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

[David Hildenbrand <david@redhat.com>]

> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1398,10 +1398,7 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>   	}
>   
>   	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
> -	if (pfn_t_devmap(pfn))
> -		entry = pmd_mkdevmap(entry);
> -	else
> -		entry = pmd_mkspecial(entry);
> +	entry = pmd_mkspecial(entry);
>   	if (write) {


I just stumbled over this, and I think there is something off here in 
the PMD/PUD case.

vmf_insert_folio_pmd() does a folio_get() + folio_add_file_rmap_pmd().

But then, we go ahead and turn this into a special mapping by setting it 
pmd_mkdevmap()/pmd_mkspecial().

Consequently, vm_normal_page_pmd() would ignore them, not following the 
rules documented for vm_normal_page() and behaving differently than 
vmf_insert_page_mkwrite()->insert_page().


folio_add_file_rmap_pmd() should never set these things special/devmap 
in the first place :/

What am I missing?

Note that fs/dax.c calls vmf_insert_folio_pmd() for PMDs and 
vmf_insert_page_mkwrite() for PTEs.

Consequently, PTEs will never be marked special (corner case, shared 
zeropage), but PMDs would always.

Hm?

-- 
Cheers,

David / dhildenb