Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[Ryan Roberts]

On 11/04/2024 13:23, Lance Yang wrote:
> On Thu, Apr 11, 2024 at 7:27 PM Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx
>>>>> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@xxxxxxxxxx
>>>>>
>>>>> Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
>>>>> ---
>>>>>   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.

> 
> Thanks,
> Lance