Re: [PATCH v4 02/16] mm: Batch-copy PTE ranges during fork()

[Ryan Roberts <ryan.roberts@arm.com>]

On 20/12/2023 13:13, David Hildenbrand wrote:
> On 20.12.23 14:10, Ryan Roberts wrote:
>> On 20/12/2023 13:06, David Hildenbrand wrote:
>>> On 20.12.23 13:04, Ryan Roberts wrote:
>>>> On 20/12/2023 11:58, David Hildenbrand wrote:
>>>>> On 20.12.23 12:51, Ryan Roberts wrote:
>>>>>> On 20/12/2023 11:36, David Hildenbrand wrote:
>>>>>>> On 20.12.23 12:28, Ryan Roberts wrote:
>>>>>>>> On 20/12/2023 10:56, David Hildenbrand wrote:
>>>>>>>>> On 20.12.23 11:41, Ryan Roberts wrote:
>>>>>>>>>> On 20/12/2023 10:16, David Hildenbrand wrote:
>>>>>>>>>>> On 20.12.23 11:11, Ryan Roberts wrote:
>>>>>>>>>>>> On 20/12/2023 09:54, David Hildenbrand wrote:
>>>>>>>>>>>>> On 20.12.23 10:51, Ryan Roberts wrote:
>>>>>>>>>>>>>> On 20/12/2023 09:17, David Hildenbrand wrote:
>>>>>>>>>>>>>>> On 19.12.23 18:42, Ryan Roberts wrote:
>>>>>>>>>>>>>>>> On 19/12/2023 17:22, David Hildenbrand wrote:
>>>>>>>>>>>>>>>>> On 19.12.23 09:30, Ryan Roberts wrote:
>>>>>>>>>>>>>>>>>> On 18/12/2023 17:47, David Hildenbrand wrote:
>>>>>>>>>>>>>>>>>>> On 18.12.23 11:50, Ryan Roberts wrote:
>>>>>>>>>>>>>>>>>>>> Convert copy_pte_range() to copy a batch of ptes in one go. A
>>>>>>>>>>>>>>>>>>>> given
>>>>>>>>>>>>>>>>>>>> batch is determined by the architecture with the new helper,
>>>>>>>>>>>>>>>>>>>> pte_batch_remaining(), and maps a physically contiguous
>>>>>>>>>>>>>>>>>>>> block of
>>>>>>>>>>>>>>>>>>>> memory,
>>>>>>>>>>>>>>>>>>>> all belonging to the same folio. A pte batch is then
>>>>>>>>>>>>>>>>>>>> write-protected in
>>>>>>>>>>>>>>>>>>>> one go in the parent using the new helper,
>>>>>>>>>>>>>>>>>>>> ptep_set_wrprotects()
>>>>>>>>>>>>>>>>>>>> and is
>>>>>>>>>>>>>>>>>>>> set in one go in the child using the new helper,
>>>>>>>>>>>>>>>>>>>> set_ptes_full().
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> The primary motivation for this change is to reduce the number
>>>>>>>>>>>>>>>>>>>> of tlb
>>>>>>>>>>>>>>>>>>>> maintenance operations that the arm64 backend has to perform
>>>>>>>>>>>>>>>>>>>> during
>>>>>>>>>>>>>>>>>>>> fork, as it is about to add transparent support for the
>>>>>>>>>>>>>>>>>>>> "contiguous
>>>>>>>>>>>>>>>>>>>> bit"
>>>>>>>>>>>>>>>>>>>> in its ptes. By write-protecting the parent using the new
>>>>>>>>>>>>>>>>>>>> ptep_set_wrprotects() (note the 's' at the end) function, the
>>>>>>>>>>>>>>>>>>>> backend
>>>>>>>>>>>>>>>>>>>> can avoid having to unfold contig ranges of PTEs, which is
>>>>>>>>>>>>>>>>>>>> expensive,
>>>>>>>>>>>>>>>>>>>> when all ptes in the range are being write-protected.
