Re: [REGRESSION] mm/mprotect: 2x+ slowdown for >=400KiB regions since PTE batching (cac1db8c3aad)

[Dev Jain <dev.jain@arm.com>]

On 17/02/26 11:13 pm, Luke Yang wrote:
> On Mon, Feb 16, 2026 at 03:42:08PM +0530, Dev Jain wrote:
>> On 13/02/26 10:56 pm, David Hildenbrand (Arm) wrote:
>>> On 2/13/26 18:16, Suren Baghdasaryan wrote:
>>>> On Fri, Feb 13, 2026 at 4:24 PM Pedro Falcato <pfalcato@suse.de> wrote:
>>>>> On Fri, Feb 13, 2026 at 04:47:29PM +0100, David Hildenbrand (Arm) wrote:
>>>>>> Hi!
>>>>>>
>>>>>>
>>>>>> Micro-benchmark results are nice. But what is the real word impact?
>>>>>> IOW, why
>>>>>> should we care?
>>>>> Well, mprotect is widely used in thread spawning, code JITting,
>>>>> and even process startup. And we don't want to pay for a feature we can't
>>>>> even use (on x86).
>>>> I agree. When I straced Android's zygote a while ago, mprotect() came
>>>> up #30 in the list of most frequently used syscalls and one of the
>>>> most used mm-related syscalls due to its use during process creation.
>>>> However, I don't know how often it's used on VMAs of size >=400KiB.
>>> See my point? :) If this is apparently so widespread then finding a real
>>> reproducer is likely not a problem. Otherwise it's just speculation.
>>>
>>> It would also be interesting to know whether the reproducer ran with any
>>> sort of mTHP enabled or not. 
>> Yes. Luke, can you experiment with the following microbenchmark:
>>
>> https://pastebin.com/3hNtYirT
>>
>> and see if there is an optimization for pte-mapped 2M folios, before and
>> after the commit?
>>
>> (set transparent_hugepages/enabled=always, hugepages-2048Kb/enabled=always)
> ----------
> amd-epyc2-rome
>
> # before commit
> $ uname -r
> 6.16.0-65.eln150.x86_64
> # after commit
> $ uname -r
> 6.17.0-0.rc1.17.eln150.x86_64
>
> $ cat /sys/kernel/mm/transparent_hugepage/enabled
> [always] madvise never
>
> $ cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> [always] inherit madvise never
>
> Before commit: Total = 6895988972
> After commit: Total = 2303697782
> Percentage change: -66.6%
>
> ----------
> amd-epyc3-milanx
>
> # before commit
> $ uname -r
> 6.16.0-65.eln150.x86_64
> # after commit
> $ uname -r
> 6.17.0-0.rc1.17.eln150.x86_64
>
> $ cat /sys/kernel/mm/transparent_hugepage/enabled
> [always] madvise never
>
> $ cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> [always] inherit madvise never
>
> Before commit: Total = 4006750392
> After commit: Total = 1497733191
> Percentage change: -62.6%
> ----------

Thanks. So after all, batching improves stuff :)

>
> Luke
>