Message-ID: <d0e81c75-ad63-4e37-9948-3ae89bc94334@redhat.com>
Date: Fri, 12 Sep 2025 14:25:31 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 12.09.25 14:19, Kiryl Shutsemau wrote:
> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
>> The following series provides khugepaged with the capability to collapse
>> anonymous memory regions to mTHPs.
>>
>> To achieve this we generalize the khugepaged functions to no longer depend
>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
>> pages that are occupied (!none/zero). After the PMD scan is done, we do
>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
>> range. The restriction on max_ptes_none is removed during the scan, to make
>> sure we account for the whole PMD range. When no mTHP size is enabled, the
>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
>> by the attempted collapse order to determine how full a mTHP must be to be
>> eligible for the collapse to occur. If a mTHP collapse is attempted, but
>> contains swapped out, or shared pages, we don't perform the collapse. It is
>> now also possible to collapse to mTHPs without requiring the PMD THP size
>> to be enabled.
>>
>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
>> mTHP collapses to prevent collapse "creep" behavior. This prevents
>> constantly promoting mTHPs to the next available size, which would occur
>> because a collapse introduces more non-zero pages that would satisfy the
>> promotion condition on subsequent scans.
> 
> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.
> 

I am all for not adding any more ugliness on top of all the ugliness we 
added in the past.

I will soon propose deprecating that parameter in favor of something 
that makes a bit more sense.

In essence, we'll likely have an "eagerness" parameter that ranges from 
0 to 10. 10 is essentially "always collapse" and 0 "never collapse if 
not all is populated".

In between we will have more flexibility on how to set these values.

Likely 9 will be around 50% to not even motivate the user to set 
something that does not make sense (creep).

Of course, the old parameter will have to stick around in compat mode.

-- 
Cheers

David / dhildenb

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