linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <80c50bf4-27b1-483c-9977-2519369c2630@redhat.com>
Date: Fri, 12 Sep 2025 16:56:47 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 12.09.25 16:35, Kiryl Shutsemau wrote:
> On Fri, Sep 12, 2025 at 04:28:09PM +0200, David Hildenbrand wrote:
>> On 12.09.25 15:47, David Hildenbrand 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.
>>>
>>> BTW, I thought further about this and I agree: if we count zero-filled
>>> pages towards none_or_zero one we can avoid the "creep" problem.
>>>
>>> The scanning-for-zero part is rather nasty, though.
>>
>> Aaand, thinking again from the other direction, this would mean that just
>> because pages became zero after some time that we would no longer collapse
>> because none_or_zero would then be higher. Hm ....
>>
>> How I hate all of this so very very much :)
> 
> This is not new. Shrinker has the same problem: it cannot distinguish
> between hot 4k that happened to be zero from the 4k that is there just
> because of we faulted in 2M a time.

Right. And so far that problem is isolated to the shrinker.

To me so far "none_or_zero" really meant "will I consume more memory 
when collapsing". That's not true for zero-filled pages, obviously.

-- 
Cheers

David / dhildenb