Message-ID: <ed1d1281-ece3-4d2c-8e58-aaeb436d3927@linux.alibaba.com>
Date: Tue, 20 May 2025 18:09:16 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for
 mTHP support

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]

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

> I can no longer reproduce this issue, that's why I posted... although
> I should have followed up, and looked into what the original issue
> was. Nothing really sticks out so perhaps something in mm-new was
> broken and pulled out... not sure.
> 
> It should now follow the expected behavior, which is that no mTHP
> collapse occurs because if the PMD size is disabled so is khugepaged
> collapse.
> 
> Lmk if you are still experiencing this issue please.
> 
> Cheers,
> -- Nico
>>
>> [1]
>> https://lore.kernel.org/all/83a66442-b7c7-42e7-af4e-fd211d8ed6f8@linux.alibaba.com/
>>

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