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Message-ID: <54d645de-d031-4efc-a1ba-042f709cd549@redhat.com>
Date: Wed, 20 Dec 2023 13:54:59 +0100
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Subject: Re: [PATCH v4 02/16] mm: Batch-copy PTE ranges during fork()
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To: Ryan Roberts <ryan.roberts@arm.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>,
 Oliver Upton <oliver.upton@linux.dev>, James Morse <james.morse@arm.com>,
 Suzuki K Poulose <suzuki.poulose@arm.com>, Zenghui Yu
 <yuzenghui@huawei.com>, Andrey Ryabinin <ryabinin.a.a@gmail.com>,
 Alexander Potapenko <glider@google.com>,
 Andrey Konovalov <andreyknvl@gmail.com>, Dmitry Vyukov <dvyukov@google.com>,
 Vincenzo Frascino <vincenzo.frascino@arm.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@arm.com>,
 Matthew Wilcox <willy@infradead.org>, Yu Zhao <yuzhao@google.com>,
 Mark Rutland <mark.rutland@arm.com>, Kefeng Wang
 <wangkefeng.wang@huawei.com>, John Hubbard <jhubbard@nvidia.com>,
 Zi Yan <ziy@nvidia.com>, Barry Song <21cnbao@gmail.com>,
 Alistair Popple <apopple@nvidia.com>, Yang Shi <shy828301@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20231218105100.172635-1-ryan.roberts@arm.com>
 <20231218105100.172635-3-ryan.roberts@arm.com>
 <0bef5423-6eea-446b-8854-980e9c23a948@redhat.com>
 <db1be625-33e4-4d07-8500-3f7d3c8f9937@arm.com>
 <be8b5181-be2c-4800-ba53-c65a6c3ed803@redhat.com>
 <dd227e51-c4b2-420b-a92a-65da85ab4018@arm.com>
 <7c0236ad-01f3-437f-8b04-125d69e90dc0@redhat.com>
 <9a58b1a2-2c13-4fa0-8ffa-2b3d9655f1b6@arm.com>
 <28968568-f920-47ac-b6fd-87528ffd8f77@redhat.com>
 <10b0b562-c1c0-4a66-9aeb-a6bff5c218f6@arm.com>
 <8f8023cb-3c31-4ead-a9e6-03a10e9490c6@redhat.com>
 <da16a7e5-76dd-4150-9ade-54b0d227a1e1@arm.com>
 <699cb1db-51eb-460e-9ceb-1ce08ca03050@redhat.com>
 <da29a4c6-61f6-4203-9c82-9ce6e1c32552@arm.com>
 <2a8c5b6c-f5ae-43b2-99aa-6d10e79b76e1@redhat.com>
 <ade26f27-03af-4ad7-ad81-38b482f7572c@arm.com>
 <3194b8a5-3f72-4d9e-a267-fbdad32ad864@redhat.com>
 <f2f420cf-678d-466d-ac30-bc8251f16632@arm.com>
From: David Hildenbrand <david@redhat.com>
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Organization: Red Hat
In-Reply-To: <f2f420cf-678d-466d-ac30-bc8251f16632@arm.com>
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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.

I think it was something stupid: I would get "17" from folio_pte_batch() 
for an order-4 folio, but only sometimes. The rmap sanity checks were 
definitely worth it :)

I guess we hit the case "next mapped folio is actually the next physical 
folio" and the detection for that was off by one.

diff --git a/mm/memory.c b/mm/memory.c
index 187d1b9b70e2..2af34add7ed7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -975,7 +975,7 @@ static inline int folio_pte_batch(struct folio 
*folio, unsigned long addr,
                  * corner cases the next PFN might fall into a different
                  * folio.
                  */
-               if (pte_pfn(pte) == folio_end_pfn - 1)
+               if (pte_pfn(pte) == folio_end_pfn)
                         break;

Briefly tested, have to do more testing.

I only tested with order-9, which means max_nr would cap at 512. 
Shouldn't affect the performance measurements, will redo them.

-- 
Cheers,

David / dhildenb