linux-kernel - Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes

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Message-ID: <3737e6e5-9569-464c-8cd0-1ec9888be04b@redhat.com>
Date: Fri, 5 Sep 2025 17:04:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>
Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with
 max_ptes_none default

On 05.09.25 17:01, Usama Arif wrote:
> 
> 
> On 05/09/2025 15:58, David Hildenbrand wrote:
>> On 05.09.25 16:53, Usama Arif wrote:
>>>
>>>
>>> On 05/09/2025 15:46, David Hildenbrand wrote:
>>>> [...]
>>>>
>>>>>
>>>>> The reason I did this is for the case if you change max_ptes_none after the THP is added
>>>>> to deferred split list but *before* memory pressure, i.e. before the shrinker runs,
>>>>> so that its considered for splitting.
>>>>
>>>> Yeah, I was assuming that was the reason why the shrinker is enabled as default.
>>>>
>>>> But in any sane system, the admin would enable the shrinker early. If not, we can look into handling it differently.
>>>
>>> Yes, I do this as well, i.e. have a low value from the start.
>>>
>>> Does it make sense to disable shrinker if max_ptes_none is 511? It wont shrink
>>> the usecase you are describing below, but we wont encounter the increased CPU usage.>
>>
>> I don't really see why we should do that.
>>
>> If the shrinker is a problem than the shrinker should be disabled. But if it is enabled, we should be shrinking as documented.
>>
>> Without more magic around our THP toggles (we want less) :)
>>
>> Shrinking happens when we are under memory pressure, so I am not really sure how relevant the scanning bit is, and if it is relevant enought to change the shrinker default.
>>
> 
> yes agreed, I also dont have numbers to back up my worry, its all theoretical :)

BTW, I was also wondering if we should just always add all THP to the 
deferred split list, and make the split toggle just affect whether we 
process them or not (scan or not).

I mean, as a default we add all of them to the list already right now, 
even though nothing would ever get reclaimed as default.

What's your take?

-- 
Cheers

David / dhildenb