Message-ID: <cd8e7f1c-a563-4ae9-a0fb-b0d04a4c35b4@redhat.com>
Date: Mon, 15 Sep 2025 12:22:07 +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 15.09.25 11:22, Kiryl Shutsemau wrote:
> On Fri, Sep 12, 2025 at 05:31:51PM -0600, Nico Pache wrote:
>> On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> 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".
>> Hi David,
>>
>> Do you have any reason for 0-10, I'm guessing these will map to
>> different max_ptes_none values.
>> I suggest 0-5, mapping to 0,32,64,128,255,511
> 
> That's too x86-64 specific.
> 
> And the whole idea is not to map to directly, but give kernel wiggle
> room to play.

Initially we will start out simple and map it directly. But yeah, the 
idea is to give us some more room later.

I had something logarithmic in mind which would roughly be (ignoring the 
the weird -1 for simplicity and expressing it as "used" instead of 
none-or-zero)

0 -> ~100% used (~0% none)
1 -> ~50% used (~50% none)
2 -> ~25% used (~75% none)
3 -> ~12.5% used (~87.5% none)
4 -> ~11.25% used (~88,75% none)
...
10 -> ~0% used (~100% none)

Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy.

-- 
Cheers

David / dhildenb

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