linux-kernel - Re: [PATCH v10 00/13] khugepaged: mTHP support

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Message-ID: <95012dfc-d82d-4ae2-b4cd-1e8dcf15e44b@redhat.com>
Date: Thu, 21 Aug 2025 22:43:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Nico Pache <npache@...hat.com>
Cc: Dev Jain <dev.jain@....com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, ziy@...dia.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 ryan.roberts@....com, corbet@....net, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
 akpm@...ux-foundation.org, baohua@...nel.org, willy@...radead.org,
 peterx@...hat.com, wangkefeng.wang@...wei.com, usamaarif642@...il.com,
 sunnanyong@...wei.com, vishal.moola@...il.com,
 thomas.hellstrom@...ux.intel.com, yang@...amperecomputing.com,
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 rdunlap@...radead.org, hughd@...gle.com
Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

> 
> The one thing we absolutely cannot have is a default that causes this
> 'creeping' behaviour. This feels like shipping something that is broken and
> alluding to it in the documentation.
> 
> I spoke to David off-list and he gave some insight into this and perhaps
> some reasonable means of avoiding an additional tunable.
> 
> I don't want to rehash what he said as I think it's more productive for him
> to reply when he has time but broadly I think how we handle this needs
> careful consideration.
> 
> To me it's clear that some sense of ratio is just immediately very very
> confusing, but then again this interface is already confusing, as with much
> of THP.
> 
> Anyway I'll let David respond here so we don't loop around before he has a
> chance to add his input.
> 
> Cheers, Lorenzo
> 

[Resending because Thunderbird decided to use the wrong smtp server]

I've been summoned.

As raised in the past, I would initially only support specific values here like

0 				  : Never collapse with any pte_none/zeropage
511 (HPAGE_PMD_NR - 1) / default  : Always collapse, ignoring pte_none/zeropage

Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), but not sure
if we have to add that for now.

Because, as raised in the past, I'm afraid nobody on this earth has a clue how
to set this parameter to values different to 0 (don't waste memory with khugepaged)
and 511 (page fault behavior).


If any other value is set, essentially
	pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse");

for now and just disable it.

-- 
Cheers

David / dhildenb