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

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Message-ID: <aTp01WOPU0dYT7yx@casper.infradead.org>
Date: Thu, 11 Dec 2025 07:37:57 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
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 Wed, Dec 10, 2025 at 11:37:51AM -0500, Zi Yan wrote:
> 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. :)

You can join the fun at meet.lpc.events, or there's apparently a youtube
stream.

> > 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).

Maybe we disagree about what words mean ;-)  When I said "mTHP" what
I meant was "support for TLB entries which cover more than one page".
I have no objection to supporting large folio allocation for anon memory
because I think that's beneficial even if there's no hardware support
for TLB entries that cover intermediate sizes between PMD and PTE.