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

[Wei Yang <richard.weiyang@gmail.com>]

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
}

>>>
>>> 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.
>
>

-- 
Wei Yang
Help you, Help me