Re: [PATCH 3/6] arm64/mm: Make pgd_pgtable_alloc() call pte_alloc_one() always

[Will Deacon <will.deacon@arm.com>]

On Thu, Jan 24, 2019 at 10:50:17AM +0530, Anshuman Khandual wrote:
> pgd_pgtable_alloc() provides page allocation function while creating all
> levels of page table (e.g PGD, PUD, CONT PMD) for various kernel mappings.
> It calls __get_free_page() and initializes page with pagetable_page_ctor().
> pte_alloc_one() already provides a standard interface for allocating a page
> table page and initializes it with pagetable_page_ctor(). This removes the
> redundancy and instead make it call pte_alloc_one() directly.
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  arch/arm64/mm/mmu.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index b6f5aa5..2dbd723 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -372,13 +372,22 @@ static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
>  
>  static phys_addr_t pgd_pgtable_alloc(void)
>  {
> -	void *ptr = (void *)__get_free_page(PGALLOC_GFP);
> -	if (!ptr || !pgtable_page_ctor(virt_to_page(ptr)))
> -		BUG();
> +
> +	pgtable_t ptr;
> +
> +	/*
> +	 * pgd_pgtable_alloc() is called while creating kernel mappings
> +	 * through __create_pgd_mapping() might not install it through
> +	 * swapper_pg_dir (&init_mm). Even then init_mm is passed here
> +	 * just to indicate that the allocation is kernel specific not
> +	 * for the user space page tables.
> +	 */
> +	ptr = pte_alloc_one(&init_mm);
> +	BUG_ON(!ptr);
>  
>  	/* Ensure the zeroed page is visible to the page table walker */
>  	dsb(ishst);
> -	return __pa(ptr);
> +	return page_to_phys(ptr);

Are you sure this is safe? afaict, this code is used to set up the linear
mapping which happens before the mem_map is initialised. I don't think
page_to_phys() can be used at this point. What am I missing?

Will

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