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Date: Thu, 11 Apr 2024 13:20:11 +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>,
 akpm@linux-foundation.org
Cc: 21cnbao@gmail.com, mhocko@suse.com, fengwei.yin@intel.com,
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 wangkefeng.wang@huawei.com, songmuchun@bytedance.com, peterx@redhat.com,
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References: <20240408042437.10951-1-ioworker0@gmail.com>
 <20240408042437.10951-2-ioworker0@gmail.com>
 <38c4add8-53a2-49ca-9f1b-f62c2ee3e764@arm.com>
From: David Hildenbrand <david@redhat.com>
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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)
> 
> 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:
> 
> 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.

Yes. Maybe better use proper __bitwise flags, the compiler should be 
smart enough to optimize either way.

-- 
Cheers,

David / dhildenb