Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid_size

[Longpeng (Mike, Cloud Infrastructure Service Product Dept.)]

On 2020/3/19 6:52, Mike Kravetz wrote:
> On 3/18/20 3:15 PM, Dave Hansen wrote:
>> Hi Mike,
>>
>> The series looks like a great idea to me.  One nit on the x86 bits,
>> though...
>>
>>> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
>>> index 5bfd5aef5378..51e6208fdeec 100644
>>> --- a/arch/x86/mm/hugetlbpage.c
>>> +++ b/arch/x86/mm/hugetlbpage.c
>>> @@ -181,16 +181,25 @@ hugetlb_get_unmapped_area(struct file *file, unsigned 
>>> long addr,
>>>  #endif /* CONFIG_HUGETLB_PAGE */
>>>  
>>>  #ifdef CONFIG_X86_64
>>> +bool __init arch_hugetlb_valid_size(unsigned long long size)
>>> +{
>>> +   if (size == PMD_SIZE)
>>> +           return true;
>>> +   else if (size == PUD_SIZE && boot_cpu_has(X86_FEATURE_GBPAGES))
>>> +           return true;
>>> +   else
>>> +           return false;
>>> +}
>>
>> I'm pretty sure it's possible to have a system without 2M/PMD page
>> support.  We even have a handy-dandy comment about it in
>> arch/x86/include/asm/required-features.h:
>>
>>      #ifdef CONFIG_X86_64
>>      #ifdef CONFIG_PARAVIRT
>>      /* Paravirtualized systems may not have PSE or PGE available */
>>      #define NEED_PSE        0
>>      ...
>>
>> I *think* you need an X86_FEATURE_PSE check here to be totally correct.
>>
>>      if (size == PMD_SIZE && cpu_feature_enabled(X86_FEATURE_PSE))
>>              return true;
>>
>> BTW, I prefer cpu_feature_enabled() to boot_cpu_has() because it
>> includes disabled-features checking.  I don't think any of it matters
>> for these specific features, but I generally prefer it on principle.
> 
> Sounds good.  I'll incorporate those changes into a v2, unless someone
> else with has a different opinion.
> 
> BTW, this patch should not really change the way the code works today.
> It is mostly a movement of code.  Unless I am missing something, the
> existing code will always allow setup of PMD_SIZE hugetlb pages.
> 
Hi Mike,

Inspired by Dave's opinion, it seems the x86-specific hugepages_supported should
also need to use cpu_feature_enabled instead.

Also, I wonder if the hugepages_supported is correct ? There're two arch
specific hugepages_supported:
x86:
#define hugepages_supported() boot_cpu_has(X86_FEATURE_PSE)
and
s390:
#define hugepages_supported() (MACHINE_HAS_EDAT1)

Is it possible that x86 has X86_FEATURE_GBPAGES but hasn't X86_FEATURE_GBPAGES
or s390 has MACHINE_HAS_EDAT2 but hasn't MACHINE_HAS_EDAT1 ?

---
Regards,
Longpeng(Mike)