>>>>>>>>>>>>>>>>>>>> Similarly, by
>>>>>>>>>>>>>>>>>>>> using set_ptes_full() rather than set_pte_at() to set up
>>>>>>>>>>>>>>>>>>>> ptes in
>>>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>>> child, the backend does not need to fold a contiguous range
>>>>>>>>>>>>>>>>>>>> once
>>>>>>>>>>>>>>>>>>>> they
>>>>>>>>>>>>>>>>>>>> are all populated - they can be initially populated as a
>>>>>>>>>>>>>>>>>>>> contiguous
>>>>>>>>>>>>>>>>>>>> range in the first place.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> This code is very performance sensitive, and a significant
>>>>>>>>>>>>>>>>>>>> amount of
>>>>>>>>>>>>>>>>>>>> effort has been put into not regressing performance for the
>>>>>>>>>>>>>>>>>>>> order-0
>>>>>>>>>>>>>>>>>>>> folio case. By default, pte_batch_remaining() is compile
>>>>>>>>>>>>>>>>>>>> constant 1,
>>>>>>>>>>>>>>>>>>>> which enables the compiler to simplify the extra loops that are
>>>>>>>>>>>>>>>>>>>> added
>>>>>>>>>>>>>>>>>>>> for batching and produce code that is equivalent (and equally
>>>>>>>>>>>>>>>>>>>> performant) as the previous implementation.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> This change addresses the core-mm refactoring only and a
>>>>>>>>>>>>>>>>>>>> separate
>>>>>>>>>>>>>>>>>>>> change
>>>>>>>>>>>>>>>>>>>> will implement pte_batch_remaining(), ptep_set_wrprotects() and
>>>>>>>>>>>>>>>>>>>> set_ptes_full() in the arm64 backend to realize the performance
>>>>>>>>>>>>>>>>>>>> improvement as part of the work to enable contpte mappings.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> To ensure the arm64 is performant once implemented, this
>>>>>>>>>>>>>>>>>>>> change is
>>>>>>>>>>>>>>>>>>>> very
>>>>>>>>>>>>>>>>>>>> careful to only call ptep_get() once per pte batch.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> The following microbenchmark results demonstate that there
>>>>>>>>>>>>>>>>>>>> is no
>>>>>>>>>>>>>>>>>>>> significant performance change after this patch. Fork is called
>>>>>>>>>>>>>>>>>>>> in a
>>>>>>>>>>>>>>>>>>>> tight loop in a process with 1G of populated memory and the
>>>>>>>>>>>>>>>>>>>> time
>>>>>>>>>>>>>>>>>>>> for
>>>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>>>> function to execute is measured. 100 iterations per run, 8 runs
>>>>>>>>>>>>>>>>>>>> performed on both Apple M2 (VM) and Ampere Altra (bare metal).
>>>>>>>>>>>>>>>>>>>> Tests
>>>>>>>>>>>>>>>>>>>> performed for case where 1G memory is comprised of order-0
>>>>>>>>>>>>>>>>>>>> folios and
>>>>>>>>>>>>>>>>>>>> case where comprised of pte-mapped order-9 folios. Negative is
>>>>>>>>>>>>>>>>>>>> faster,
>>>>>>>>>>>>>>>>>>>> positive is slower, compared to baseline upon which the
>>>>>>>>>>>>>>>>>>>> series is
>>>>>>>>>>>>>>>>>>>> based:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> | Apple M2 VM   | order-0 (pte-map) | order-9 (pte-map) |
>>>>>>>>>>>>>>>>>>>> | fork          |-------------------|-------------------|
