Subject: Re: [RFC PATCH v2 11/20] powerpc/mm: Complement huge_pte_alloc() for all non HUGEPD setups

From: Oscar Salvador <[email-masked]>
Date: Sat, 25 May 2024 06:29:48 +0200

On Fri, May 17, 2024 at 09:00:05PM +0200, Christophe Leroy wrote:
> huge_pte_alloc() for non-HUGEPD targets is reserved for 8xx at the
> moment. In order to convert other targets for non-HUGEPD, complement
> huge_pte_alloc() to support any standard cont-PxD setup.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/mm/hugetlbpage.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 42b12e1ec851..f8aefa1e7363 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -195,11 +195,34 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  		      unsigned long addr, unsigned long sz)
>  {
> -	pmd_t *pmd = pmd_off(mm, addr);
> +	pgd_t *pgd;
> +	p4d_t *p4d;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +
> +	addr &= ~(sz - 1);
> +	pgd = pgd_offset(mm, addr);
> +
> +	p4d = p4d_offset(pgd, addr);
> +	if (sz >= PGDIR_SIZE)
> +		return (pte_t *)p4d;
> +
> +	pud = pud_alloc(mm, p4d, addr);
> +	if (!pud)
> +		return NULL;
> +	if (sz >= PUD_SIZE)
> +		return (pte_t *)pud;
> +
> +	pmd = pmd_alloc(mm, pud, addr);
> +	if (!pmd)
> +		return NULL;
>  
>  	if (sz < PMD_SIZE)
>  		return pte_alloc_huge(mm, pmd, addr, sz);
>  
> +	if (!IS_ENABLED(CONFIG_PPC_8xx))
> +		return (pte_t *)pmd;

So only 8xx has cont-PMD for hugepages?

> +
>  	if (sz != SZ_8M)
>  		return NULL;

Since this function is the core for allocation huge pages, I think it would
benefit from a comment at the top explaining the possible layouts.
e.g: Who can have cont-{P4d,PUD,PMD} etc.
A brief explanation of the possible scheme for all powerpc platforms.

That would help people looking into this in a future.

-- 
Oscar Salvador
SUSE Labs