Re: [PATCH 4/4] mm/huge_memory: use the large-page PAT encoding for huge pfn mappings

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 11/11/26 3:06 PM, Eliav Farber wrote:
> insert_pmd() and insert_pud() build the entry from the pgprot that
> vmf_insert_pfn_pmd()/_pud() took from vma->vm_page_prot and passed
> through pfnmap_setup_cachemode_pfn(). That pgprot carries the PTE-level
> cache mode encoding, but x86 keeps the high PAT bit at bit 7 for a PTE
> (_PAGE_BIT_PAT) and at bit 12 for a PMD or PUD (_PAGE_BIT_PAT_LARGE),
> where bit 7 is _PAGE_PSE.
> 
> A memtype that needs that bit, write-through or write-protected, loses
> it: the cache mode index of the huge entry drops from 4-7 to 0-3, so a
> write-through range is mapped write-back. The other x86 paths that build

here contradicts the cover letter, shouldn't it say the same as the 
cover letter since its the one right?

> a large entry from a PTE pgprot convert it first, see populate_pmd() and
> populate_pud().
> 
> Convert in the two huge pfn paths. Add a generic pgprot_4k_2_large()
> that returns its argument unchanged, so that only architectures which
> encode the levels differently have to define it, and x86's existing
> definition is picked up.
> 
> The folio paths keep using vma->vm_page_prot as is. They map
> struct-page-backed memory, which x86 maps write-back.
> 
> Fixes: 6857be5fecae ("mm: introduce ARCH_SUPPORTS_HUGE_PFNMAP and special bits to pmd/pud")

I think that 5cad465d7fa6 ("mm: add vmf_insert_pfn_pmd()") would fit 
better, IMHO.

> Signed-off-by: Eliav Farber <farbere@amazon.com>
> ---
>   arch/x86/include/asm/pgtable_types.h | 2 ++
>   include/linux/pgtable.h              | 9 +++++++++
>   mm/huge_memory.c                     | 4 ++--
>   3 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index b1a669e775f4..b1ffd72e47ef 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -497,6 +497,8 @@ static inline pgprotval_t protval_4k_2_large(pgprotval_t val)
>   	return (val & ~(_PAGE_PAT | _PAGE_PAT_LARGE)) |
>   		((val & _PAGE_PAT) << (_PAGE_BIT_PAT_LARGE - _PAGE_BIT_PAT));
>   }
> +
> +#define pgprot_4k_2_large pgprot_4k_2_large
>   static inline pgprot_t pgprot_4k_2_large(pgprot_t pgprot)
>   {
>   	return __pgprot(protval_4k_2_large(pgprot_val(pgprot)));
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 780fe849ff8b..112814d31f0c 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1787,6 +1787,15 @@ void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
>   #define pgprot_mhp(prot)	(prot)
>   #endif
>   
> +/*
> + * Convert a pgprot built for a PTE so that it can be used in a PMD or PUD
> + * entry. Only needed on architectures that encode attributes differently
> + * between the levels; elsewhere the pgprot is used as is.
> + */
> +#ifndef pgprot_4k_2_large
> +#define pgprot_4k_2_large(prot)	(prot)
> +#endif
> +
>   #ifdef CONFIG_MMU
>   #ifndef pgprot_modify
>   #define pgprot_modify pgprot_modify
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 052c50dadc48..945d480c1512 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1675,7 +1675,7 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>   			add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
>   		}
>   	} else {
> -		entry = pmd_mkhuge(pfn_pmd(fop.pfn, prot));
> +		entry = pmd_mkhuge(pfn_pmd(fop.pfn, pgprot_4k_2_large(prot)));
>   		entry = pmd_mkspecial(entry);
>   	}
>   	if (write) {
> @@ -1804,7 +1804,7 @@ static vm_fault_t insert_pud(struct vm_area_struct *vma, unsigned long addr,
>   		folio_add_file_rmap_pud(fop.folio, &fop.folio->page, vma);
>   		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PUD_NR);
>   	} else {
> -		entry = pud_mkhuge(pfn_pud(fop.pfn, prot));
> +		entry = pud_mkhuge(pfn_pud(fop.pfn, pgprot_4k_2_large(prot)));
>   		entry = pud_mkspecial(entry);
>   	}
>   	if (write) {

the code LGTM in general.

-- 
cheers,
jose a. p-a