Message-ID: <59641180-a0d9-400c-aaeb-0c9e93954bf5@redhat.com>
Date: Fri, 12 Sep 2025 17:45:26 +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 17:41, Kiryl Shutsemau wrote:
> On Fri, Sep 12, 2025 at 04:56:47PM +0200, David Hildenbrand wrote:
>> 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.
> 
> Well, KSM can reclaim these zero-filled memory until we collapse it.

KSM is used so rarely (for good reasons) that I would never ever build 
an argument based on its existence :P

But yes: during the very first shrinker discussion I raised that KSM can 
do the same thing. Obviously that was not good enough.

-- 
Cheers

David / dhildenb

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