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

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Message-ID: <6b966403-91e0-4f06-86a9-a4f7780b9557@kernel.org>
Date: Wed, 26 Nov 2025 15:46:40 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>, Wei Yang <richard.weiyang@...il.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 15:22, Ryan Roberts wrote:
> On 26/11/2025 13:47, Wei Yang wrote:
>> On Wed, Nov 26, 2025 at 01:03:42PM +0000, Ryan Roberts wrote:
>>> On 26/11/2025 12:35, David Hildenbrand (Red Hat) wrote:
>> [...]
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> I've just come across this patch and wanted to mention that we could also
>>>>>>>> benefit from this improved absraction for some features we are looking at for
>>>>>>>> arm64. As you mention, Anshuman had a go but hit some roadblocks.
>>>>>>>>
>>>>>>>> The main issue is that the compiler was unable to optimize away the
>>>>>>>> READ_ONCE()s
>>>>>>>> for the case where certain levels of the pgtable are folded. But it can
>>>>>>>> optimize
>>>>>>>> the plain C dereferences. There were complaints the the generated code for arm
>>>>>>>> (32) and powerpc was significantly impacted due to having many more
>>>>>>>> (redundant)
>>>>>>>> loads.
>>>>>>>>
>>>>>>>
>>>>>>> We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort
>>>>>>> this out internally?
>>>>>>>
>>>>>>
>>>>>> Just stumbled over the reply from Christope:
>>>>>>
>>>>>> https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org
>>>>>>
>>>>>> And wonder if we could handle that somehow directly in the pgdp_get() etc.
>>>
>>> I certainly don't like the suggestion of doing the is_folded() test outside the
>>> helper, but if we can push that logic down into pXdp_get() that would be pretty
>>> neat. Anshuman and I did briefly play with the idea of doing a C dereference if
>>> the level is folded and a READ_ONCE() otherwise, all inside each pXdp_get()
>>> helper. Although we never proved it to be correct. I struggle with the model for
>>> folding. Do you want to optimize out all-but-the-highest level's access or
>>> all-but-the-lowest level's access? Makes my head hurt...
>>>
>>>
>>
>> You mean sth like:
>>
>> static inline pmd_t pmdp_get(pmd_t *pmdp)
>> {
>> #ifdef __PAGETABLE_PMD_FOLDED
>> 	return *pmdp;
>> #else
>> 	return READ_ONCE(*pmdp);
>> #endif
>> }
> 
> Yes. But I'm not convinced it's correct.

Yeah, I'm also still trying to understand how it could work.

> 
> I *think* (but please correct me if I'm wrong) if the PMD is folded, the PUD and
> P4D must also be folded, and you effectively have a 2 level pgtable consisting
> of the PGD table and the PTE table. p4dp_get(), pudp_get() and pmdp_get() are
> all effectively duplicating the load of the pgd entry? So assuming pgdp_get()
> was already called and used READ_ONCE(), you might hope the compiler will just
> drop the other loads and just use the value returned by READ_ONCE(). But I doubt
> there is any guarantee of that and you might be in a situation where pgdp_get()
> never even got called (perhaps you already have the pmd pointer).
With __PAGETABLE_PMD_FOLDED we treat the PUD to be fake-present, like

static inline int pud_present(pud_t pud)	{ return 1; }

And obtaining the pmd_t* is essentially  cast of the pud_t*

static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address)
{
	return (pmd_t *)pud;
}

So in that case we might want to have the READ_ONCE() remove from the 
pudp_get(), not the pmdp_get()?

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 :)

-- 
Cheers

David