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Date: Thu, 11 Apr 2024 15:55:08 +0200
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Subject: Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in
 madvise_free
To: Ryan Roberts <ryan.roberts@arm.com>, Lance Yang <ioworker0@gmail.com>
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From: David Hildenbrand <david@redhat.com>
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On 11.04.24 15:51, Ryan Roberts wrote:
> On 11/04/2024 13:23, Lance Yang wrote:
>> On Thu, Apr 11, 2024 at 7:27 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>
>>> On 11/04/2024 12:20, David Hildenbrand wrote:
>>>> On 11.04.24 13:11, Ryan Roberts wrote:
>>>>> On 08/04/2024 05:24, Lance Yang wrote:
>>>>>> This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
>>>>>> (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
>>>>>> splitting if the large folio is fully mapped within the target range.
>>>>>>
>>>>>> If a large folio is locked or shared, or if we fail to split it, we just
>>>>>> leave it in place and advance to the next PTE in the range. But note that
>>>>>> the behavior is changed; previously, any failure of this sort would cause
>>>>>> the entire operation to give up. As large folios become more common,
>>>>>> sticking to the old way could result in wasted opportunities.
>>>>>>
>>>>>> On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
>>>>>> the same size results in the following runtimes for madvise(MADV_FREE) in
>>>>>> seconds (shorter is better):
>>>>>>
>>>>>> Folio Size |   Old    |   New    | Change
>>>>>> ------------------------------------------
>>>>>>         4KiB | 0.590251 | 0.590259 |    0%
>>>>>>        16KiB | 2.990447 | 0.185655 |  -94%
>>>>>>        32KiB | 2.547831 | 0.104870 |  -95%
>>>>>>        64KiB | 2.457796 | 0.052812 |  -97%
>>>>>>       128KiB | 2.281034 | 0.032777 |  -99%
>>>>>>       256KiB | 2.230387 | 0.017496 |  -99%
>>>>>>       512KiB | 2.189106 | 0.010781 |  -99%
>>>>>>      1024KiB | 2.183949 | 0.007753 |  -99%
>>>>>>      2048KiB | 0.002799 | 0.002804 |    0%
>>>>>>
>>>>>> [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@arm.com
>>>>>> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
>>>>>>
>>>>>> Signed-off-by: Lance Yang <ioworker0@gmail.com>
>>>>>> ---
>>>>>>    include/linux/pgtable.h |  34 +++++++++
>>>>>>    mm/internal.h           |  12 +++-
>>>>>>    mm/madvise.c            | 149 ++++++++++++++++++++++------------------
>>>>>>    mm/memory.c             |   4 +-
>>>>>>    4 files changed, 129 insertions(+), 70 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>> index 0f4b2faa1d71..4dd442787420 100644
>>>>>> --- a/include/linux/pgtable.h
>>>>>> +++ b/include/linux/pgtable.h
>>>>>> @@ -489,6 +489,40 @@ static inline pte_t ptep_get_and_clear(struct mm_struct
>>>>>> *mm,
>>>>>>    }
>>>>>>    #endif
>>>>>>    +#ifndef mkold_clean_ptes
>>>>>> +/**
>>>>>> + * mkold_clean_ptes - Mark PTEs that map consecutive pages of the same folio
>>>>>> + *        as old and clean.
>>>>>> + * @mm: Address space the pages are mapped into.
>>>>>> + * @addr: Address the first page is mapped at.
>>>>>> + * @ptep: Page table pointer for the first entry.
>>>>>> + * @nr: Number of entries to mark old and clean.
>>>>>> + *
>>>>>> + * May be overridden by the architecture; otherwise, implemented by
>>>>>> + * get_and_clear/modify/set for each pte in the range.
>>>>>> + *
>>>>>> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
>>>>>> + * some PTEs might be write-protected.
>>>>>> + *
>>>>>> + * Context: The caller holds the page table lock.  The PTEs map consecutive
>>>>>> + * pages that belong to the same folio.  The PTEs are all in the same PMD.
>>>>>> + */
>>>>>> +static inline void mkold_clean_ptes(struct mm_struct *mm, unsigned long addr,
>>>>>> +                    pte_t *ptep, unsigned int nr)
>>>>>
>>
>> Thanks for the suggestions, Ryan, David!
>>
>>>>> Just thinking out loud, I wonder if it would be cleaner to convert mkold_ptes()
>>>>> (which I added as part of swap-out) to something like:
>>
>> Yeah, this is definitely cleaner than before.
>>
>>>>>
>>>>> clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr,
>>>>>                 pte_t *ptep, unsigned int nr,
>>>>>                 bool clear_young, bool clear_dirty);
>>>>>
>>>>> Then we can use the same function for both use cases and also have the ability
>>>>> to only clear dirty in future if we ever need it. The other advantage is that we
>>>>> only need to plumb a single function down the arm64 arch code. As it currently
>>>>> stands, those 2 functions would be duplicating most of their code.
>>
>> Agreed. It's indeed a good idea to use a single function for both use cases.
>>
>>>>
>>>> Yes. Maybe better use proper __bitwise flags, the compiler should be smart
>>>> enough to optimize either way.
>>
>> Nice. I'll use the __bitwise flags as the input.
>>
>>>
>>> Agreed. I was also thinking perhaps it makes sense to start using output bitwise
>>> flags for folio_pte_batch() since this patch set takes us up to 3 optional bool
>>> pointers for different things. Might be cleaner to have input flags to tell it
>>> what we care about and output flags to highlight those things. I guess the
>>> compiler should be able to optimize in the same way.
>>>
>>
>> Should I start using output bitwise flags for folio_pte_batch() in
>> this patch set?
> 
> I don't think its crucial (yet). I'd leave it as you have done it for now,
> unless David shouts.

Let's do that separately, and investigate if the compiler actually is 
smart enough ... :)

-- 
Cheers,

David / dhildenb