Re: [PATCH 1/4] hugetlb: skip to end of PT page mapping when pte not present

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 6/18/2022 1:17 AM, Mike Kravetz wrote:
> On 06/17/22 10:15, Peter Xu wrote:
>> Hi, Mike,
>>
>> On Thu, Jun 16, 2022 at 02:05:15PM -0700, Mike Kravetz wrote:
>>> @@ -6877,6 +6896,39 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>>>   	return (pte_t *)pmd;
>>>   }
>>>   
>>> +/*
>>> + * Return a mask that can be used to update an address to the last huge
>>> + * page in a page table page mapping size.  Used to skip non-present
>>> + * page table entries when linearly scanning address ranges.  Architectures
>>> + * with unique huge page to page table relationships can define their own
>>> + * version of this routine.
>>> + */
>>> +unsigned long hugetlb_mask_last_page(struct hstate *h)
>>> +{
>>> +	unsigned long hp_size = huge_page_size(h);
>>> +
>>> +	switch (hp_size) {
>>> +	case P4D_SIZE:
>>> +		return PGDIR_SIZE - P4D_SIZE;
>>> +	case PUD_SIZE:
>>> +		return P4D_SIZE - PUD_SIZE;
>>> +	case PMD_SIZE:
>>> +		return PUD_SIZE - PMD_SIZE;
>>> +	default:
>>
>> Should we add a WARN_ON_ONCE() if it should never trigger?
>>
> 
> Sure.  I will add this.
> 
>>> +		break; /* Should never happen */
>>> +	}
>>> +
>>> +	return ~(0UL);
>>> +}
>>> +
>>> +#else
>>> +
>>> +/* See description above.  Architectures can provide their own version. */
>>> +__weak unsigned long hugetlb_mask_last_page(struct hstate *h)
>>> +{
>>> +	return ~(0UL);
>>
>> I'm wondering whether it's better to return 0 rather than ~0 by default.
>> Could an arch with !CONFIG_ARCH_WANT_GENERAL_HUGETLB wrongly skip some
>> valid address ranges with ~0, or perhaps I misread?
> 
> Thank you, thank you, thank you Peter!
> 
> Yes, the 'default' return for hugetlb_mask_last_page() should be 0.  If
> there is no 'optimization', we do not want to modify the address so we
> want to OR with 0 not ~0.  My bad, I must have been thinking AND instead
> of OR.
> 
> I will change here as well as in Baolin's patch.

Ah, I also overlooked this. Thanks Peter, and thanks Mike for updating.