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

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Message-ID: <3c857cdb-01d0-4884-85c1-dfae46d8e4a0@gmail.com>
Date: Fri, 5 Sep 2025 16:16:41 +0100
From: Usama Arif <usamaarif642@...il.com>
To: David Hildenbrand <david@...hat.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 05/09/2025 16:04, David Hildenbrand wrote:
> 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?
> 

hmm I probably didnt understand what you meant to say here:
we already add all of them to the list in __do_huge_pmd_anonymous_page and collapse_huge_page and
shrink_underused sets/clears split_underused_thp in deferred_split_folio decides whether we process or not.

In deferred_split_folio, if split_underused_thp is false, we dont add them to the list (unless partially_mapped).

Unless you are referring to non pmd mapped THPs?