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

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

On 11/26/25 15:37, Lorenzo Stoakes wrote:
> On Wed, Nov 26, 2025 at 02:22:13PM +0000, 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.
>>
>> 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).
> 
> Yeah, it kinda sucks to bake that assumption in too even if we can prove it
> currently _is_ correct, and it becomes tricky because to somebody observing this
> they might well think 'oh so we don't need to think about tearing here' but in
> reality we are just assuming somebody already thought about it for us :)

Looking at include/asm-generic/pgtable-nopmd.h, PUD entries there are

* always present (pud_present() == 1)
* always a page table (pud_leaf() == 0)

And pmd_offset() is just a typecast.

So I wonder if that means that we can make pudp_get() be a simple load 
(!READ_ONCE) because nobody should possibly do something with that value 
as we must perform the pmd_present() checks etc. later and obtain the 
PMD through a READ_ONCE().

So far my thinking, maybe it's flawed :)

-- 
Cheers

David