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Subject: Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid_size
To: Dave Hansen <dave.hansen@intel.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
 linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
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References: <20200318220634.32100-1-mike.kravetz@oracle.com>
 <20200318220634.32100-2-mike.kravetz@oracle.com>
 <831a0773-1ba6-4d72-44b9-7472123b8528@intel.com>
From: Mike Kravetz <mike.kravetz@oracle.com>
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Date: Wed, 18 Mar 2020 15:52:46 -0700
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Cc: Albert Ou <aou@eecs.berkeley.edu>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

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.
-- 
Mike Kravetz