linux-kernel - Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time

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Message-ID: <1dfa0bd6-9658-4d29-ae5d-baad0a7a9572@kernel.org>
Date: Wed, 11 Feb 2026 20:55:37 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Matthew Wilcox <willy@...radead.org>, Usama Arif <usama.arif@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, lorenzo.stoakes@...cle.com,
 linux-mm@...ck.org, fvdl@...gle.com, hannes@...xchg.org, riel@...riel.com,
 shakeel.butt@...ux.dev, kas@...nel.org, baohua@...nel.org, dev.jain@....com,
 baolin.wang@...ux.alibaba.com, npache@...hat.com, Liam.Howlett@...cle.com,
 ryan.roberts@....com, vbabka@...e.cz, lance.yang@...ux.dev,
 linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time

On 2/11/26 20:28, Matthew Wilcox wrote:
> On Wed, Feb 11, 2026 at 04:49:45AM -0800, Usama Arif wrote:
>> - Memory reclaim (try_to_unmap()): Returns false, folio rotated back
>> LRU, retried in next reclaim cycle.
> 
> I was advised to ask my stupid question ...
> 
> Why do we still try to split the PMD in reclaim?  I understand we're
> about to swap the folio out and we'll need to put a swap entry in the page
> table so we can find it again.  But can't we now store swap entries at the
> PMD level, or are we still forced to store 512 entries at the PTE level?

Yes. We don't support PMD swap entries yet.

I don't know all historical details. I suspect there are some rough 
edges around swapin (assume we cannot swapin a 2M THP), and maybe it was 
just easier to not deal with splitting of PMD swap entries (which we 
would similarly have to support).

For sure an interesting project to look into.

-- 
Cheers,

David