptep + 1 exists */
>> +static inline bool maybe_contiguous_pte_pfns(pte_t *ptep, pte_t pte)
>> +{
>> +    pte_t *next_ptep, next_pte;
>> +
>> +    if (pte_batch_hint(ptep, pte) != 1)
>> +        return true;
>> +
>> +    next_ptep = ptep + 1;
>> +    next_pte = ptep_get(next_ptep);
>> +    if (!pte_present(next_pte))
>> +        return false;
>> +
>> +    return unlikely(pte_pfn(next_pte) - pte_pfn(pte) == PAGE_SIZE);
>> +}
> 
> So, where we want to use that is:
> 
> if (pte_present(old_pte)) {
>      if ((max_nr != 1) && maybe_contiguous_pte_pfns(old_ptep, old_pte)) {
>          struct folio *folio = vm_normal_folio(vma, old_addr, old_pte);
> 
>          if (folio && folio_test_large(folio))
>              nr = folio_pte_batch(folio, old_addr, old_ptep,
>                           old_pte, max_nr, fpb_flags, NULL, NULL, NULL);
>      }
> }
> 
> where we won't need the folio later. But want it all part of the same 
> folio?
> 
> 
> And the simpler version would be
> 
> 
> if (pte_present(old_pte)) {
>      if (max_nr != 1) {
>          struct folio *folio = vm_normal_folio(vma, old_addr, old_pte);
> 
>          if (folio && folio_test_large(folio))
>              nr = folio_pte_batch(folio, old_addr, old_ptep,
>                           old_pte, max_nr, fpb_flags, NULL, NULL, NULL);
>      }
> }
> 
> 
> Two things come to mind:
> 
> (1) Do we *really* care about the vm_normal_folio() + folio_test_large() 
> call that much, that you
> have to add this optimization ahead of times ? :)

For my mprotect series, I see a regression of almost (7.7 - 7.65)/7.7 = 
0.65% for the small folio case. I am happy to remove this 
micro-optimization if that is the preference.

> 
> (2) Do we really need "must be part of the same folio", or could be just 
> batch over present
> ptes that map consecutive PFNs? In that case, a helper that avoids 
> folio_pte_batch() completely
> might be better.
> 
I am not sure I get you here. folio_pte_batch() seems to be the simplest 
thing we can do as being done around in the code elsewhere, I am not 
aware of any alternate.