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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 11/26/25 15:52, Lorenzo Stoakes wrote:
> On Wed, Nov 26, 2025 at 03:46:40PM +0100, David Hildenbrand (Red Hat) wrote:
>> 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()?
> 
> 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.

> 
>>
>> 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 :)

-- 
Cheers

David