linux-kernel - Re: [PATCH v2 3/5] mm: Default implementation of arch_wants_pte

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Message-ID: <dd9ea461-df2d-afe1-a67c-c73ac1cb96b4@arm.com>
Date:   Tue, 4 Jul 2023 14:23:20 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yu Zhao <yuzhao@...gle.com>, "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 3/5] mm: Default implementation of
 arch_wants_pte_order()

On 04/07/2023 13:36, Ryan Roberts wrote:
> On 04/07/2023 04:59, Yu Zhao wrote:
>> On Mon, Jul 3, 2023 at 9:02 PM Yu Zhao <yuzhao@...gle.com> wrote:
>>>
>>> On Mon, Jul 3, 2023 at 8:23 PM Yin, Fengwei <fengwei.yin@...el.com> 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@....com>
>>>>> ---
>>>>>  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?

Actually, I'm not sure its a good idea to default to a fixed order. If running
on an arch with big base pages (e.g. powerpc with 64K pages?), that will soon
add up to a big chunk of memory, which could be wasteful?

PAGE_ALLOC_COSTLY_ORDER = 3 so with 64K base page, that 512K. Is that a concern?
Wouldn't it be better to define this as an absolute size? Or even the min of
PAGE_ALLOC_COSTLY_ORDER and an absolute size?


> 
> 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.
>