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

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 26/11/2025 à 15:22, Ryan Roberts a écrit :
> 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F0019d675-ce3d-4a5c-89ed-f126c45145c9%40kernel.org&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C22d0a028b1ec4a8b678108de2cf73769%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638997637481119954%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=ocR6usVgRHfue0MrtbQnDO8whINvy%2FDMAfNE3caiY8c%3D&reserved=0
>>>>>>
>>>>>> 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).

I think you can't assume pgdp_get() was already called, because some 
parts of code will directly descend to PMD level using pmd_off() or 
pmd_off_k()

static inline pmd_t *pmd_off(struct mm_struct *mm, unsigned long va)
{
	return pmd_offset(pud_offset(p4d_offset(pgd_offset(mm, va), va), va), va);
}

static inline pmd_t *pmd_off_k(unsigned long va)
{
	return pmd_offset(pud_offset(p4d_offset(pgd_offset_k(va), va), va), va);
}


Christophe