Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start()

[David Hildenbrand <david@redhat.com>]

>>>
>>> If you prefer vm_normal_page_pud() to be defined and check pud_special()
>>> there, I can do that.  But again, I don't yet see how that can make a
>>> functional difference considering the so far very limited usage of the
>>> special bit, and wonder whether we can do that on top when it became
>>> necessary (and when we start to have functional requirement of such).
>>
>> I hope my explanation why pte_special() even exists and how it is used makes
>> it clearer.
>>
>> It's not that much code to handle it like pte_special(), really. I don't
>> expect you to teach GUP-slow about vm_normal_page() etc.
> 
> One thing I can do here is I move the pmd_special() check into the existing
> vm_normal_page_pmd(), then it'll be a fixup on top of this patch:
> 
> ===8<===
> diff --git a/mm/memory.c b/mm/memory.c
> index 288f81a8698e..42674c0748cb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -672,11 +672,10 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>   {
>   	unsigned long pfn = pmd_pfn(pmd);
>   
> -	/*
> -	 * There is no pmd_special() but there may be special pmds, e.g.
> -	 * in a direct-access (dax) mapping, so let's just replicate the
> -	 * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
> -	 */
> +	/* Currently it's only used for huge pfnmaps */
> +	if (unlikely(pmd_special(pmd)))
> +		return NULL;


Better.

I'd appreciate a vm_normal_page_pud(), but I guess I have to be the one 
cleaning up the mess after you.

-- 
Cheers,

David / dhildenb