linux-kernel - Re: [RFC v2 0/3] Decoupling large folios dependency on THP

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Message-ID: <D498FB7E-1C57-47A6-BAF4-EA1BAAE75526@nvidia.com>
Date: Wed, 10 Dec 2025 11:37:51 -0500
From: Zi Yan <ziy@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Pankaj Raghav <p.raghav@...sung.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Mike Rapoport <rppt@...nel.org>,
 David Hildenbrand <david@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Michal Hocko <mhocko@...e.com>, Lance Yang <lance.yang@...ux.dev>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Nico Pache <npache@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 mcgrof@...nel.org, gost.dev@...sung.com, kernel@...kajraghav.com,
 tytso@....edu
Subject: Re: [RFC v2 0/3] Decoupling large folios dependency on THP

On 9 Dec 2025, at 23:27, Matthew Wilcox wrote:

> On Tue, Dec 09, 2025 at 11:03:23AM -0500, Zi Yan wrote:
>> I wonder if core-mm should move mTHP code out of CONFIG_TRANSPARENT_HUGEPAGE
>> and mTHP might just work. Hmm, folio split might need to be moved out of
>> mm/huge_memory.c in that case. khugepaged should work for mTHP without
>> CONFIG_TRANSPARENT_HUGEPAGE as well. OK, for anon folios, the changes might
>> be more involved.
>
> I think this is the key question to be discussed at LPC.  How much of

I am not going, so would like to get a summary afterwards. :)

> the current THP code should we say "OK, this is large folio support
> and everybody needs it" and how much is "This is PMD (or mTHP) support;
> this architecture doesn't have it, we don't need to compile it in".

I agree with most of it, except mTHP part. mTHP should be part of large
folio, since I see mTHP is anon equivalent to file backed large folio.
Both are a >0 order folio mapped by PTEs (ignoring to-be-implemented
multi-PMD mapped large folios for now).

Best Regards,
Yan, Zi