>>>>>>>>>>>>>>>>>>>> | microbench    |    mean |   stdev |    mean |   stdev |
>>>>>>>>>>>>>>>>>>>> |---------------|---------|---------|---------|---------|
>>>>>>>>>>>>>>>>>>>> | baseline      |    0.0% |    1.1% |    0.0% |    1.2% |
>>>>>>>>>>>>>>>>>>>> | after-change  |   -1.0% |    2.0% |   -0.1% |    1.1% |
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> | Ampere Altra  | order-0 (pte-map) | order-9 (pte-map) |
>>>>>>>>>>>>>>>>>>>> | fork          |-------------------|-------------------|
>>>>>>>>>>>>>>>>>>>> | microbench    |    mean |   stdev |    mean |   stdev |
>>>>>>>>>>>>>>>>>>>> |---------------|---------|---------|---------|---------|
>>>>>>>>>>>>>>>>>>>> | baseline      |    0.0% |    1.0% |    0.0% |    0.1% |
>>>>>>>>>>>>>>>>>>>> | after-change  |   -0.1% |    1.2% |   -0.1% |    0.1% |
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Tested-by: John Hubbard <jhubbard@nvidia.com>
>>>>>>>>>>>>>>>>>>>> Reviewed-by: Alistair Popple <apopple@nvidia.com>
>>>>>>>>>>>>>>>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>>>>>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>>>>>>>            include/linux/pgtable.h | 80
>>>>>>>>>>>>>>>>>>>> +++++++++++++++++++++++++++++++++++
>>>>>>>>>>>>>>>>>>>>            mm/memory.c             | 92
>>>>>>>>>>>>>>>>>>>> ++++++++++++++++++++++++++---------------
>>>>>>>>>>>>>>>>>>>>            2 files changed, 139 insertions(+), 33 deletions(-)
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>>>>>>>>>>>>>>>> index af7639c3b0a3..db93fb81465a 100644
>>>>>>>>>>>>>>>>>>>> --- a/include/linux/pgtable.h
>>>>>>>>>>>>>>>>>>>> +++ b/include/linux/pgtable.h
>>>>>>>>>>>>>>>>>>>> @@ -205,6 +205,27 @@ static inline int pmd_young(pmd_t pmd)
>>>>>>>>>>>>>>>>>>>>            #define arch_flush_lazy_mmu_mode()    do {} while
>>>>>>>>>>>>>>>>>>>> (0)
>>>>>>>>>>>>>>>>>>>>            #endif
>>>>>>>>>>>>>>>>>>>>            +#ifndef pte_batch_remaining
>>>>>>>>>>>>>>>>>>>> +/**
>>>>>>>>>>>>>>>>>>>> + * pte_batch_remaining - Number of pages from addr to next
>>>>>>>>>>>>>>>>>>>> batch
>>>>>>>>>>>>>>>>>>>> boundary.
>>>>>>>>>>>>>>>>>>>> + * @pte: Page table entry for the first page.
>>>>>>>>>>>>>>>>>>>> + * @addr: Address of the first page.
>>>>>>>>>>>>>>>>>>>> + * @end: Batch ceiling (e.g. end of vma).
>>>>>>>>>>>>>>>>>>>> + *
>>>>>>>>>>>>>>>>>>>> + * Some architectures (arm64) can efficiently modify a
>>>>>>>>>>>>>>>>>>>> contiguous
>>>>>>>>>>>>>>>>>>>> batch of
>>>>>>>>>>>>>>>>>>>> ptes.
>>>>>>>>>>>>>>>>>>>> + * In such cases, this function returns the remaining
>>>>>>>>>>>>>>>>>>>> number of
>>>>>>>>>>>>>>>>>>>> pages to
>>>>>>>>>>>>>>>>>>>> the end
>>>>>>>>>>>>>>>>>>>> + * of the current batch, as defined by addr. This can be
>>>>>>>>>>>>>>>>>>>> useful
>>>>>>>>>>>>>>>>>>>> when
>>>>>>>>>>>>>>>>>>>> iterating
>>>>>>>>>>>>>>>>>>>> + * over ptes.
>>>>>>>>>>>>>>>>>>>> + *
>>>>>>>>>>>>>>>>>>>> + * May be overridden by the architecture, else batch size is
>>>>>>>>>>>>>>>>>>>> always 1.
