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

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Message-ID: <ee5f5da3-8c6b-4381-aee8-b0fab56cbf83@arm.com>
Date: Wed, 26 Nov 2025 14:22:13 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 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 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.

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).

So I don't think it works.

Probably we either have to live with the extra loads or have 2 types of helper.

> 
>>>>
>>>> I find that kind of gross to be honest. Isn't the whole point of folding that we
>>>> don't have to think about it...
>>
>> Agreed, but if we can put it inside the default helper implementation, that
>> solves it, I think? An arch has to be careful if they are overriding the
>> defaults, but it's still well contained.
>>
>>>
>>> If we could adjust generic pgdp_get() and friends to not do a READ_ONCE() once
>>> folded we might not have to think about that in the callers.
>>>
>>> Just an idea, though, not sure if that would fly the way I envision it.
>>
>>
>