linux-kernel - Re: [RFC 01/12] mm: add PUD THP ptdesc and rmap support

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Message-ID: <ee5bd77f-87ad-4640-a974-304b488e4c64@kernel.org>
Date: Thu, 5 Feb 2026 18:40:13 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Usama Arif <usamaarif642@...il.com>, Matthew Wilcox <willy@...radead.org>
Cc: Zi Yan <ziy@...dia.com>, Kiryl Shutsemau <kas@...nel.org>,
 lorenzo.stoakes@...cle.com, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org, hannes@...xchg.org, riel@...riel.com,
 shakeel.butt@...ux.dev, 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 01/12] mm: add PUD THP ptdesc and rmap support

On 2/5/26 06:13, Usama Arif wrote:
> 
> 
> On 04/02/2026 20:21, Matthew Wilcox wrote:
>> On Thu, Feb 05, 2026 at 04:17:19AM +0000, Matthew Wilcox wrote:
>>> Why are you even talking about "the next series"?  The approach is
>>> wrong.  You need to put this POC aside and solve the problems that
>>> you've bypassed to create this POC.
> 
> 
> Ah is the issue the code duplication that Lorenzo has raised (ofcourse
> completely agree that there is quite a bit), the lru.next patch I did
> which hopefully [1] makes better, or investigating if it might be
> interferring with DAX/VFIO that Lorenzo pointed out (will ofcourse
> investigate before sending the next revision)? The mapcount work
> (I think David is working on this?) that is needed to allow splitting
> PUDs to PMD is completely a separate issue and can be tackled in parallel
> to this.

I would enjoy seeing an investigation where we see what might have to be 
done to avoid preallocating page tables for anonymous memory THPs, and 
instead, try allocating them on demand when remapping. If allocation 
fails, it's just another -ENOMEM or -EAGAIN.

That would not only reduce the page table overhead when using THPs, it 
would also avoid the preallocation of two levels like you need here.

Maybe it's doable, maybe not.

Last time I looked into it I was like "there must be a better way to 
achieve that" :)

Spinlocks might require preallocating etc.

(as raised elsewhere, staring with shmem support avoid the page table 
problem)

> 
>>
>> ... and gmail is rejecting this email as being spam.  You need to stop
>> using gmail for kernel deveopment work.
> 
> I asked a couple of folks now and it seems they got it without any issue.
> I have used it for a long time. I will try and see if something has changed.

Gmail is absolutely horrible for upstream development. For example, 
linux-mm recently un-subscribed all gmail addresses.

When I moved to my kernel.org address I thought using gmail as a backend 
would be a great choice. I was wrong and after getting daily bounce 
notifications from MLs (even though my spamfilter rules essentially 
allowed everything). So I moved to something else (I now pay 3Euro a 
month, omg! :) ).

-- 
Cheers,

David