Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls

David Hildenbrand
2025-06-05T17:04:55Z

On 05.06.25 18:30, Dan Williams wrote:
> David Hildenbrand wrote:
>> On 05.06.25 14:09, Jason Gunthorpe wrote:
>>> On Wed, Jun 04, 2025 at 07:35:24PM -0700, Dan Williams wrote:
>>>
>>>> If all dax pages are special, then vm_normal_page() should never find
>>>> them and gup should fail.
>>>>
>>>> ...oh, but vm_normal_page_p[mu]d() is not used in the gup path, and
>>>> 'special' is not set in the pte path.
>>>
>>> That seems really suboptimal?? Why would pmd and pte be different?
>>>
>>>> I think for any p[mu]d where p[mu]d_page() is ok to use should never set
>>>> 'special', right?
>>>
>>> There should be dedicated functions for installing pages and PFNs,
>>> only the PFN one would set the special bit.
>>>
>>> And certainly your tests *should* be failing as special entries should
>>> never ever be converted to struct page.
>>
>> Worth reviewing [1] where I clean that up and describe the current
>> impact. ;)
>
> Will do.
>
>> What's even worse about this pte_devmap()/pmd_devmap()/... shit (sorry!
>> but it's absolute shit) is that some pte_mkdev() set the pte special,
>> while others ... don't.
>
> As the person who started the turd rolling into this pile that Alistair
> is heroically cleaning up, I approve this characterization.
>
>> E.g., loongarch
>>
>> static inline pte_t pte_mkdevmap(pte_t pte) { pte_val(pte) |=
>> _PAGE_DEVMAP; return pte; }
>>
>> I don't even know how it can (could) survive vm_normal_page().
>
> Presently "can" because dax switched away from vmf_insert_mixed() to
> vmf_insert_page(), "could" in the past was the devmap hack to avoid
> treating VM_MIXEDMAP as !vm_normal_page().

The thing is, in vm_normal_page() if we have CONFIG_ARCH_HAS_PTE_SPECIAL
-- which loongarch sets -- if we don't see pte_special(), we will assume
that it is refcounted.

if (likely(!pte_special(pte))
goto check_pfn;

So if pte_mkdevmap() does not set pte_special(), then ...
vm_normal_page() would detect it as normal, although it isn't normal?

But maybe I am missing something important.

--
Cheers,

David / dhildenb