linux-kernel - Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes

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Message-ID: <e2a89e74-1533-4a83-8d0f-3f4821750e25@redhat.com>
Date: Tue, 28 Oct 2025 18:56:10 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, ziy@...dia.com, Liam.Howlett@...cle.com,
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Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce
 collapse_max_ptes_none helper function

[...]

> 
>> towards David's earlier simplified approach:
>> 	max_ptes_none == 511 -> collapse mTHP always
>> 	max_ptes_none != 511 -> collapse mTHP only if all PTEs are non-none/zero
> 
> Pretty sure David's suggestion was that max_ptes_none would literally get set to
> 511 if you specified 511, or 0 if you specified anything else.

We had multiple incarnations of this approach, but the first one really was:

max_ptes_none == 511 -> collapse mTHP always
max_ptes_none == 0 -> collapse mTHP only if all non-none/zero

And for the intermediate values

(1) pr_warn() when mTHPs are enabled, stating that mTHP collapse is not 
supported yet with other values
(2) treat it like max_ptes_none == 0 or (maybe better?) just disable 
mTHP collapse


I still like that approach because it let's us defer solving the creep 
problem later and doesn't add a silent capping.

Using intermediate max_ptes_none values are really only reasonable with 
the deferred shrinker today. And that one does not support mTHP even 
with this series, so it's future work either way.

-- 
Cheers

David / dhildenb