Re: [PATCH v5 22/25] mm: Add pte_batch_hint() to reduce scanning in folio_pte_batch()

[Ryan Roberts]

On 12/02/2024 13:43, David Hildenbrand wrote:
> On 02.02.24 09:07, Ryan Roberts wrote:
>> Some architectures (e.g. arm64) can tell from looking at a pte, if some
>> follow-on ptes also map contiguous physical memory with the same pgprot.
>> (for arm64, these are contpte mappings).
>>
>> Take advantage of this knowledge to optimize folio_pte_batch() so that
>> it can skip these ptes when scanning to create a batch. By default, if
>> an arch does not opt-in, folio_pte_batch() returns a compile-time 1, so
>> the changes are optimized out and the behaviour is as before.
>>
>> arm64 will opt-in to providing this hint in the next patch, which will
>> greatly reduce the cost of ptep_get() when scanning a range of contptes.
>>
>> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>   include/linux/pgtable.h | 18 ++++++++++++++++++
>>   mm/memory.c             | 20 +++++++++++++-------
>>   2 files changed, 31 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index 50f32cccbd92..cba31f177d27 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -212,6 +212,24 @@ static inline int pmd_dirty(pmd_t pmd)
>>   #define arch_flush_lazy_mmu_mode()    do {} while (0)
>>   #endif
>>   +#ifndef pte_batch_hint
>> +/**
>> + * pte_batch_hint - Number of pages that can be added to batch without scanning.
>> + * @ptep: Page table pointer for the entry.
>> + * @pte: Page table entry.
>> + *
>> + * Some architectures know that a set of contiguous ptes all map the same
>> + * contiguous memory with the same permissions. In this case, it can provide a
>> + * hint to aid pte batching without the core code needing to scan every pte.
> 
> I think we might want to document here the expectation regarding
> dirty/accessed bits. folio_pte_batch() will ignore dirty bits only with
> FPB_IGNORE_DIRTY. But especially for arm64, it makes sense to ignore them
> always when batching, because the dirty bit may target any pte part of the
> cont-pte group either way.
> 
> Maybe something like:
> 
> "
> An architecture implementation may only ignore the PTE accessed and dirty bits.
> Further, it may only ignore the dirty bit if that bit is already not
> maintained with precision per PTE inside the hinted batch, and ptep_get()
> would already have to collect it from various PTEs.
> "

I'm proposing to simplify this to:

"
An architecture implementation may ignore the PTE accessed state. Further, the
dirty state must apply atomically to all the PTEs described by the hint.
"

Which I think more accurately describes the requirement. Shout if you disagree.

> 
> I think there are some more details to it, but I'm hoping something along
> the lines above is sufficient.
> 
> 
>> +
>>   #ifndef pte_advance_pfn
>>   static inline pte_t pte_advance_pfn(pte_t pte, unsigned long nr)
>>   {
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 65fbe4f886c1..902665b27702 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -988,16 +988,21 @@ static inline int folio_pte_batch(struct folio *folio,
>> unsigned long addr,
>>   {
>>       unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
>>       const pte_t *end_ptep = start_ptep + max_nr;
>> -    pte_t expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, 1),
>> flags);
>> -    pte_t *ptep = start_ptep + 1;
>> +    pte_t expected_pte = __pte_batch_clear_ignored(pte, flags);
>> +    pte_t *ptep = start_ptep;
>>       bool writable;
>> +    int nr;
>>         if (any_writable)
>>           *any_writable = false;
>>         VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>>   -    while (ptep != end_ptep) {
>> +    nr = pte_batch_hint(ptep, pte);
>> +    expected_pte = pte_advance_pfn(expected_pte, nr);
>> +    ptep += nr;
>> +
> 
> *Maybe* it's easier to get when initializing expected_pte+ptep only once.
> 
> Like:
> 
> [...]
> pte_t expected_pte, *ptep;
> [...]
> 
> nr = pte_batch_hint(start_ptep, pte);
> expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
> ptep = start_ptep + nr;
> 
>> +    while (ptep < end_ptep) {
>>           pte = ptep_get(ptep);
>>           if (any_writable)
>>               writable = !!pte_write(pte);
>> @@ -1011,17 +1016,18 @@ static inline int folio_pte_batch(struct folio *folio,
>> unsigned long addr,
>>            * corner cases the next PFN might fall into a different
>>            * folio.
>>            */
>> -        if (pte_pfn(pte) == folio_end_pfn)
>> +        if (pte_pfn(pte) >= folio_end_pfn)
>>               break;
>>             if (any_writable)
>>               *any_writable |= writable;
>>   -        expected_pte = pte_advance_pfn(expected_pte, 1);
>> -        ptep++;
>> +        nr = pte_batch_hint(ptep, pte);
>> +        expected_pte = pte_advance_pfn(expected_pte, nr);
>> +        ptep += nr;
>>       }
>>   -    return ptep - start_ptep;
>> +    return min(ptep - start_ptep, max_nr);
>>   }
> 
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>