Re: [RFC PATCH 1/3] hugetlb: skip to end of PT page mapping when pte not present

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/31/22 22:35, Mike Kravetz wrote:
> On 5/30/22 19:04, Muchun Song wrote:
>> On Mon, May 30, 2022 at 03:56:43PM -0400, Peter Xu wrote:
>>> Hi, Mike,
>>>
>>> On Fri, May 27, 2022 at 03:58:47PM -0700, Mike Kravetz wrote:
>>>> +unsigned long hugetlb_mask_last_hp(struct hstate *h)
>>>> +{
>>>> +	unsigned long hp_size = huge_page_size(h);
>>>> +
>>>> +	if (hp_size == P4D_SIZE)
>>>> +		return PGDIR_SIZE - P4D_SIZE;
>>>> +	else if (hp_size == PUD_SIZE)
>>>> +		return P4D_SIZE - PUD_SIZE;
>>>> +	else if (hp_size == PMD_SIZE)
>>>> +		return PUD_SIZE - PMD_SIZE;
>>>> +
>>>> +	return ~(0);
>>>> +}
>>>
>>> How about:
>>>
>>> unsigned long hugetlb_mask_last_hp(struct hstate *h)
>>> {
>>> 	unsigned long hp_size = huge_page_size(h);
>>>
>>> 	return hp_size * (PTRS_PER_PTE - 1);
>>> }
>>>
>>
>> +1
>>  
> 
> 
> I like this as well, but I wonder if we should put something like the
> following in just to be safe.
> 
> BUILD_BUG_ON(PTRS_PER_PTE != PTRS_PER_PMD);
> BUILD_BUG_ON(PTRS_PER_PTE != PTRS_PER_PUD);
> BUILD_BUG_ON(PTRS_PER_PTE != PTRS_PER_P4D);
Exactly, assuming that PTRS_PER_PTE entries are present per page table page for
all supported HugeTLB levels might be bit risky, particularly for the top level.
Hence rather than all these additional BUILD_BUG_ON() checks for using standard
page table page entries i.e PTRS_PER_PTE, it might be better to just stick with
the original check either via 'if else' as proposed or better with a switch case.