>>>>>>>>>>>>>>>>>>>> + */
>>>>>>>>>>>>>>>>>>>> +static inline unsigned int pte_batch_remaining(pte_t pte,
>>>>>>>>>>>>>>>>>>>> unsigned
>>>>>>>>>>>>>>>>>>>> long
>>>>>>>>>>>>>>>>>>>> addr,
>>>>>>>>>>>>>>>>>>>> +                        unsigned long end)
>>>>>>>>>>>>>>>>>>>> +{
>>>>>>>>>>>>>>>>>>>> +    return 1;
>>>>>>>>>>>>>>>>>>>> +}
>>>>>>>>>>>>>>>>>>>> +#endif
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> It's a shame we now lose the optimization for all other
>>>>>>>>>>>>>>>>>>> archtiectures.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Was there no way to have some basic batching mechanism that
>>>>>>>>>>>>>>>>>>> doesn't
>>>>>>>>>>>>>>>>>>> require
>>>>>>>>>>>>>>>>>>> arch
>>>>>>>>>>>>>>>>>>> specifics?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I tried a bunch of things but ultimately the way I've done it
>>>>>>>>>>>>>>>>>> was the
>>>>>>>>>>>>>>>>>> only
>>>>>>>>>>>>>>>>>> way
>>>>>>>>>>>>>>>>>> to reduce the order-0 fork regression to 0.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> My original v3 posting was costing 5% extra and even my first
>>>>>>>>>>>>>>>>>> attempt
>>>>>>>>>>>>>>>>>> at an
>>>>>>>>>>>>>>>>>> arch-specific version that didn't resolve to a compile-time
>>>>>>>>>>>>>>>>>> constant 1
>>>>>>>>>>>>>>>>>> still
>>>>>>>>>>>>>>>>>> cost an extra 3%.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> I'd have thought that something very basic would have worked
>>>>>>>>>>>>>>>>>>> like:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> * Check if PTE is the same when setting the PFN to 0.
>>>>>>>>>>>>>>>>>>> * Check that PFN is consecutive
>>>>>>>>>>>>>>>>>>> * Check that all PFNs belong to the same folio
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I haven't tried this exact approach, but I'd be surprised if I
>>>>>>>>>>>>>>>>>> can
>>>>>>>>>>>>>>>>>> get
>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>> regression under 4% with this. Further along the series I spent a
>>>>>>>>>>>>>>>>>> lot of
>>>>>>>>>>>>>>>>>> time
>>>>>>>>>>>>>>>>>> having to fiddle with the arm64 implementation; every
>>>>>>>>>>>>>>>>>> conditional and
>>>>>>>>>>>>>>>>>> every
>>>>>>>>>>>>>>>>>> memory read (even when in cache) was a problem. There is just so
>>>>>>>>>>>>>>>>>> little in
>>>>>>>>>>>>>>>>>> the
>>>>>>>>>>>>>>>>>> inner loop that every instruction matters. (At least on Ampere
>>>>>>>>>>>>>>>>>> Altra
>>>>>>>>>>>>>>>>>> and
>>>>>>>>>>>>>>>>>> Apple
>>>>>>>>>>>>>>>>>> M2).
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Of course if you're willing to pay that 4-5% for order-0 then the
>>>>>>>>>>>>>>>>>> benefit to
>>>>>>>>>>>>>>>>>> order-9 is around 10% in my measurements. Personally though, I'd
>>>>>>>>>>>>>>>>>> prefer to
>>>>>>>>>>>>>>>>>> play
>>>>>>>>>>>>>>>>>> safe and ensure the common order-0 case doesn't regress, as you
>>>>>>>>>>>>>>>>>> previously
>>>>>>>>>>>>>>>>>> suggested.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> I just hacked something up, on top of my beloved rmap
>>>>>>>>>>>>>>>>> cleanup/batching
>>>>>>>>>>>>>>>>> series. I
>>>>>>>>>>>>>>>>> implemented very generic and simple batching for large folios
>>>>>>>>>>>>>>>>> (all PTE
>>>>>>>>>>>>>>>>> bits
>>>>>>>>>>>>>>>>> except the PFN have to match).
