linux-kernel - Re: [PATCH v3 06/22] mm: Always use page table accessor functions

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Message-ID: <a1d25bde-3ab6-46b5-a957-db80da7e737b@kernel.org>
Date: Wed, 26 Nov 2025 16:12:59 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Ryan Roberts <ryan.roberts@....com>, Wei Yang
 <richard.weiyang@...il.com>, Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 linux-riscv@...ts.infradead.org, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org, devicetree@...r.kernel.org,
 Suren Baghdasaryan <surenb@...gle.com>, linux-kernel@...r.kernel.org,
 Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
 Conor Dooley <conor@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Alexandre Ghiti <alex@...ti.fr>, Emil Renner Berthing <kernel@...il.dk>,
 Rob Herring <robh+dt@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Julia Lawall <Julia.Lawall@...ia.fr>, Nicolas Palix <nicolas.palix@...g.fr>,
 Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions

On 11/26/25 16:08, Lorenzo Stoakes wrote:
> On Wed, Nov 26, 2025 at 03:56:13PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 11/26/25 15:52, Lorenzo Stoakes wrote:
>>>
>>> Would the pmdp_get() never get invoked then? Or otherwise wouldn't that end up
>>> requiring a READ_ONCE() further up the stack?
>>
>> See my other reply, I think the pmdp_get() is required because all pud_*
>> functions are just simple stubs.
> 
> OK, thought you were saying we should push further down the stack? Or up
> depending on how you view these things :P as in READ_ONCE at leaf?

I think at leaf because I think the previous ones should essentially be 
only used by stubs.

But I haven't fully digested how this is all working. Or supposed to work.

I'm trying to chew through the arch/arm/include/asm/pgtable-2level.h 
example to see if I can make sense of it,

> 
> Anyway. I am now designating you the expert at this ;)

Oh no. :)

> 
>>
>>>
>>>>
>>>> IOW, push the READ_ONCE() down to the lowest level so the previous ones
>>>> (that will get essentially ignore?) will get folded into the last
>>>> READ_ONCE()?
>>>>
>>>> But my head still hurts and I am focusing on something else concurrently :)
>>>
>>> Even if we could make this work, I don't love that there's some implicit
>>> assumption there that could easily break later on.
>>>
>>> I'd rather we kept it as stupid/obvious as possible...
>>
>> Looking at include/asm-generic/pgtable-nopmd.h I am not sure we are talking
>> about implicit assumptions here. It's kind-of the design that the pud_t
>> values are dummies, so why shoul the pudp_get() give you any guarantees.
>>
>> At least that's my current understanding, which might be very flawed :)
> 
> I mean I'm waving my hands around like I'm working on an aircraft carrier here
> so if you're _sure_ it's _absolutely_ safe then fine :)

Well, not yet ... :)

-- 
Cheers

David