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

[Luke Yang]

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@xxxxxxx> 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%
----------

Luke