linux-kernel - Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan

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Message-ID: <13f8e557-da9d-4ccf-9372-6acdc865586a@redhat.com>
Date: Tue, 20 May 2025 12:26:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for
 mTHP support

On 20.05.25 12:09, Baolin Wang wrote:
> Sorry for late reply.
> 
> On 2025/5/17 14:47, Nico Pache wrote:
>> On Thu, May 15, 2025 at 9:20 PM Baolin Wang
>> <baolin.wang@...ux.alibaba.com> wrote:
>>>
>>>
>>>
>>> On 2025/5/15 11:22, Nico Pache wrote:
>>>> khugepaged scans anons PMD ranges for potential collapse to a hugepage.
>>>> To add mTHP support we use this scan to instead record chunks of utilized
>>>> sections of the PMD.
>>>>
>>>> khugepaged_scan_bitmap uses a stack struct to recursively scan a bitmap
>>>> that represents chunks of utilized regions. We can then determine what
>>>> mTHP size fits best and in the following patch, we set this bitmap while
>>>> scanning the anon PMD. A minimum collapse order of 2 is used as this is
>>>> the lowest order supported by anon memory.
>>>>
>>>> max_ptes_none is used as a scale to determine how "full" an order must
>>>> be before being considered for collapse.
>>>>
>>>> When attempting to collapse an order that has its order set to "always"
>>>> lets always collapse to that order in a greedy manner without
>>>> considering the number of bits set.
>>>>
>>>> Signed-off-by: Nico Pache <npache@...hat.com>
>>>
>>> Sigh. You still haven't addressed or explained the issues I previously
>>> raised [1], so I don't know how to review this patch again...
>> Can you still reproduce this issue?
> 
> Yes, I can still reproduce this issue with today's (5/20) mm-new branch.
> 
> I've disabled PMD-sized THP in my system:
> [root]# cat /sys/kernel/mm/transparent_hugepage/enabled
> always madvise [never]
> [root]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> always inherit madvise [never]

Thanks for the easy reproducer, Baolin! It's certainly something that 
must be fixed.

> 
> And I tried calling madvise() with MADV_COLLAPSE for anonymous memory,
> and I can still see it collapsing to a PMD-sized THP.

This almost sounds like it could be converted into an easy selftest.

Baolin, do you have other ideas for easy selftests? It might be good to 
include some in the next version.

I can think of: enable only a single size, then MADV_COLLAPSE X times 
and see if it worked. etc.

-- 
Cheers,

David / dhildenb