Re: [PATCH] arm64: mm: Use generic enum pgtable_level

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 16/03/2026 15:45, David Hildenbrand (Arm) wrote:
> On 3/16/26 15:38, Kevin Brodsky wrote:
>> On 16/03/2026 15:22, Ryan Roberts wrote:
>>>>  static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
>>>> -				       enum pgtable_type pgtable_type)
>>>> +				       enum pgtable_level pgtable_level)
>>>>  {
>>>>  	/* Page is zeroed by init_clear_pgtable() so don't duplicate effort. */
>>>>  	struct ptdesc *ptdesc = pagetable_alloc(gfp & ~__GFP_ZERO, 0);
>>>> @@ -539,40 +539,42 @@ static phys_addr_t __pgd_pgtable_alloc(struct mm_struct *mm, gfp_t gfp,
>>>>  
>>>>  	pa = page_to_phys(ptdesc_page(ptdesc));
>>>>  
>>>> -	switch (pgtable_type) {
>>>> -	case TABLE_PTE:
>>>> +	switch (pgtable_level) {
>>>> +	case PGTABLE_LEVEL_PTE:
>>>>  		BUG_ON(!pagetable_pte_ctor(mm, ptdesc));
>>>>  		break;
>>>> -	case TABLE_PMD:
>>>> +	case PGTABLE_LEVEL_PMD:
>>>>  		BUG_ON(!pagetable_pmd_ctor(mm, ptdesc));
>>>>  		break;
>>>> -	case TABLE_PUD:
>>>> +	case PGTABLE_LEVEL_PUD:
>>>>  		pagetable_pud_ctor(ptdesc);
>>>>  		break;
>>>> -	case TABLE_P4D:
>>>> +	case PGTABLE_LEVEL_P4D:
>>>>  		pagetable_p4d_ctor(ptdesc);
>>>>  		break;
>>>> +	default:
>>>> +		break;
>>> nit: I think we should either explicitly support pgd or explicitly bug/warn. Now
>>> that the enum has PGTABLE_LEVEL_PGD it looks legit to call __pgd_pgtable_alloc()
>>> to allocate one. But it will currently silently fail to call pagetable_pgd_ctor().
>> I hesitated there as well, eventually I concluded that we're dealing
>> with kernel page tables so we'll never allocate a PGD anyway...
>>
>>> Probably simplest just to call BUG() in the default path?
>> ... but that's certainly fine by me :)
> If we could force it to be inline, we could turn it into a BUILD_BUG().

I don't think this would help, pgd_pgtable_alloc_init_mm() is passed
around as a function pointer so the compiler can't know the value of
pgtable_level at compile time.

> 	VM_WARN_ON()
>
> might be good enough I guess.

I think so, not calling a ctor at PGD level has no real consequence (it
wasn't done at all until recently).

- Kevin