Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when MADV_COLD/MADV_FREE

[Wei Yang <richard.weiyang@gmail.com>]

On Sun, Dec 21, 2025 at 12:25:44PM +0800, Vernon Yang wrote:
>On Sun, Dec 21, 2025 at 02:10:44AM +0000, Wei Yang wrote:
>> On Fri, Dec 19, 2025 at 09:58:17AM +0100, David Hildenbrand (Red Hat) wrote:
>> >On 12/19/25 06:29, Vernon Yang wrote:
>> >> On Thu, Dec 18, 2025 at 10:31:58AM +0100, David Hildenbrand (Red Hat) wrote:
>> >> > On 12/15/25 10:04, Vernon Yang wrote:
>> >> > > For example, create three task: hot1 -> cold -> hot2. After all three
>> >> > > task are created, each allocate memory 128MB. the hot1/hot2 task
>> >> > > continuously access 128 MB memory, while the cold task only accesses
>> >> > > its memory briefly andthen call madvise(MADV_COLD). However, khugepaged
>> >> > > still prioritizes scanning the cold task and only scans the hot2 task
>> >> > > after completing the scan of the cold task.
>> >> > >
>> >> > > So if the user has explicitly informed us via MADV_COLD/FREE that this
>> >> > > memory is cold or will be freed, it is appropriate for khugepaged to
>> >> > > scan it only at the latest possible moment, thereby avoiding unnecessary
>> >> > > scan and collapse operations to reducing CPU wastage.
>> >> > >
>> >> > > Here are the performance test results:
>> >> > > (Throughput bigger is better, other smaller is better)
>> >> > >
>> >> > > Testing on x86_64 machine:
>> >> > >
>> >> > > | task hot2           | without patch | with patch    |  delta  |
>> >> > > |---------------------|---------------|---------------|---------|
>> >> > > | total accesses time |  3.14 sec     |  2.92 sec     | -7.01%  |
>> >> > > | cycles per access   |  4.91         |  2.07         | -57.84% |
>> >> > > | Throughput          |  104.38 M/sec |  112.12 M/sec | +7.42%  |
>> >> > > | dTLB-load-misses    |  288966432    |  1292908      | -99.55% |
>> >> > >
>> >> > > Testing on qemu-system-x86_64 -enable-kvm:
>> >> > >
>> >> > > | task hot2           | without patch | with patch    |  delta  |
>> >> > > |---------------------|---------------|---------------|---------|
>> >> > > | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
>> >> > > | cycles per access   |  7.23         |  2.12         | -70.68% |
>> >> > > | Throughput          |  97.88 M/sec  |  110.76 M/sec | +13.16% |
>> >> > > | dTLB-load-misses    |  237406497    |  3189194      | -98.66% |
>> >> >
>> >> > Again, I also don't like that because you make assumptions on a full process
>> >> > based on some part of it's address space.
>> >> >
>> >> > E.g., if a library issues a MADV_COLD on some part of the memory the library
>> >> > manages, why should the remaining part of the process suffer as well?
>> >>
>> >> Yes, you make a good point, thanks!
>> >>
>> >> > This seems to be an heuristic focused on some specific workloads, no?
>> >>
>> >> Right.
>> >>
>> >> Could we use the VM_NOHUGEPAGE flag to indicate that this region should
>> >> not be collapsed, so that khugepaged can simply skip this VMA during
>> >> scanning? This way, it won't affect the remaining part of the task's
>> >> memory regions.
>> >
>> >I thought we would skip these regions already properly in khugeapged, or
>> >maybe I misunderstood your question.
>> >
>>
>> I think we should, but seems we didn't do this for anonymous memory during
>> khugepaged.
>>
>> We check the vma with thp_vma_allowable_order() during scan.
>>
>>   * For anonymous memory during khugepaged, if we always enable 2M collapse,
>>     we will scan this vma. Even VM_NOHUGEPAGE is set.
>>
>>   * For other cases, it looks good since __thp_vma_allowable_order() will skip
>>     this vma with vma_thp_disabled().
>
>Hi David, Wei,
>
>The khugepaged has already checked the VM_NOHUGEPAGE flag for anonymous
>memory during scan, as below:
>
>khugepaged_scan_mm_slot()
>    thp_vma_allowable_order()
>        thp_vma_allowable_orders()

Oops, you are right. It only bypass __thp_vma_allowable_order() if orders is
0.

>            __thp_vma_allowable_orders()
>                vma_thp_disabled() {
>                     if (vm_flags & VM_NOHUGEPAGE)
>                         return true;
>                }
>
>REAL ISSUE: when madvise(MADV_COLD)，not set VM_NOHUGEPAGE flag to vma,
>so the khugepaged will continue scan this vma.
>
>I set VM_NOHUGEPAGE flag to vma when madvise(MADV_COLD), the test has
>been successful. I will send it in the next version.
>
>--
>Thanks,
>Vernon

-- 
Wei Yang
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