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

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Message-ID: <6c8a85b6-232e-4341-8df1-501b9d73e14e@redhat.com>
Date: Fri, 12 Sep 2025 17:51:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kas@...nel.org>, Pedro Falcato <pfalcato@...e.de>
Cc: Johannes Weiner <hannes@...xchg.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 12.09.25 17:44, Kiryl Shutsemau wrote:
> On Fri, Sep 12, 2025 at 04:39:02PM +0100, Kiryl Shutsemau wrote:
>>> Shower thought: it might be in these cases especially where the FreeBSD
>>> reservation system comes in handy - best effort allocating a THP, but not
>>> actually mapping it as such until you really _know_ it is hot - and until
>>> then, memory reclaim can just break your THP down if it really needs to.
>>
>> This is just silly. All downsides without benefit until maybe later. And
>> for short-lived processes the "later" never comes.
> 
> The right way out is to get better info on access pattern from hardware.
> For instance, if we move access bit out of page table entry and make it
> independent of the actually mapping size that would give us much better
> view on what actually is going on.

We discussed this a couple of times in the past, the problem is that it 
does not help anybody really if all but a handful piece of hardware 
provides such a features.

Long long long term I agree, short term we cannot really build core 
infrastructure around any of that.

-- 
Cheers

David / dhildenb