Re: [PATCH RFC 03/11] x86/mm: Add lookup_pgtable_in_pgd()

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Thu, Mar 13, 2025 at 06:11:22PM +0000, Brendan Jackman wrote:
> This is the same thing as lookup_address_in_pgd(), but it returns the
> pagetable unconditionally instead of returning NULL when the pagetable
> is none. This will be used for looking up and modifying pages that are
> *_none() in order to map memory into the ASI restricted address space.
> 
> For a [PATCH], if this logic is needed, the surrounding code should
> probably first be somewhat refactored. It now looks pretty repetitive,
> and it's confusing that lookup_address_in_pgd() returns NULL when
> pmd_none() but note when pte_none(). For now here's something that
> works.

My first instinct reading this is that lookup_address_in_pgd() should be
calling lookup_pgtable_in_pgd(), but I didn't look too closely.

> 
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
>  arch/x86/include/asm/pgtable_types.h |  2 ++
>  arch/x86/mm/pat/set_memory.c         | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 4b804531b03c3ce5cc48f0a75cb75d58b985777a..e09b509e525534f31c986d705e07b25dd9c04cb7 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -572,6 +572,8 @@ extern pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
>  				    unsigned int *level);
>  pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>  				  unsigned int *level, bool *nx, bool *rw);
> +extern pte_t *lookup_pgtable_in_pgd(pgd_t *pgd, unsigned long address,
> +				    unsigned int *level);
>  extern pmd_t *lookup_pmd_address(unsigned long address);
>  extern phys_addr_t slow_virt_to_phys(void *__address);
>  extern int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn,
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index ef4514d64c0524e5854fa106e3f37ff1e1ba10a2..d066bf2c9e93e126757bd32a7a666db89b2488b6 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -658,6 +658,40 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
>  	return new;
>  }
>  
> +/*
> + * Lookup the page table entry for a virtual address in a specific pgd. Return
> + * the pointer to the entry, without implying that any mapping actually exists
> + * (the returned value may be zero).
> + */
> +pte_t *lookup_pgtable_in_pgd(pgd_t *pgd, unsigned long address, unsigned int *level)
> +{
> +	p4d_t *p4d;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +
> +	*level = PG_LEVEL_256T;
> +	if (pgd_none(*pgd))
> +		return (pte_t *)pgd;
> +
> +	*level = PG_LEVEL_512G;
> +	p4d = p4d_offset(pgd, address);
> +	if (p4d_none(*p4d) || p4d_leaf(*p4d) || !p4d_present(*p4d))
> +		return (pte_t *)p4d;
> +
> +	*level = PG_LEVEL_1G;
> +	pud = pud_offset(p4d, address);
> +	if (pud_none(*pud) || pud_leaf(*pud) || !pud_present(*pud))
> +		return (pte_t *)pud;
> +
> +	*level = PG_LEVEL_2M;
> +	pmd = pmd_offset(pud, address);
> +	if (pmd_none(*pmd) || pmd_leaf(*pmd) || !pmd_present(*pmd))
> +		return (pte_t *)pmd;
> +
> +	*level = PG_LEVEL_4K;
> +	return pte_offset_kernel(pmd, address);
> +}
> +
>  /*
>   * Lookup the page table entry for a virtual address in a specific pgd.
>   * Return a pointer to the entry (or NULL if the entry does not exist),
> 
> -- 
> 2.49.0.rc1.451.g8f38331e32-goog
>