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Some very quick testing (don't trust each last % ) on Intel(R)
>>>>>>>>>>>>>>>>> Xeon(R)
>>>>>>>>>>>>>>>>> Silver
>>>>>>>>>>>>>>>>> 4210R CPU.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> order-0: 0.014210 -> 0.013969
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> -> Around 1.7 % faster
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> order-9: 0.014373 -> 0.009149
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> -> Around 36.3 % faster
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Well I guess that shows me :)
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> I'll do a review and run the tests on my HW to see if it concurs.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I pushed a simple compile fixup (we need pte_next_pfn()).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I've just been trying to compile and noticed this. Will take a
>>>>>>>>>>>>>> look at
>>>>>>>>>>>>>> your
>>>>>>>>>>>>>> update.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> But upon review, I've noticed the part that I think makes this
>>>>>>>>>>>>>> difficult
>>>>>>>>>>>>>> for
>>>>>>>>>>>>>> arm64 with the contpte optimization; You are calling ptep_get() for
>>>>>>>>>>>>>> every
>>>>>>>>>>>>>> pte in
>>>>>>>>>>>>>> the batch. While this is functionally correct, once arm64 has the
>>>>>>>>>>>>>> contpte
>>>>>>>>>>>>>> changes, its ptep_get() has to read every pte in the contpte block in
>>>>>>>>>>>>>> order to
>>>>>>>>>>>>>> gather the access and dirty bits. So if your batching function
>>>>>>>>>>>>>> ends up
>>>>>>>>>>>>>> wealking
>>>>>>>>>>>>>> a 16 entry contpte block, that will cause 16 x 16 reads, which kills
>>>>>>>>>>>>>> performance. That's why I added the arch-specific
>>>>>>>>>>>>>> pte_batch_remaining()
>>>>>>>>>>>>>> function; this allows the core-mm to skip to the end of the contpte
>>>>>>>>>>>>>> block and
>>>>>>>>>>>>>> avoid ptep_get() for the 15 tail ptes. So we end up with 16
>>>>>>>>>>>>>> READ_ONCE()s
>>>>>>>>>>>>>> instead
>>>>>>>>>>>>>> of 256.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I considered making a ptep_get_noyoungdirty() variant, which would
>>>>>>>>>>>>>> avoid
>>>>>>>>>>>>>> the
>>>>>>>>>>>>>> bit
>>>>>>>>>>>>>> gathering. But we have a similar problem in zap_pte_range() and that
>>>>>>>>>>>>>> function
>>>>>>>>>>>>>> needs the dirty bit to update the folio. So it doesn't work there.
>>>>>>>>>>>>>> (see
>>>>>>>>>>>>>> patch 3
>>>>>>>>>>>>>> in my series).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I guess you are going to say that we should combine both
>>>>>>>>>>>>>> approaches, so
>>>>>>>>>>>>>> that
>>>>>>>>>>>>>> your batching loop can skip forward an arch-provided number of ptes?
>>>>>>>>>>>>>> That
>>>>>>>>>>>>>> would
>>>>>>>>>>>>>> certainly work, but feels like an orthogonal change to what I'm
>>>>>>>>>>>>>> trying to
>>>>>>>>>>>>>> achieve :). Anyway, I'll spend some time playing with it today.
>>>>>>>>>>>>>
>>>>>>>>>>>>> You can overwrite the function or add special-casing internally, yes.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Right now, your patch is called "mm: Batch-copy PTE ranges during
>>>>>>>>>>>>> fork()"
>>>>>>>>>>>>> and it
>>>>>>>>>>>>> doesn't do any of that besides preparing for some arm64 work.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Well it allows an arch to opt-in to batching. But I see your point.
>>>>>>>>>>>>
>>>>>>>>>>>> How do you want to handle your patches? Do you want to clean them up
>>>>>>>>>>>> and
>>>>>>>>>>>> I'll
>>>>>>>>>>>> base my stuff on top? Or do you want me to take them and sort it all
>>>>>>>>>>>> out?
>>>>>>>>>>>
>>>>>>>>>>> Whatever you prefer, it was mostly a quick prototype to see if we can
>>>>>>>>>>> achieve
>>>>>>>>>>> decent performance.
>>>>>>>>>>
>>>>>>>>>> I'm about to run it on Altra and M2. But I assume it will show similar
>>>>>>>>>> results.
>>>>>>>>
>>>>>>>> OK results in, not looking great, which aligns with my previous experience.
>>>>>>>> That
>>>>>>>> said, I'm seeing some "BUG: Bad page state in process gmain  pfn:12a094" so
>>>>>>>> perhaps these results are not valid...
>>>>>>>
>>>>>>> I didn't see that so far on x86, maybe related to the PFN fixup?
