Re: [RFC PATCH 6/7] mm: memory: add mTHP support for wp

[David Hildenbrand <david@redhat.com>]

On 14.08.25 13:38, Vernon Yang wrote:
> Currently pagefaults on anonymous pages support mthp, and hardware
> features (such as arm64 contpte) can be used to store multiple ptes in
> one TLB entry, reducing the probability of TLB misses. However, when the
> process is forked and the cow is triggered again, the above optimization
> effect is lost, and only 4KB is requested once at a time.
> 
> Therefore, make pagefault write-protect copy support mthp to maintain the
> optimization effect of TLB and improve the efficiency of cow pagefault.
> 
> vm-scalability usemem shows a great improvement,
> test using: usemem -n 32 --prealloc --prefault 249062617
> (result unit is KB/s, bigger is better)
> 
> |    size     | w/o patch | w/ patch  |  delta  |
> |-------------|-----------|-----------|---------|
> | baseline 4K | 723041.63 | 717643.21 | -0.75%  |
> | mthp 16K    | 732871.14 | 799513.18 | +9.09%  |
> | mthp 32K    | 746060.91 | 836261.83 | +12.09% |
> | mthp 64K    | 747333.18 | 855570.43 | +14.48% |

You're missing two of the most important metrics: COW latency and memory 
waste.

Just imagine what happens if you have PMD-sized THP.

I would suggest you explore why Redis used to recommend to disable THPs 
(hint: tail latency due to COW of way-too-large chunks before we do what 
we do today).

So staring at usemem micro-benchmark results is a bit misleading.

As discussed in the past, I would actually suggest to

a) Let khugepaged deal with fixing this up later, keeping CoW path
    simpler and faster.
b) If we really really have to do this during fault time, limit it to
    some order (might even be have to be configurable).

I really think we should keep CoW latency low and instead let khugepaged 
fix that up later. (Nico is working on mTHP collapse support)

[are you handling having a mixture of PageAnonExclusive within a folio 
properly? Only staring at R/O PTEs is usually insufficient to determine 
whether you can COW or whether you must reuse].

-- 
Cheers

David / dhildenb