linux-kernel - Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when MADV_COLD/MADV

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Message-ID: <mcf4mydmyz6e5ioeseaawhfro2snox4hyfr2irpx4kxkagx2k7@uszrm24ulffg>
Date: Sun, 21 Dec 2025 12:25:44 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Wei Yang <richard.weiyang@...il.com>, 
	"David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, ziy@...dia.com, 
	baohua@...nel.org, lance.yang@...ux.dev, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when
 MADV_COLD/MADV_FREE

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()
            __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