>>>>>>
>>>>>> All I've done is define PFN_PTE_SHIFT for arm64 on top of your latest patch:
>>>>>>
>>>>>> diff --git a/arch/arm64/include/asm/pgtable.h
>>>>>> b/arch/arm64/include/asm/pgtable.h
>>>>>> index b19a8aee684c..9eb0fd693df9 100644
>>>>>> --- a/arch/arm64/include/asm/pgtable.h
>>>>>> +++ b/arch/arm64/include/asm/pgtable.h
>>>>>> @@ -359,6 +359,8 @@ static inline void set_ptes(struct mm_struct *mm,
>>>>>>     }
>>>>>>     #define set_ptes set_ptes
>>>>>>     +#define PFN_PTE_SHIFT          PAGE_SHIFT
>>>>>> +
>>>>>>     /*
>>>>>>      * Huge pte definitions.
>>>>>>      */
>>>>>>
>>>>>>
>>>>>> As an aside, I think there is a bug in arm64's set_ptes() for PA > 48-bit
>>>>>> case. But that won't affect this.
>>>>>>
>>>>>>
>>>>>> With VM_DEBUG on, this is the first warning I see during boot:
>>>>>>
>>>>>>
>>>>>> [    0.278110] page:00000000c7ced4e8 refcount:12 mapcount:0
>>>>>> mapping:00000000b2f9739b index:0x1a8 pfn:0x1bff30
>>>>>> [    0.278742] head:00000000c7ced4e8 order:2 entire_mapcount:0
>>>>>> nr_pages_mapped:2 pincount:0
>>>>>
>>>>> ^ Ah, you are running with mTHP. Let me play with that.
>>>>
>>>> Err... Its in mm-unstable, but I'm not enabling any sizes. It should only be
>>>> set
>>>> up for PMD-sized THP.
>>>>
>>>> I am using XFS though, so I imagine its a file folio.
>>>>
>>>> I've rebased your rmap cleanup and fork batching to the version of mm-unstable
>>>> that I was doing all my other testing with so I could compare numbers. But its
>>>> not very old (perhaps a week). All the patches applied without any conflict.
>>>
>>>
>>> It would also be interesting to know if the compiler on arm64 decides to do
>>> something stupid: like not inline wrprotect_ptes().
>>>
>>> Because with an effective unlikely(folio_test_large(folio)) we shouldn't see
>>> that much overhead.
>>>
>>
>> What version of gcc are you using? I must confess I'm using the Ubuntu 20.04
>> default version:
>>
>> aarch64-linux-gnu-gcc (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0
>>
>> Perhaps I should grab something a bit newer?
>>
> 
> gcc version 13.2.1 20231011 (Red Hat 13.2.1-4) (GCC)
> 
> From Fedora 38. So "a bit" newer :P
> 

I'll retry with newer toolchain.

FWIW, with the code fix and the original compiler:

Fork, order-0, Apple M2:
| kernel                |   mean_rel |   std_rel |
|:----------------------|-----------:|----------:|
| mm-unstable           |       0.0% |      0.8% |
| hugetlb-rmap-cleanups |       1.3% |      2.0% |
| fork-batching         |       4.3% |      1.0% |

Fork, order-9, Apple M2:
| kernel                |   mean_rel |   std_rel |
|:----------------------|-----------:|----------:|
| mm-unstable           |       0.0% |      0.8% |
| hugetlb-rmap-cleanups |       0.9% |      0.9% |
| fork-batching         |     -37.3% |      1.0% |

Fork, order-0, Ampere Altra:
| kernel                |   mean_rel |   std_rel |
|:----------------------|-----------:|----------:|
| mm-unstable           |       0.0% |      0.7% |
| hugetlb-rmap-cleanups |       3.2% |      0.7% |
| fork-batching         |       5.5% |      1.1% |

Fork, order-9, Ampere Altra:
| kernel                |   mean_rel |   std_rel |
|:----------------------|-----------:|----------:|
| mm-unstable           |       0.0% |      0.1% |
| hugetlb-rmap-cleanups |       0.5% |      0.1% |
| fork-batching         |     -10.4% |      0.1% |