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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/19/26 13:15, Pedro Falcato wrote:
> On Wed, Feb 18, 2026 at 01:24:28PM +0100, David Hildenbrand (Arm) wrote:
>> On 2/18/26 12:58, Pedro Falcato wrote:
>>>
>>> I don't understand what you're looking for. an mprotect-based workload? those
>>> obviously don't really exist, apart from something like a JIT engine cranking
>>> out a lot of mprotect() calls in an aggressive fashion. Or perhaps some of that
>>> usage of mprotect that our DB friends like to use sometimes (discussed in
>>> $OTHER_CONTEXTS), though those are generally hugepages.
>>>
>>
>> Anything besides a homemade micro-benchmark that highlights why we should
>> care about this exact fast and repeated sequence of events.
>>
>> I'm surprise that such a "large regression" does not show up in any other
>> non-home-made benchmark that people/bots are running. That's really what I
>> am questioning.
> 
> I don't know, perhaps there isn't a will-it-scale test for this. That's
> alright. Even the standard will-it-scale and stress-ng tests people use
> to detect regressions usually have glaring problems and are insanely
> microbenchey.

My theory is that most heavy (high frequency where it would really hit performance)
mprotect users (like JITs) perform mprotect on very small ranges (e.g., single page),
where all the other overhead (syscall, TLB flush) dominates.

That's why I was wondering which use cases that behave similar to the reproducer exist.

> 
>>
>> Having that said, I'm all for optimizing it if there is a real problem
>> there.
>>
>>> I don't see how this can justify large performance regressions in a system
>>> call, for something every-architecture-not-named-arm64 does not have.
>> Take a look at the reported performance improvements on AMD with large
>> folios.
> 
> Sure, but pte-mapped 2M folios is almost a worst-case (why not a PMD at that
> point...)

Well, 1M and all the way down will similarly benefit. 2M is just always the extreme case.

> 
>>
>> The issue really is that small folios don't perform well, on any
>> architecture. But to detect large vs. small folios we need the ... folio.
>>
>> So once we optimize for small folios (== don't try to detect large folios)
>> we'll degrade large folios.
> 
> I suspect it's not that huge of a deal. Worst case you can always provide a
> software PTE_CONT bit that would e.g be set when mapping a large folio. Or
> perhaps "if this pte has a PFN, and the next pte has PFN + 1, then we're
> probably in a large folio, thus do the proper batching stuff". I think that
> could satisfy everyone. There are heuristics we can use, and perhaps
> pte_batch_hint() does not need to be that simple and useless in the !arm64
> case then. I'll try to look into a cromulent solution for everyone.

Software bits are generally -ENOSPC, but maybe we are lucky on some architectures.

We'd run into similar issues like aarch64 when shattering contiguity etc, so
there is quite some complexity too it that might not be worth it.

> 
> (shower thought: do we always get wins when batching large folios, or do these
> need to be of a significant order to get wins?)

For mprotect(), I don't know. For fork() and unmap() batching there was always a
win even with order-2 folios. (never measured order-1, because they don't apply to
anonymous memory)

I assume for mprotect() it depends whether we really needed the folio before, or
whether it's just not required like for mremap().

> 
> But personally I would err on the side of small folios, like we did for mremap()
> a few months back.

The following (completely untested) might make most people happy by looking up
the folio only if (a) required or (b) if the architecture indicates that there is a large folio.

I assume for some large folio use cases it might perform worse than before. But for
the write-upgrade case with large anon folios the performance improvement should remain.

Not sure if some regression would remain for which we'd have to special-case the implementation
to take a separate path for nr_ptes == 1.

Maybe you had something similar already:


diff --git a/mm/mprotect.c b/mm/mprotect.c
index c0571445bef7..0b3856ad728e 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -211,6 +211,25 @@ static void set_write_prot_commit_flush_ptes(struct vm_area_struct *vma,
         commit_anon_folio_batch(vma, folio, page, addr, ptep, oldpte, ptent, nr_ptes, tlb);
  }
  
+static bool mprotect_wants_folio_for_pte(unsigned long cp_flags, pte_t *ptep,
+               pte_t pte, unsigned long max_nr_ptes)
+{
+       /* NUMA hinting needs decide whether working on the folio is ok. */
+       if (cp_flags & MM_CP_PROT_NUMA)
+               return true;
+
+       /* We want the folio for possible write-upgrade. */
+       if (!pte_write(pte) && (cp_flags & MM_CP_TRY_CHANGE_WRITABLE))
+               return true;
+
+       /* There is nothing to batch. */
+       if (max_nr_ptes == 1)
+               return false;
+
+       /* For guaranteed large folios it's usually a win. */
+       return pte_batch_hint(ptep, pte) > 1;
+}
+
  static long change_pte_range(struct mmu_gather *tlb,
                 struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr,
                 unsigned long end, pgprot_t newprot, unsigned long cp_flags)
@@ -241,16 +260,18 @@ static long change_pte_range(struct mmu_gather *tlb,
                         const fpb_t flags = FPB_RESPECT_SOFT_DIRTY | FPB_RESPECT_WRITE;
                         int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
                         struct folio *folio = NULL;
-                       struct page *page;
+                       struct page *page = NULL;
                         pte_t ptent;
  
                         /* Already in the desired state. */
                         if (prot_numa && pte_protnone(oldpte))
                                 continue;
  
-                       page = vm_normal_page(vma, addr, oldpte);
-                       if (page)
-                               folio = page_folio(page);
+                       if (mprotect_wants_folio_for_pte(cp_flags, pte, oldpte, max_nr_ptes)) {
+                               page = vm_normal_page(vma, addr, oldpte);
+                               if (page)
+                                       folio = page_folio(page);
+                       }
  
                         /*
                          * Avoid trapping faults against the zero or KSM


-- 
Cheers,

David