Re: [PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

[Ryan Roberts]

On 04/07/2023 04:59, Yu Zhao wrote:
> On Mon, Jul 3, 2023 at 9:02 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>
>> On Mon, Jul 3, 2023 at 8:23 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>
>>>
>>>
>>> On 7/3/2023 9:53 PM, Ryan Roberts wrote:
>>>> arch_wants_pte_order() can be overridden by the arch to return the
>>>> preferred folio order for pte-mapped memory. This is useful as some
>>>> architectures (e.g. arm64) can coalesce TLB entries when the physical
>>>> memory is suitably contiguous.
>>>>
>>>> The first user for this hint will be FLEXIBLE_THP, which aims to
>>>> allocate large folios for anonymous memory to reduce page faults and
>>>> other per-page operation costs.
>>>>
>>>> Here we add the default implementation of the function, used when the
>>>> architecture does not define it, which returns the order corresponding
>>>> to 64K.
>>>>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>> ---
>>>>  include/linux/pgtable.h | 13 +++++++++++++
>>>>  1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>> index a661a17173fa..f7e38598f20b 100644
>>>> --- a/include/linux/pgtable.h
>>>> +++ b/include/linux/pgtable.h
>>>> @@ -13,6 +13,7 @@
>>>>  #include <linux/errno.h>
>>>>  #include <asm-generic/pgtable_uffd.h>
>>>>  #include <linux/page_table_check.h>
>>>> +#include <linux/sizes.h>
>>>>
>>>>  #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
>>>>       defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
>>>> @@ -336,6 +337,18 @@ static inline bool arch_has_hw_pte_young(void)
>>>>  }
>>>>  #endif
>>>>
>>>> +#ifndef arch_wants_pte_order
>>>> +/*
>>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
>>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
>>>> + * to be at least order-2.
>>>> + */
>>>> +static inline int arch_wants_pte_order(struct vm_area_struct *vma)
>>>> +{
>>>> +     return ilog2(SZ_64K >> PAGE_SHIFT);
>>> Default value which is not related with any silicon may be: PAGE_ALLOC_COSTLY_ORDER?
>>>
>>> Also, current pcp list support cache page with order 0...PAGE_ALLOC_COSTLY_ORDER, 9.
>>> If the pcp could cover the page, the pressure to zone lock will be reduced by pcp.
>>
>> The value of PAGE_ALLOC_COSTLY_ORDER is reasonable but again it's a
>> s/w policy not a h/w preference. Besides, I don't think we can include
>> mmzone.h in pgtable.h.
> 
> I think we can make a compromise:
> 1. change the default implementation of arch_has_hw_pte_young() to return 0, and
> 2. in memory.c, we can try PAGE_ALLOC_COSTLY_ORDER for archs that
> don't override arch_has_hw_pte_young(), or if its return value is too
> large to fit.
> This should also take care of the regression, right?

I think you are suggesting that we use 0 as a sentinel which we then translate
to PAGE_ALLOC_COSTLY_ORDER? I already have a max_anon_folio_order() function in
memory.c (actually it is currently a macro defined as arch_wants_pte_order()).

So it would become (I'll talk about the vma concern separately in the thread
where you raised it):

static inline int max_anon_folio_order(struct vm_area_struct *vma)
{
	int order = arch_wants_pte_order(vma);

	return order ? order : PAGE_ALLOC_COSTLY_ORDER;
}

Correct?

I don't see how it fixes the regression (assume you're talking about
Speedometer) though? On arm64 arch_wants_pte_order() will still be returning
order-4.